csharplang/spec/variables.md

Variables

Variables represent storage locations. Every variable has a type that determines what values can be stored in the variable. C# is a type-safe language, and the C# compiler guarantees that values stored in variables are always of the appropriate type. The value of a variable can be changed through assignment or through use of the ++ and -- operators.

A variable must be definitely assigned (Definite assignment) before its value can be obtained.

As described in the following sections, variables are either initially assigned or initially unassigned. An initially assigned variable has a well-defined initial value and is always considered definitely assigned. An initially unassigned variable has no initial value. For an initially unassigned variable to be considered definitely assigned at a certain location, an assignment to the variable must occur in every possible execution path leading to that location.

Variable categories

C# defines seven categories of variables: static variables, instance variables, array elements, value parameters, reference parameters, output parameters, and local variables. The sections that follow describe each of these categories.

In the example

class A
{
    public static int x;
    int y;

    void F(int[] v, int a, ref int b, out int c) {
        int i = 1;
        c = a + b++;
    }
}

x is a static variable, y is an instance variable, v[0] is an array element, a is a value parameter, b is a reference parameter, c is an output parameter, and i is a local variable.

Static variables

A field declared with the static modifier is called a static variable. A static variable comes into existence before execution of the static constructor (Static constructors) for its containing type, and ceases to exist when the associated application domain ceases to exist.

The initial value of a static variable is the default value (Default values) of the variable's type.

For purposes of definite assignment checking, a static variable is considered initially assigned.

Instance variables

A field declared without the static modifier is called an instance variable.

Instance variables in classes

An instance variable of a class comes into existence when a new instance of that class is created, and ceases to exist when there are no references to that instance and the instance's destructor (if any) has executed.

The initial value of an instance variable of a class is the default value (Default values) of the variable's type.

For the purpose of definite assignment checking, an instance variable of a class is considered initially assigned.

Instance variables in structs

An instance variable of a struct has exactly the same lifetime as the struct variable to which it belongs. In other words, when a variable of a struct type comes into existence or ceases to exist, so too do the instance variables of the struct.

The initial assignment state of an instance variable of a struct is the same as that of the containing struct variable. In other words, when a struct variable is considered initially assigned, so too are its instance variables, and when a struct variable is considered initially unassigned, its instance variables are likewise unassigned.

Array elements

The elements of an array come into existence when an array instance is created, and cease to exist when there are no references to that array instance.

The initial value of each of the elements of an array is the default value (Default values) of the type of the array elements.

For the purpose of definite assignment checking, an array element is considered initially assigned.

Value parameters

A parameter declared without a ref or out modifier is a value parameter.

A value parameter comes into existence upon invocation of the function member (method, instance constructor, accessor, or operator) or anonymous function to which the parameter belongs, and is initialized with the value of the argument given in the invocation. A value parameter normally ceases to exist upon return of the function member or anonymous function. However, if the value parameter is captured by an anonymous function (Anonymous function expressions), its life time extends at least until the delegate or expression tree created from that anonymous function is eligible for garbage collection.

For the purpose of definite assignment checking, a value parameter is considered initially assigned.

Reference parameters

A parameter declared with a ref modifier is a reference parameter.

A reference parameter does not create a new storage location. Instead, a reference parameter represents the same storage location as the variable given as the argument in the function member or anonymous function invocation. Thus, the value of a reference parameter is always the same as the underlying variable.

The following definite assignment rules apply to reference parameters. Note the different rules for output parameters described in Output parameters.

• A variable must be definitely assigned (Definite assignment) before it can be passed as a reference parameter in a function member or delegate invocation.
• Within a function member or anonymous function, a reference parameter is considered initially assigned.

Within an instance method or instance accessor of a struct type, the this keyword behaves exactly as a reference parameter of the struct type (This access).

Output parameters

A parameter declared with an out modifier is an output parameter.

An output parameter does not create a new storage location. Instead, an output parameter represents the same storage location as the variable given as the argument in the function member or delegate invocation. Thus, the value of an output parameter is always the same as the underlying variable.

The following definite assignment rules apply to output parameters. Note the different rules for reference parameters described in Reference parameters.

• A variable need not be definitely assigned before it can be passed as an output parameter in a function member or delegate invocation.
• Following the normal completion of a function member or delegate invocation, each variable that was passed as an output parameter is considered assigned in that execution path.
• Within a function member or anonymous function, an output parameter is considered initially unassigned.
• Every output parameter of a function member or anonymous function must be definitely assigned (Definite assignment) before the function member or anonymous function returns normally.

Within an instance constructor of a struct type, the this keyword behaves exactly as an output parameter of the struct type (This access).

Local variables

A local variable is declared by a local_variable_declaration, which may occur in a block, a for_statement, a switch_statement or a using_statement; or by a foreach_statement or a specific_catch_clause for a try_statement.

The lifetime of a local variable is the portion of program execution during which storage is guaranteed to be reserved for it. This lifetime extends at least from entry into the block, for_statement, switch_statement, using_statement, foreach_statement, or specific_catch_clause with which it is associated, until execution of that block, for_statement, switch_statement, using_statement, foreach_statement, or specific_catch_clause ends in any way. (Entering an enclosed block or calling a method suspends, but does not end, execution of the current block, for_statement, switch_statement, using_statement, foreach_statement, or specific_catch_clause.) If the local variable is captured by an anonymous function (Captured outer variables), its lifetime extends at least until the delegate or expression tree created from the anonymous function, along with any other objects that come to reference the captured variable, are eligible for garbage collection.

If the parent block, for_statement, switch_statement, using_statement, foreach_statement, or specific_catch_clause is entered recursively, a new instance of the local variable is created each time, and its local_variable_initializer, if any, is evaluated each time.

A local variable introduced by a local_variable_declaration is not automatically initialized and thus has no default value. For the purpose of definite assignment checking, a local variable introduced by a local_variable_declaration is considered initially unassigned. A local_variable_declaration may include a local_variable_initializer, in which case the variable is considered definitely assigned only after the initializing expression (Declaration statements).

Within the scope of a local variable introduced by a local_variable_declaration, it is a compile-time error to refer to that local variable in a textual position that precedes its local_variable_declarator. If the local variable declaration is implicit (Local variable declarations), it is also an error to refer to the variable within its local_variable_declarator.

A local variable introduced by a foreach_statement or a specific_catch_clause is considered definitely assigned in its entire scope.

The actual lifetime of a local variable is implementation-dependent. For example, a compiler might statically determine that a local variable in a block is only used for a small portion of that block. Using this analysis, the compiler could generate code that results in the variable's storage having a shorter lifetime than its containing block.

The storage referred to by a local reference variable is reclaimed independently of the lifetime of that local reference variable (Automatic memory management).

Default values

The following categories of variables are automatically initialized to their default values:

• Static variables.
• Instance variables of class instances.
• Array elements.

The default value of a variable depends on the type of the variable and is determined as follows:

• For a variable of a value_type, the default value is the same as the value computed by the value_type's default constructor (Default constructors).
• For a variable of a reference_type, the default value is null.

Initialization to default values is typically done by having the memory manager or garbage collector initialize memory to all-bits-zero before it is allocated for use. For this reason, it is convenient to use all-bits-zero to represent the null reference.

Definite assignment

At a given location in the executable code of a function member, a variable is said to be definitely assigned if the compiler can prove, by a particular static flow analysis (Precise rules for determining definite assignment), that the variable has been automatically initialized or has been the target of at least one assignment. Informally stated, the rules of definite assignment are:

• An initially assigned variable (Initially assigned variables) is always considered definitely assigned.
• An initially unassigned variable (Initially unassigned variables) is considered definitely assigned at a given location if all possible execution paths leading to that location contain at least one of the following:
  • A simple assignment (Simple assignment) in which the variable is the left operand.
  • An invocation expression (Invocation expressions) or object creation expression (Object creation expressions) that passes the variable as an output parameter.
  • For a local variable, a local variable declaration (Local variable declarations) that includes a variable initializer.

The formal specification underlying the above informal rules is described in Initially assigned variables, Initially unassigned variables, and Precise rules for determining definite assignment.

The definite assignment states of instance variables of a struct_type variable are tracked individually as well as collectively. In addition to the rules above, the following rules apply to struct_type variables and their instance variables:

• An instance variable is considered definitely assigned if its containing struct_type variable is considered definitely assigned.
• A struct_type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.

Definite assignment is a requirement in the following contexts:

• A variable must be definitely assigned at each location where its value is obtained. This ensures that undefined values never occur. The occurrence of a variable in an expression is considered to obtain the value of the variable, except when
  • the variable is the left operand of a simple assignment,
  • the variable is passed as an output parameter, or
  • the variable is a struct_type variable and occurs as the left operand of a member access.
• A variable must be definitely assigned at each location where it is passed as a reference parameter. This ensures that the function member being invoked can consider the reference parameter initially assigned.
• All output parameters of a function member must be definitely assigned at each location where the function member returns (through a return statement or through execution reaching the end of the function member body). This ensures that function members do not return undefined values in output parameters, thus enabling the compiler to consider a function member invocation that takes a variable as an output parameter equivalent to an assignment to the variable.
• The this variable of a struct_type instance constructor must be definitely assigned at each location where that instance constructor returns.

Initially assigned variables

The following categories of variables are classified as initially assigned:

• Static variables.
• Instance variables of class instances.
• Instance variables of initially assigned struct variables.
• Array elements.
• Value parameters.
• Reference parameters.
• Variables declared in a catch clause or a foreach statement.

Initially unassigned variables

The following categories of variables are classified as initially unassigned:

• Instance variables of initially unassigned struct variables.
• Output parameters, including the this variable of struct instance constructors.
• Local variables, except those declared in a catch clause or a foreach statement.

Precise rules for determining definite assignment

In order to determine that each used variable is definitely assigned, the compiler must use a process that is equivalent to the one described in this section.

The compiler processes the body of each function member that has one or more initially unassigned variables. For each initially unassigned variable v, the compiler determines a definite assignment state for v at each of the following points in the function member:

• At the beginning of each statement
• At the end point (End points and reachability) of each statement
• On each arc which transfers control to another statement or to the end point of a statement
• At the beginning of each expression
• At the end of each expression

The definite assignment state of v can be either:

• Definitely assigned. This indicates that on all possible control flows to this point, v has been assigned a value.
• Not definitely assigned. For the state of a variable at the end of an expression of type bool, the state of a variable that isn't definitely assigned may (but doesn't necessarily) fall into one of the following sub-states:
  • Definitely assigned after true expression. This state indicates that v is definitely assigned if the boolean expression evaluated as true, but is not necessarily assigned if the boolean expression evaluated as false.
  • Definitely assigned after false expression. This state indicates that v is definitely assigned if the boolean expression evaluated as false, but is not necessarily assigned if the boolean expression evaluated as true.

The following rules govern how the state of a variable v is determined at each location.

General rules for statements

• v is not definitely assigned at the beginning of a function member body.
• v is definitely assigned at the beginning of any unreachable statement.
• The definite assignment state of v at the beginning of any other statement is determined by checking the definite assignment state of v on all control flow transfers that target the beginning of that statement. If (and only if) v is definitely assigned on all such control flow transfers, then v is definitely assigned at the beginning of the statement. The set of possible control flow transfers is determined in the same way as for checking statement reachability (End points and reachability).
• The definite assignment state of v at the end point of a block, checked, unchecked, if, while, do, for, foreach, lock, using, or switch statement is determined by checking the definite assignment state of v on all control flow transfers that target the end point of that statement. If v is definitely assigned on all such control flow transfers, then v is definitely assigned at the end point of the statement. Otherwise; v is not definitely assigned at the end point of the statement. The set of possible control flow transfers is determined in the same way as for checking statement reachability (End points and reachability).

Block statements, checked, and unchecked statements

The definite assignment state of v on the control transfer to the first statement of the statement list in the block (or to the end point of the block, if the statement list is empty) is the same as the definite assignment statement of v before the block, checked, or unchecked statement.

Expression statements

For an expression statement stmt that consists of the expression expr:

• v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
• If v if definitely assigned at the end of expr, it is definitely assigned at the end point of stmt; otherwise; it is not definitely assigned at the end point of stmt.

Declaration statements

• If stmt is a declaration statement without initializers, then v has the same definite assignment state at the end point of stmt as at the beginning of stmt.
• If stmt is a declaration statement with initializers, then the definite assignment state for v is determined as if stmt were a statement list, with one assignment statement for each declaration with an initializer (in the order of declaration).

If statements

For an if statement stmt of the form:

if ( expr ) then_stmt else else_stmt

• v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
• If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to then_stmt and to either else_stmt or to the end-point of stmt if there is no else clause.
• If v has the state "definitely assigned after true expression" at the end of expr, then it is definitely assigned on the control flow transfer to then_stmt, and not definitely assigned on the control flow transfer to either else_stmt or to the end-point of stmt if there is no else clause.
• If v has the state "definitely assigned after false expression" at the end of expr, then it is definitely assigned on the control flow transfer to else_stmt, and not definitely assigned on the control flow transfer to then_stmt. It is definitely assigned at the end-point of stmt if and only if it is definitely assigned at the end-point of then_stmt.
• Otherwise, v is considered not definitely assigned on the control flow transfer to either the then_stmt or else_stmt, or to the end-point of stmt if there is no else clause.

Switch statements

In a switch statement stmt with a controlling expression expr:

• The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
• The definite assignment state of v on the control flow transfer to a reachable switch block statement list is the same as the definite assignment state of v at the end of expr.

While statements

For a while statement stmt of the form:

while ( expr ) while_body

• v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
• If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to while_body and to the end point of stmt.
• If v has the state "definitely assigned after true expression" at the end of expr, then it is definitely assigned on the control flow transfer to while_body, but not definitely assigned at the end-point of stmt.
• If v has the state "definitely assigned after false expression" at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt, but not definitely assigned on the control flow transfer to while_body.

Do statements

For a do statement stmt of the form:

do do_body while ( expr ) ;

• v has the same definite assignment state on the control flow transfer from the beginning of stmt to do_body as at the beginning of stmt.
• v has the same definite assignment state at the beginning of expr as at the end point of do_body.
• If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt.
• If v has the state "definitely assigned after false expression" at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt.

For statements

Definite assignment checking for a for statement of the form:

for ( for_initializer ; for_condition ; for_iterator ) embedded_statement

is done as if the statement were written:

{
    for_initializer ;
    while ( for_condition ) {
        embedded_statement ;
        for_iterator ;
    }
}

If the for_condition is omitted from the for statement, then evaluation of definite assignment proceeds as if for_condition were replaced with true in the above expansion.

Break, continue, and goto statements

The definite assignment state of v on the control flow transfer caused by a break, continue, or goto statement is the same as the definite assignment state of v at the beginning of the statement.

Throw statements

For a statement stmt of the form

throw expr ;

The definite assignment state of v at the beginning of expr is the same as the definite assignment state of v at the beginning of stmt.

Return statements

For a statement stmt of the form

return expr ;

• The definite assignment state of v at the beginning of expr is the same as the definite assignment state of v at the beginning of stmt.
• If v is an output parameter, then it must be definitely assigned either:
  • after expr
  • or at the end of the finally block of a try-finally or try-catch-finally that encloses the return statement.

For a statement stmt of the form:

return ;

• If v is an output parameter, then it must be definitely assigned either:
  • before stmt
  • or at the end of the finally block of a try-finally or try-catch-finally that encloses the return statement.

Try-catch statements

For a statement stmt of the form:

try try_block
catch(...) catch_block_1
...
catch(...) catch_block_n

• The definite assignment state of v at the beginning of try_block is the same as the definite assignment state of v at the beginning of stmt.
• The definite assignment state of v at the beginning of catch_block_i (for any i) is the same as the definite assignment state of v at the beginning of stmt.
• The definite assignment state of v at the end-point of stmt is definitely assigned if (and only if) v is definitely assigned at the end-point of try_block and every catch_block_i (for every i from 1 to n).

Try-finally statements

For a try statement stmt of the form:

try try_block finally finally_block

• The definite assignment state of v at the beginning of try_block is the same as the definite assignment state of v at the beginning of stmt.
• The definite assignment state of v at the beginning of finally_block is the same as the definite assignment state of v at the beginning of stmt.
• The definite assignment state of v at the end-point of stmt is definitely assigned if (and only if) at least one of the following is true:
  • v is definitely assigned at the end-point of try_block
  • v is definitely assigned at the end-point of finally_block

If a control flow transfer (for example, a goto statement) is made that begins within try_block, and ends outside of try_block, then v is also considered definitely assigned on that control flow transfer if v is definitely assigned at the end-point of finally_block. (This is not an only if—if v is definitely assigned for another reason on this control flow transfer, then it is still considered definitely assigned.)

Try-catch-finally statements

Definite assignment analysis for a try-catch-finally statement of the form:

try try_block
catch(...) catch_block_1
...
catch(...) catch_block_n
finally *finally_block*

is done as if the statement were a try-finally statement enclosing a try-catch statement:

try {
    try try_block
    catch(...) catch_block_1
    ...
    catch(...) catch_block_n
}
finally finally_block

The following example demonstrates how the different blocks of a try statement (The try statement) affect definite assignment.

class A
{
    static void F() {
        int i, j;
        try {
            goto LABEL;
            // neither i nor j definitely assigned
            i = 1;
            // i definitely assigned
        }

        catch {
            // neither i nor j definitely assigned
            i = 3;
            // i definitely assigned
        }

        finally {
            // neither i nor j definitely assigned
            j = 5;
            // j definitely assigned
            }
        // i and j definitely assigned
        LABEL:;
        // j definitely assigned

    }
}

Foreach statements

For a foreach statement stmt of the form:

foreach ( type identifier in expr ) embedded_statement

• The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
• The definite assignment state of v on the control flow transfer to embedded_statement or to the end point of stmt is the same as the state of v at the end of expr.

Using statements

For a using statement stmt of the form:

using ( resource_acquisition ) embedded_statement

• The definite assignment state of v at the beginning of resource_acquisition is the same as the state of v at the beginning of stmt.
• The definite assignment state of v on the control flow transfer to embedded_statement is the same as the state of v at the end of resource_acquisition.

Lock statements

For a lock statement stmt of the form:

lock ( expr ) embedded_statement

• The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
• The definite assignment state of v on the control flow transfer to embedded_statement is the same as the state of v at the end of expr.

Yield statements

For a yield return statement stmt of the form:

yield return expr ;

• The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
• The definite assignment state of v at the end of stmt is the same as the state of v at the end of expr.
• A yield break statement has no effect on the definite assignment state.

General rules for simple expressions

The following rule applies to these kinds of expressions: literals (Literals), simple names (Simple names), member access expressions (Member access), non-indexed base access expressions (Base access), typeof expressions (The typeof operator), default value expressions (Default value expressions) and nameof expressions (Nameof expressions).

• The definite assignment state of v at the end of such an expression is the same as the definite assignment state of v at the beginning of the expression.

General rules for expressions with embedded expressions

The following rules apply to these kinds of expressions: parenthesized expressions (Parenthesized expressions), element access expressions (Element access), base access expressions with indexing (Base access), increment and decrement expressions (Postfix increment and decrement operators, Prefix increment and decrement operators), cast expressions (Cast expressions), unary +, -, ~, * expressions, binary +, -, *, /, %, <<, >>, <, <=, >, >=, ==, !=, is, as, &, |, ^ expressions (Arithmetic operators, Shift operators, Relational and type-testing operators, Logical operators), compound assignment expressions (Compound assignment), checked and unchecked expressions (The checked and unchecked operators), plus array and delegate creation expressions (The new operator).

Each of these expressions has one or more sub-expressions that are unconditionally evaluated in a fixed order. For example, the binary % operator evaluates the left hand side of the operator, then the right hand side. An indexing operation evaluates the indexed expression, and then evaluates each of the index expressions, in order from left to right. For an expression expr, which has sub-expressions e1, e2, ..., eN, evaluated in that order:

• The definite assignment state of v at the beginning of e1 is the same as the definite assignment state at the beginning of expr.
• The definite assignment state of v at the beginning of ei (i greater than one) is the same as the definite assignment state at the end of the previous sub-expression.
• The definite assignment state of v at the end of expr is the same as the definite assignment state at the end of eN

Invocation expressions and object creation expressions

For an invocation expression expr of the form:

primary_expression ( arg1 , arg2 , ... , argN )

or an object creation expression of the form:

new type ( arg1 , arg2 , ... , argN )

• For an invocation expression, the definite assignment state of v before primary_expression is the same as the state of v before expr.
• For an invocation expression, the definite assignment state of v before arg1 is the same as the state of v after primary_expression.
• For an object creation expression, the definite assignment state of v before arg1 is the same as the state of v before expr.
• For each argument argi, the definite assignment state of v after argi is determined by the normal expression rules, ignoring any ref or out modifiers.
• For each argument argi for any i greater than one, the definite assignment state of v before argi is the same as the state of v after the previous arg.
• If the variable v is passed as an out argument (i.e., an argument of the form out v) in any of the arguments, then the state of v after expr is definitely assigned. Otherwise; the state of v after expr is the same as the state of v after argN.
• For array initializers (Array creation expressions), object initializers (Object initializers), collection initializers (Collection initializers) and anonymous object initializers (Anonymous object creation expressions), the definite assignment state is determined by the expansion that these constructs are defined in terms of.

Simple assignment expressions

For an expression expr of the form w = expr_rhs:

• The definite assignment state of v before expr_rhs is the same as the definite assignment state of v before expr.
• The definite assignment state of v after expr is determined by:
  • If w is the same variable as v, then the definite assignment state of v after expr is definitely assigned.
  • Otherwise, if the assignment occurs within the instance constructor of a struct type, if w is a property access designating an automatically implemented property P on the instance being constructed and v is the hidden backing field of P, then the definite assignment state of v after expr is definitely assigned.
  • Otherwise, the definite assignment state of v after expr is the same as the definite assignment state of v after expr_rhs.

&& (conditional AND) expressions

For an expression expr of the form expr_first && expr_second:

• The definite assignment state of v before expr_first is the same as the definite assignment state of v before expr.
• The definite assignment state of v before expr_second is definitely assigned if the state of v after expr_first is either definitely assigned or "definitely assigned after true expression". Otherwise, it is not definitely assigned.
• The definite assignment state of v after expr is determined by:
  • If expr_first is a constant expression with the value false, then the definite assignment state of v after expr is the same as the definite assignment state of v after expr_first.
  • Otherwise, if the state of v after expr_first is definitely assigned, then the state of v after expr is definitely assigned.
  • Otherwise, if the state of v after expr_second is definitely assigned, and the state of v after expr_first is "definitely assigned after false expression", then the state of v after expr is definitely assigned.
  • Otherwise, if the state of v after expr_second is definitely assigned or "definitely assigned after true expression", then the state of v after expr is "definitely assigned after true expression".
  • Otherwise, if the state of v after expr_first is "definitely assigned after false expression", and the state of v after expr_second is "definitely assigned after false expression", then the state of v after expr is "definitely assigned after false expression".
  • Otherwise, the state of v after expr is not definitely assigned.

In the example

class A
{
    static void F(int x, int y) {
        int i;
        if (x >= 0 && (i = y) >= 0) {
            // i definitely assigned
        }
        else {
            // i not definitely assigned
        }
        // i not definitely assigned
    }
}

the variable i is considered definitely assigned in one of the embedded statements of an if statement but not in the other. In the if statement in method F, the variable i is definitely assigned in the first embedded statement because execution of the expression (i = y) always precedes execution of this embedded statement. In contrast, the variable i is not definitely assigned in the second embedded statement, since x >= 0 might have tested false, resulting in the variable i being unassigned.

|| (conditional OR) expressions

For an expression expr of the form expr_first || expr_second:

• The definite assignment state of v before expr_first is the same as the definite assignment state of v before expr.
• The definite assignment state of v before expr_second is definitely assigned if the state of v after expr_first is either definitely assigned or "definitely assigned after false expression". Otherwise, it is not definitely assigned.
• The definite assignment statement of v after expr is determined by:
  • If expr_first is a constant expression with the value true, then the definite assignment state of v after expr is the same as the definite assignment state of v after expr_first.
  • Otherwise, if the state of v after expr_first is definitely assigned, then the state of v after expr is definitely assigned.
  • Otherwise, if the state of v after expr_second is definitely assigned, and the state of v after expr_first is "definitely assigned after true expression", then the state of v after expr is definitely assigned.
  • Otherwise, if the state of v after expr_second is definitely assigned or "definitely assigned after false expression", then the state of v after expr is "definitely assigned after false expression".
  • Otherwise, if the state of v after expr_first is "definitely assigned after true expression", and the state of v after expr_second is "definitely assigned after true expression", then the state of v after expr is "definitely assigned after true expression".
  • Otherwise, the state of v after expr is not definitely assigned.

In the example

class A
{
    static void G(int x, int y) {
        int i;
        if (x >= 0 || (i = y) >= 0) {
            // i not definitely assigned
        }
        else {
            // i definitely assigned
        }
        // i not definitely assigned
    }
}

the variable i is considered definitely assigned in one of the embedded statements of an if statement but not in the other. In the if statement in method G, the variable i is definitely assigned in the second embedded statement because execution of the expression (i = y) always precedes execution of this embedded statement. In contrast, the variable i is not definitely assigned in the first embedded statement, since x >= 0 might have tested true, resulting in the variable i being unassigned.

! (logical negation) expressions

For an expression expr of the form ! expr_operand:

• The definite assignment state of v before expr_operand is the same as the definite assignment state of v before expr.
• The definite assignment state of v after expr is determined by:
  • If the state of v after *expr_operand *is definitely assigned, then the state of v after expr is definitely assigned.
  • If the state of v after *expr_operand *is not definitely assigned, then the state of v after expr is not definitely assigned.
  • If the state of v after *expr_operand *is "definitely assigned after false expression", then the state of v after expr is "definitely assigned after true expression".
  • If the state of v after *expr_operand *is "definitely assigned after true expression", then the state of v after expr is "definitely assigned after false expression".

?? (null coalescing) expressions

For an expression expr of the form expr_first ?? expr_second:

• The definite assignment state of v before expr_first is the same as the definite assignment state of v before expr.
• The definite assignment state of v before expr_second is the same as the definite assignment state of v after expr_first.
• The definite assignment statement of v after expr is determined by:
  • If expr_first is a constant expression (Constant expressions) with value null, then the state of v after expr is the same as the state of v after expr_second.
• Otherwise, the state of v after expr is the same as the definite assignment state of v after expr_first.

?: (conditional) expressions

For an expression expr of the form expr_cond ? expr_true : expr_false:

• The definite assignment state of v before expr_cond is the same as the state of v before expr.
• The definite assignment state of v before expr_true is definitely assigned if and only if one of the following holds:
  • expr_cond is a constant expression with the value false
  • the state of v after expr_cond is definitely assigned or "definitely assigned after true expression".
• The definite assignment state of v before expr_false is definitely assigned if and only if one of the following holds:
  • expr_cond is a constant expression with the value true
  • the state of v after expr_cond is definitely assigned or "definitely assigned after false expression".
• The definite assignment state of v after expr is determined by:
  • If expr_cond is a constant expression (Constant expressions) with value true then the state of v after expr is the same as the state of v after expr_true.
  • Otherwise, if expr_cond is a constant expression (Constant expressions) with value false then the state of v after expr is the same as the state of v after expr_false.
  • Otherwise, if the state of v after expr_true is definitely assigned and the state of v after expr_false is definitely assigned, then the state of v after expr is definitely assigned.
  • Otherwise, the state of v after expr is not definitely assigned.

Anonymous functions

For a lambda_expression or anonymous_method_expression expr with a body (either block or expression) body:

• The definite assignment state of an outer variable v before body is the same as the state of v before expr. That is, definite assignment state of outer variables is inherited from the context of the anonymous function.
• The definite assignment state of an outer variable v after expr is the same as the state of v before expr.

The example

delegate bool Filter(int i);

void F() {
    int max;

    // Error, max is not definitely assigned
    Filter f = (int n) => n < max;

    max = 5;
    DoWork(f);
}

generates a compile-time error since max is not definitely assigned where the anonymous function is declared. The example

delegate void D();

void F() {
    int n;
    D d = () => { n = 1; };

    d();

    // Error, n is not definitely assigned
    Console.WriteLine(n);
}

also generates a compile-time error since the assignment to n in the anonymous function has no affect on the definite assignment state of n outside the anonymous function.

Variable references

A variable_reference is an expression that is classified as a variable. A variable_reference denotes a storage location that can be accessed both to fetch the current value and to store a new value.

variable_reference
    : expression
    ;

In C and C++, a variable_reference is known as an lvalue.

Atomicity of variable references

Reads and writes of the following data types are atomic: bool, char, byte, sbyte, short, ushort, uint, int, float, and reference types. In addition, reads and writes of enum types with an underlying type in the previous list are also atomic. Reads and writes of other types, including long, ulong, double, and decimal, as well as user-defined types, are not guaranteed to be atomic. Aside from the library functions designed for that purpose, there is no guarantee of atomic read-modify-write, such as in the case of increment or decrement.