* Update records spec with new warning * Update proposals/csharp-9.0/records.md Co-authored-by: Fred Silberberg <fred@silberberg.xyz> Co-authored-by: Fred Silberberg <fred@silberberg.xyz>
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Records
This proposal tracks the specification for the C# 9 records feature, as agreed to by the C# language design team.
The syntax for a record is as follows:
record_declaration
: attributes? class_modifier* 'partial'? 'record' identifier type_parameter_list?
parameter_list? record_base? type_parameter_constraints_clause* record_body
;
record_base
: ':' class_type argument_list?
| ':' interface_type_list
| ':' class_type argument_list? interface_type_list
;
record_body
: '{' class_member_declaration* '}'
| ';'
;
Record types are reference types, similar to a class declaration. It is an error for a record to provide
a record_base
argument_list
if the record_declaration
does not contain a parameter_list
.
At most one partial type declaration of a partial record may provide a parameter_list
.
It is an error for a parameter_list
to be empty.
Record parameters cannot use ref
, out
or this
modifiers (but in
and params
are allowed).
Inheritance
Records cannot inherit from classes, unless the class is object
, and classes cannot inherit from records. Records can inherit from other records.
Members of a record type
In addition to the members declared in the record body, a record type has additional synthesized members. Members are synthesized unless a member with a "matching" signature is declared in the record body or an accessible concrete non-virtual member with a "matching" signature is inherited. Two members are considered matching if they have the same signature or would be considered "hiding" in an inheritance scenario. It is an error for a member of a record to be named "Clone".
The synthesized members are as follows:
Equality members
If the record is derived from object
, the record type includes a synthesized readonly property equivalent to a property declared as follows:
Type EqualityContract { get; };
The property is private
if the record type is sealed
. Otherwise, the property is virtual
and protected
.
The property can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
.
If the record type is derived from a base record type Base
, the record type includes a synthesized readonly property equivalent to a property declared as follows:
protected override Type EqualityContract { get; };
The property can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
. It is an error if either synthesized, or explicitly declared property doesn't override a property with this signature in the record type Base
(for example, if the property is missing in the Base
, or sealed, or not virtual, etc.).
The synthesized property returns typeof(R)
where R
is the record type.
The record type implements System.IEquatable<R>
and includes a synthesized strongly-typed overload of Equals(R? other)
where R
is the record type.
The method is public
, and the method is virtual
unless the record type is sealed
.
The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
.
If either Equals(R? other)
or GetHashCode
are user-defined (not synthesized), but not both, a warning is produced.
public virtual bool Equals(R? other);
The synthesized Equals(R?)
returns true
if and only if each of the following are true
:
other
is notnull
, and- For each instance field
fieldN
in the record type that is not inherited, the value ofSystem.Collections.Generic.EqualityComparer<TN>.Default.Equals(fieldN, other.fieldN)
whereTN
is the field type, and - If there is a base record type, the value of
base.Equals(other)
(a non-virtual call topublic virtual bool Equals(Base? other)
); otherwise the value ofEqualityContract == other.EqualityContract
.
The record type includes synthesized ==
and !=
operators equivalent to operators declared as follows:
pubic static bool operator==(R? r1, R? r2)
=> (object)r1 == r2 || (r1?.Equals(r2) ?? false);
public static bool operator!=(R? r1, R? r2)
=> !(r1 == r2);
The Equals
method called by the ==
operator is the Equals(R? other)
method specified above. The !=
operator delegates to the ==
operator. It is an error if the operators are declared explicitly.
If the record type is derived from a base record type Base
, the record type includes a synthesized override equivalent to a method declared as follows:
public sealed override bool Equals(Base? other);
It is an error if the override is declared explicitly. It is an error if the method doesn't override a method with same signature in record type Base
(for example, if the method is missing in the Base
, or sealed, or not virtual, etc.).
The synthesized override returns Equals((object?)other)
.
The record type includes a synthesized override equivalent to a method declared as follows:
public override bool Equals(object? obj);
It is an error if the override is declared explicitly. It is an error if the method doesn't override object.Equals(object? obj)
(for example, due to shadowing in intermediate base types, etc.).
The synthesized override returns Equals(other as R)
where R
is the record type.
The record type includes a synthesized override equivalent to a method declared as follows:
public override int GetHashCode();
The method can be declared explicitly.
It is an error if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
. It is an error if either synthesized, or explicitly declared method doesn't override object.GetHashCode()
(for example, due to shadowing in intermediate base types, etc.).
A warning is reported if one of Equals(R?)
and GetHashCode()
is explicitly declared but the other method is not explicit.
The synthesized override of GetHashCode()
returns an int
result of a deterministic function combining the following values:
- For each instance field
fieldN
in the record type that is not inherited, the value ofSystem.Collections.Generic.EqualityComparer<TN>.Default.GetHashCode(fieldN)
whereTN
is the field type, and - If there is a base record type, the value of
base.GetHashCode()
; otherwise the value ofSystem.Collections.Generic.EqualityComparer<System.Type>.Default.GetHashCode(EqualityContract)
.
For example, consider the following record types:
record R1(T1 P1);
record R2(T1 P1, T2 P2) : R1(P1);
record R3(T1 P1, T2 P2, T3 P3) : R2(P1, P2);
For those record types, the synthesized equality members would be something like:
class R1 : IEquatable<R1>
{
public T1 P1 { get; init; }
protected virtual Type EqualityContract => typeof(R1);
public override bool Equals(object? obj) => Equals(obj as R1);
public virtual bool Equals(R1? other)
{
return !(other is null) &&
EqualityContract == other.EqualityContract &&
EqualityComparer<T1>.Default.Equals(P1, other.P1);
}
pubic static bool operator==(R1? r1, R1? r2)
=> (object)r1 == r2 || (r1?.Equals(r2) ?? false);
public static bool operator!=(R1? r1, R1? r2)
=> !(r1 == r2);
public override int GetHashCode()
{
return Combine(EqualityComparer<Type>.Default.GetHashCode(EqualityContract),
EqualityComparer<T1>.Default.GetHashCode(P1));
}
}
class R2 : R1, IEquatable<R2>
{
public T2 P2 { get; init; }
protected override Type EqualityContract => typeof(R2);
public override bool Equals(object? obj) => Equals(obj as R2);
public sealed override bool Equals(R1? other) => Equals((object?)other);
public virtual bool Equals(R2? other)
{
return base.Equals((R1?)other) &&
EqualityComparer<T2>.Default.Equals(P2, other.P2);
}
pubic static bool operator==(R2? r1, R2? r2)
=> (object)r1 == r2 || (r1?.Equals(r2) ?? false);
public static bool operator!=(R2? r1, R2? r2)
=> !(r1 == r2);
public override int GetHashCode()
{
return Combine(base.GetHashCode(),
EqualityComparer<T2>.Default.GetHashCode(P2));
}
}
class R3 : R2, IEquatable<R3>
{
public T3 P3 { get; init; }
protected override Type EqualityContract => typeof(R3);
public override bool Equals(object? obj) => Equals(obj as R3);
public sealed override bool Equals(R2? other) => Equals((object?)other);
public virtual bool Equals(R3? other)
{
return base.Equals((R2?)other) &&
EqualityComparer<T3>.Default.Equals(P3, other.P3);
}
pubic static bool operator==(R3? r1, R3? r2)
=> (object)r1 == r2 || (r1?.Equals(r2) ?? false);
public static bool operator!=(R3? r1, R3? r2)
=> !(r1 == r2);
public override int GetHashCode()
{
return Combine(base.GetHashCode(),
EqualityComparer<T3>.Default.GetHashCode(P3));
}
}
Copy and Clone members
A record type contains two copying members:
- A constructor taking a single argument of the record type. It is referred to as a "copy constructor".
- A synthesized public parameterless instance "clone" method with a compiler-reserved name
The purpose of the copy constructor is to copy the state from the parameter to the new instance being created. This constructor doesn't run any instance field/property initializers present in the record declaration. If the constructor is not explicitly declared, a constructor will be synthesized by the compiler. If the record is sealed, the constructor will be private, otherwise it will be protected. An explicitly declared copy constructor must be either public or protected, unless the record is sealed. The first thing the constructor must do, is to call a copy constructor of the base, or a parameter-less object constructor if the record inherits from object. An error is reported if a user-defined copy constructor uses an implicit or explicit constructor initializer that doesn't fulfill this requirement. After a base copy constructor is invoked, a synthesized copy constructor copies values for all instance fields implicitly or explicitly declared within the record type.
If a virtual "clone" method is present in the base record, the synthesized "clone" method overrides it and the return type of the method is the current containing type if the "covariant returns" feature is supported and the override return type otherwise. An error is produced if the base record clone method is sealed. If a virtual "clone" method is not present in the base record, the return type of the clone method is the containing type and the method is virtual, unless the record is sealed or abstract. If the containing record is abstract, the synthesized clone method is also abstract. If the "clone" method is not abstract, it returns the result of a call to a copy constructor.
Printing members: PrintMembers and ToString methods
If the record is derived from object
, the record includes a synthesized method equivalent to a method declared as follows:
bool PrintMembers(System.StringBuilder builder);
The method is private
if the record type is sealed
. Otherwise, the method is virtual
and protected
.
The method:
- for each of the record's printable members (public field and property members), appends that member's name followed by " = " followed by the member's value:
this.member
, separated with ", ", - return true if the record has printable members.
If the record type is derived from a base record Base
, the record includes a synthesized override equivalent to a method declared as follows:
protected override bool PrintMembers(StringBuilder builder);
If the record has no printable members, the method calls the base PrintMembers
method with one argument (its builder
parameter) and returns the result.
Otherwise, the method:
- calls the base
PrintMembers
method with one argument (itsbuilder
parameter), - if the
PrintMembers
method returned true, append ", " to the builder, - for each of the record's printable members, appends that member's name followed by " = " followed by the member's value:
this.member
(orthis.member.ToString()
for value types), separated with ", ", - return true.
The PrintMembers
method can be declared explicitly.
It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
.
The record includes a synthesized method equivalent to a method declared as follows:
public override string ToString();
The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed
. It is an error if either synthesized, or explicitly declared method doesn't override object.ToString()
(for example, due to shadowing in intermediate base types, etc.).
The synthesized method:
- creates a
StringBuilder
instance, - appends the record name to the builder, followed by " { ",
- invokes the record's
PrintMembers
method giving it the builder, followed by " " if it returned true, - appends "}",
- returns the builder's contents with
builder.ToString()
.
For example, consider the following record types:
record R1(T1 P1);
record R2(T1 P1, T2 P2, T3 P3) : R1(P1);
For those record types, the synthesized printing members would be something like:
class R1 : IEquatable<R1>
{
public T1 P1 { get; init; }
protected virtual bool PrintMembers(StringBuilder builder)
{
builder.Append(nameof(P1));
builder.Append(" = ");
builder.Append(this.P1); // or builder.Append(this.P1); if P1 has a value type
return true;
}
public override string ToString()
{
var builder = new StringBuilder();
builder.Append(nameof(R1));
builder.Append(" { ");
if (PrintMembers(builder))
builder.Append(" ");
builder.Append("}");
return builder.ToString();
}
}
class R2 : R1, IEquatable<R2>
{
public T2 P2 { get; init; }
public T3 P3 { get; init; }
protected override void PrintMembers(StringBuilder builder)
{
if (base.PrintMembers(builder))
builder.Append(", ");
builder.Append(nameof(P2));
builder.Append(" = ");
builder.Append(this.P2); // or builder.Append(this.P2); if P2 has a value type
builder.Append(", ");
builder.Append(nameof(P3));
builder.Append(" = ");
builder.Append(this.P3); // or builder.Append(this.P3); if P3 has a value type
return true;
}
public override string ToString()
{
var builder = new StringBuilder();
builder.Append(nameof(R2));
builder.Append(" { ");
if (PrintMembers(builder))
builder.Append(" ");
builder.Append("}");
return builder.ToString();
}
}
Positional record members
In addition to the above members, records with a parameter list ("positional records") synthesize additional members with the same conditions as the members above.
Primary Constructor
A record type has a public constructor whose signature corresponds to the value parameters of the type declaration. This is called the primary constructor for the type, and causes the implicitly declared default class constructor, if present, to be suppressed. It is an error to have a primary constructor and a constructor with the same signature already present in the class.
At runtime the primary constructor
-
executes the instance initializers appearing in the class-body
-
invokes the base class constructor with the arguments provided in the
record_base
clause, if present
If a record has a primary constructor, any user-defined constructor, except "copy constructor" must have an
explicit this
constructor initializer.
Parameters of the primary constructor as well as members of the record are in scope within the argument_list
of the record_base
clause and within initializers of instance fields or properties. Instance members would
be an error in these locations (similar to how instance members are in scope in regular constructor initializers
today, but an error to use), but the parameters of the primary constructor would be in scope and useable and
would shadow members. Static members would also be useable, similar to how base calls and initializers work in
ordinary constructors today.
Expression variables declared in the argument_list
are in scope within the argument_list
. The same shadowing
rules as within an argument list of a regular constructor initializer apply.
Properties
For each record parameter of a record type declaration there is a corresponding public property member whose name and type are taken from the value parameter declaration.
For a record:
- A public
get
andinit
auto-property is created (see separateinit
accessor specification). An inheritedabstract
property with matching type is overridden. It is an error if the inherited property does not havepublic
overridableget
andinit
accessors. The auto-property is initialized to the value of the corresponding primary constructor parameter. Attributes can be applied to the synthesized auto-property and its backing field by usingproperty:
orfield:
targets for attributes syntactically applied to the corresponding record parameter.
Deconstruct
A positional record synthesizes a public void-returning instance method called Deconstruct with an out parameter declaration for each parameter of the primary constructor declaration. Each parameter of the Deconstruct method has the same type as the corresponding parameter of the primary constructor declaration. The body of the method assigns each parameter of the Deconstruct method to the value from an instance member access to a member of the same name. The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or is static.
with
expression
A with
expression is a new expression using the following syntax.
with_expression
: switch_expression
| switch_expression 'with' '{' member_initializer_list? '}'
;
member_initializer_list
: member_initializer (',' member_initializer)*
;
member_initializer
: identifier '=' expression
;
A with
expression is not permitted as a statement.
A with
expression allows for "non-destructive mutation", designed to
produce a copy of the receiver expression with modifications in assignments
in the member_initializer_list
.
A valid with
expression has a receiver with a non-void type. The receiver type must be a record.
On the right hand side of the with
expression is a member_initializer_list
with a sequence
of assignments to identifier, which must be an accessible instance field or property of the receiver's
type.
First, receiver's "clone" method (specified above) is invoked and its result is converted to the
receiver's type. Then, each member_initializer
is processed the same way as an assignment to
a field or property access of the result of the conversion. Assignments are processed in lexical order.