csharplang/proposals/csharp-6.0/struct-autoprop-init.md

Relaxed rules for auto-properties in structs

Before C# 6.0 you could not write code like this:

struct S 
{ 
    int X { get; set; } 
    int Y { get; set; } 
    public S(int x, int y) 
    { 
        this.X = x; 
        this.Y = y; 
    } 
} 

error CS0188: The 'this' object cannot be used before all of its fields are assigned to
error CS0843: Backing field for automatically implemented property 'S.X' must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer. 
error CS0843: Backing field for automatically implemented property 'S.Y' must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer. 

A workaround was to invoke the default constructor:

struct S 
{ 
    int X { get; set; } 
    int Y { get; set; } 
    public S(int x, int y) : this() 
    { 
        this.X = x; 
        this.Y = y; 
    } 
} 

or to initialize this to the default value:

struct S 
{ 
    int X { get; set; } 
    int Y { get; set; } 
    public S(int x, int y) 
    { 
        this = default(S); 
        this.X = x; 
        this.Y = y; 
    } 
} 

Unfortunately, both methods effectively disabled definite assignment analysis for instance fields for the given constructor.

C# 6.0 relaxes these rules by tracking assignments to instance auto-properties as if they were assignments to their backing fields, so the code in the beginning of this section is now valid.