Struct Weirdness

Over on StackOverflow, saw an interesting question that taught me something new about C# 3.0.

The question Automatic Properties and Structures Don’t Mix? asked why this code doesn’t work:

struct MyStruct
{
    public MyStruct ( int size )
    {
        this.Size = size; // <-- Compile-Time Error!
    }

    public int Size{get; set;}
}

Turns out that the solution is to properly chain the declared constructor with the default parameterless constructor:

struct MyStruct
{
    public MyStruct(int size)
        : this()   // Add this for the fix
    {
        this.Size = size; // <-- Compile-Time Error!
    }

    public int Size { get; set; }
}

Which just goes to show that taking shortcuts (like not properly chaining constructors) can turn around and bite.

Interestingly, though, the C# compiler gives you all the information you need to solve this as a part of the error messages (emphasis added):

• error CS0188: The ‘this’ object cannot be used before all of its fields are assigned to

• error CS0843: Backing field for automatically implemented property ‘WindowsApplicationSample.MyStruct.Size’ must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer.

Someone posted another solution, taken from a Codeplex project - though in this case I think the “solution” is a problem in it’s own right … check it out.

public struct TempTuple<TFirst, TSecond>
{
    public TempTuple(TFirst first, TSecond second)
    {
        this = new TempTuple<TFirst, TSecond>(); // Kung fu!
        this.First = first;
        this.Second = second;
    }

    public TFirst First { get; private set; }

    public TSecond Second { get; private set; }
}

Look at the “Kung fu!” like - assignment to this - Yikes!