Roslyn: Warning when there is a struct constructor with default parameters

[YohDeadfall]

The following code compiles without any warnings or errors. The compiler correctly generates IL and uses initobj to initialize a struct, but there is a constructor with a default parameter. So it seems like it should be invoked:

public class Program {
    public static void Main() => new TestStruct().Value.ToString();
}

public struct TestStruct
{
    public TestStruct(string value = "default") => Value = value;
    public string Value { get; }
}

    .method public hidebysig static 
        void Main () cil managed 
    {
        // Method begins at RVA 0x2050
        // Code size 21 (0x15)
        .maxstack 2
        .locals init (
            [0] valuetype TestStruct
        )

        IL_0000: ldloca.s 0
        IL_0002: dup
        IL_0003: initobj TestStruct
        IL_0009: call instance string TestStruct::get_Value()
        IL_000e: callvirt instance string [mscorlib]System.Object::ToString()
        IL_0013: pop
        IL_0014: ret
    } // end of method Program::Main

If the type is changed to be a class, then the constructor with the default parameter is invoked:

public class TestStruct
{
    public TestStruct(string value = "default") => Value = value;
    public string Value { get; }
}

    .method public hidebysig static 
        void Main () cil managed 
    {
        // Method begins at RVA 0x2050
        // Code size 22 (0x16)
        .maxstack 8

        IL_0000: ldstr "default"
        IL_0005: newobj instance void TestStruct::.ctor(string)
        IL_000a: call instance string TestStruct::get_Value()
        IL_000f: callvirt instance string [mscorlib]System.Object::ToString()
        IL_0014: pop
        IL_0015: ret
    } // end of method Program::Main

Makes sense to generate a warning or even an error to remove inconsistency between class and structure instantiations.

[Joe4evr]

See also dotnet/csharplang#146.

[sharwell]

The struct has two constructors:

public struct TestStruct
{
    public TestStruct() { } // this one defined by the compiler (or runtime)
    public TestStruct(string value = "default") => Value = value;
    public string Value { get; }
}

A class with the same two constructors will be constructed in the same way as the structure.

[canton7]

I suppose it might make sense to raise a warning if:

1. There is an (explicit or implicit) parameteress constructor, and
2. There's another constructor, which has a single parameter, which has a default value

In this case, I think the default value is pointless and might indicate user error.

I suppose the same could be said about two method overloads: if there's a parameterless method, and another method which takes a single parameter with a default value, that default value is pointless as Method() will always invoke the parameterless method. I suppose you might want to be careful if that method is part of an interface implementation though.

[jaredpar]

Closing as the compiler is operating as the language design intends here and that this seems like an unlikely candidate for a warning wave.