struct in a different assembly used without initialization

[sgurukrupa]

The following C# program doesn't compile

class Program
{
    public struct MetaWorkspace
    {
        public string Name { get; set; }
        public string Id { get; set; }
    }

    static void Main(string[] args)
    {
        MetaWorkspace ws;
        ws.Id = "abc";
    }
}

The compiler complains: error CS0165: Use of unassigned local variable 'ws'

The compiler is Microsoft (R) Visual C# Compiler version 3.10.0-4.21329.37 and the version of .NET Framework is 4.7.2

But, the same program compiles successfully when the MetaWorkspace struct definition is moved into a different assembly (DLL) and is included as a reference. How is this so? If this is not a compiler error, am I still dealing with a fully defined MetaWorkspace object or have I tread into the undefined zone?

[spydacarnage]

The reason why you're getting that error is because you're declaring the local ws variable, but you're not explicitly assigning anything to it. The same applies in the following code, where you are using x before explicitly assigning anything to it, even though you would think it has a default value of 0:

int x;
string s = x.ToString();

On the other hand, if you'd declared your MetaWorkspace ws; at the class-level, it does auto-initialise, which is probably what you are seeing when you use it in your second case.

You can see more information on this here: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/cs0165

[ContentsMayVary]

@spydacarnage The question isn't why error CS0165 is generated; the question is why CS0165 isn't generated when MetaWorkspace is defined in a different assembly.

[spydacarnage]

It doesn't make a difference for me - I get the same error.

Hence why I wondered if, in using it from the separate assembly, the code also changed slightly...

[sgurukrupa]

The following program compiles successfully when MetaWorkspace is defined in a different assembly.

class Program
{
    static void Main(string[] args)
    {
        MetaWorkspace ws;
        ws.Id = "abc";
    }
}

[ufcpp]

dotnet/roslyn#29319 ?

[ContentsMayVary]

Looks like dotnet/roslyn#29319 is indeed the same issue.

[ContentsMayVary]

I can reproduce this using .net 5.0 with Visual Studio 16.11.2. When the struct is defined in another assembly I get a warning Warning CS8887 Use of unassigned local variable 'ws' and if it's defined in the same assembly I get an error Error CS0165 Use of unassigned local variable 'ws'. (Note: I disabled Resharper for this test because it also gives an error/warning.) This question was first asked on StackOverflow: https://stackoverflow.com/questions/69220541/struct-in-a-different-assembly-used-without-initialization#comment122586849_69220541