Invalid "Use of unassigned variable".

[CheloXL]

Version Used:
Using .net5/preview 8

Steps to Reproduce:
The following code was compiling just fine with the 3.1 sdk:

private static string? FindRequestValue(HttpRequest? request, string value)
{
	if (request?.Headers?.TryGetValue(value, out var headers) != true)
	{
		return null;
	}

	return headers.Count > 0 ? headers[0] : null;
}

Expected Behavior:
Code compiles

Actual Behavior:
Error compiling: Use of unassigned local variable 'headers' in the return line.

[Youssef1313]

Is "TryGetValue" a method that you defined?

I cannot find it in the API reference: https://docs.microsoft.com/en-us/dotnet/api/system.collections.specialized.namevaluecollection?view=net-5.0

Also HttpRequest doesn't seem to be available (https://docs.microsoft.com/en-us/dotnet/api/system.web.httprequest?view=net5.0).

Here is another repro to simplify tracking this for the compiler team:

#nullable enable

using System;

public class MyClass
{
    public bool TryGetSomething(out string something)
    {
        something = "Something";
        return true;
    }
}

public class C
{     
    public static void MyMethod(MyClass? myHeader)
    {
        if (myHeader?.TryGetSomething(out var something) != true)
        {
            return;
        }
        Console.WriteLine(something);
    }
}

[CheloXL]

The project is a netcoreapp3.1. You have to include in your csproj the following line: <FrameworkReference Include="Microsoft.AspNetCore.App" /> to have access to the HttpRequest.

Anyways, the problem is related to the code analysis.

If I change the code to

if (request?.Headers == null)
{
	return null;
}

if (!request.Headers.TryGetValue(value, out var headers))
{
	return null;
}

return headers.Count > 0 ? headers[0] : null;

The method compiles without issues.

[RikkiGibson]

Duplicate of #45582.

Many people have requested this change, but a definite assignment spec change is required first.

[bpaczkowski]

@RikkiGibson I'm not sure if it's the same issue. The issue described in this report is a change in .NET 5.0, when compiled to .NET Core 3.1 it compiles without errors. That would mean that before .NET 5.0 it wasn't working as per spec, but in .NET 5.0 it was fixed, which is causing this?

[RikkiGibson]

That would mean that before .NET 5.0 it wasn't working as per spec, but in .NET 5.0 it was fixed, which is causing this?

Yeah. What's happening is most likely similar to #52834 (comment)

We expect to fix this scenario in C# 10. dotnet/csharplang#4465