Invalid "Use of unassigned variable".

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.

[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]

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 https://github.com/dotnet/roslyn/issues/52834#issuecomment-825749694

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