Nullable false positives when boolean conditional is first assigned to a named variable

[j2jensen]

Version Used: Microsoft.Net.Compilers.Toolset v3.9.0, or VS/MSBuild 16.9.4

Steps to Reproduce:

1. Write code like this. Note the lack of compiler warnings.

#nullable enable

if(!(new[] { "" }.ToDictionary(i => i.ToString())
    .TryGetValue("", out var value) && value.Any()))
{
    Console.WriteLine("No value");
}
else
{
    Console.WriteLine(value.ToString());
}

2. Now extract a named variable:

#nullable enable

var hasNonEmptyValue = new[] { "" }.ToDictionary(i => i.ToString())
    .TryGetValue("", out var value) && value.Any();
if(!hasNonEmptyValue)
{
    Console.WriteLine("No value");
}
else
{
    Console.WriteLine(value.ToString());
}

Expected Behavior:
The compiler should know that value cannot be null in the else block, just as it knows that when not using a variable.

Actual Behavior:
CS8602 warning upon calling value.ToString()
Similar behavior with other nullable-related errors if I try to pass the value as an argument, assign it to a field, etc.

[j2jensen]

I just realized this is described in this SO post, where Mads Torgensen mentions it might be revisited in C# 9. I'm guessing it wasn't?

[Youssef1313]

The current behavior is by design. This may need a detailed proposal on how things should be improved. #27011

[RikkiGibson]

It would be great if someone decided to explore this issue in https://github.com/dotnet/csharplang/discussions. In particular, how associating conditional state with boolean variables should work, and how "invalidating" such state should work when changes to the underlying variables occur, i.e.

void M(string? x)
{
    bool b = x != null;
    x = null; // perhaps conditional state associated with 'b' is now "out of date"
    if (b)
    {
        x.ToString(); // warning
    }
}