In C# indicate to static analyzers that a method guarantees non-null values

Question

With .NET 6, it is easy to create helper or extension methods that can throw an exception without dirtying up the stack trace. For example:

    [System.Diagnostics.StackTraceHidden]
    public static void ThrowIfNullOrWhitespace(this string? stringValue, Exception exception)
    {
        if (string.IsNullOrWhiteSpace(stringValue))
            throw exception;
    }

In this case, you can call that method

foo.ThrowIfNullOrWhitespace(new ArgumentException("null or whitespace"));

but subsequent uses of foo in the code still get "possible null value" squiggles in the editor.

I also see this issue in more mundane cases where methods like foo = DoSomething(foo) guarantee a non-null result internally.

Is there an attribute or some other way to tell the C# static code analyzers that these kinds of methods enforce postconditions that guarantee that foo is not null?

Answer 1

The attribute you're looking for is [NotNull], used like this:

public static void ThrowIfNullOrWhitespace([NotNull]this string? stringValue, Exception exception)

You can see literally the same example in the official documentation here.

And you can see your function implemented in the Windows Community Toolkit here.