Possibility of external functions as nullable guards?

Question

C# 8 introduced nullable reference types, which is a very cool feature. Now if you expect to get nullable values you have to write so-called guards:

Answer 1

There are a few things you can do.

You can use [DoesNotReturnIf(...)] in your guard method, to indicate that it throws if a particular condition is true or false, for example:

public static class Ensure
{
    public static void True([DoesNotReturnIf(false)] bool condition)
    {
        if (!condition)
        {
             throw new Exception("!!!");   
        }
    }
}

Then:

public void TestMethod(object? o)
{
    Ensure.True(o != null);
    Console.WriteLine(o.ToString()); // No warning
}

This works because:

[DoesNotReturnIf(bool)]: Placed on a bool parameter. Code after the call is unreachable if the parameter has the specified bool value

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Alternatively, you can declare a guard method like this:

public static class Ensure
{
    public static void NotNull([NotNull] object? o)
    {
        if (o is null)   
        {
            throw new Exception("!!!");
        }
    }
}

And use it like this:

public void TestMethod(object? o)
{
    Ensure.NotNull(o);
    Console.WriteLine(o.ToString()); // No warning
}

This works because:

[NotNull]: For outputs (ref/out parameters, return values), the output will not be null, even if the type allows it. For inputs (by-value/in parameters) the value passed is known not to be null when we return.

SharpLab with examples

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Of course, the real question is why you want to do this. If you don't expect value to be null, then declare it as object?, rather than object -- that's the point of having NRTs.