Parameter null-checking added to C# 11 Preview

The first preview of C# 11 is out, and well, I think I like what I see. I dig the new List patterns and am a fan of allowing newlines in the “holes” of interpolated strings. Parameter null-checking is a bit contentious, and it’s good that they are releasing it in preview one and asking for feedback.

In a nutshell, they want to spare us a lot of boilerplate. Code like this:

public static void M(string s)
{
    if (s is null)
    {
        throw new ArgumentNullException(nameof(s));
    }
    // Body of the method
}

Would be abbreviated by adding !! to the parameter name:

public static void M(string s!!)
{
    // Body of the method
}

Code will be generated to perform the null check. The generated null check will execute before any of the code within the method. For constructors, the null check occurs before field initialization, calls to base constructors, and calls to this constructors.

My initial reaction was, we don’t need this; we got Nullable Reference Types. NRTs however help at design time, to know whether a null is possible, while parameter null-checking is meant for runtime.

According to Kathleen Dollard, the .NET Runtime itself removed nearly 20,000 lines of code using this new null-check syntax. That’s one heck of a lot of boilerplate removed.

I don’t think I like the syntax, though. It’s super concise, which is good, and I appreciate putting the !! on the parameter rather than the type since the parameter’s value is being checked. Still, the two-punctuation character seems a bit clumsy. Someone suggested adopting notnull instead:

public void M(string s notnull) { // code }

I like this suggestion. I wouldn’t want notnull on the left of the parameter name. To the right? Count me in.