Writing new guards with extensions in C# 14

In C# we have many nice guards sitting on top of static exceptions classes like ArgumentNullException, ArgumentOutOfRangeException, etc: As ArgumentException.ThrowIfNullOrEmpty, ArgumentException.ThrowIfNullOrWhiteSpace. Now we can extend them easily!

Extensions to the rescue

The new feature of C# 14 is that we can also extend type definitions with functions. This means we can add new guards to existing classes. If we want to have this "very useful": "Throw if our string has exactly one character" semantics, we can do something like this:

static class EnumExtensions
{
    extension(ArgumentException)
    {
        public static void ThrowIfHasOneCharacter(string argument, [CallerArgumentExpression("argument")] string? paramName = null)
        {
            if (argument.Length == 1)
            {
                throw new ArgumentException($"Argument '{paramName}' has exactly one character but shouldn't have.", paramName);
            }
        }
    }
}

Which then can be used like this:

public class MyClass
{
    public void MyMethod(string argument)
    {
        ArgumentException.ThrowIfHasOneCharacter(argument);
    }
}

That fits especially well with other already existing guards:

public class MyClass
{
    public void MyMethod(string argument)
    {
        ArgumentException.ThrowIfNullOrEmpty(argument);
        ArgumentException.ThrowIfHasOneCharacter(argument);
    }
}

Of course this is an overly simplified example. But you get the idea. We get something which makes it look and feel like the existing guards.