Question

Let’s suppose I write public API in C#:

public void Method(object param) 
{
    if(param == null) 
    {
        throw new ArgumentNullException("Specified 'param' can not be null");
    }

// ... other business logic
}

I wonder is there any guarantees that I do not need to check parameter for null value if I have NOT nullable parameter (object? param) as method parameter? In other words is above example’s checking for null redundant?

Answer 1

No, reference types are always nullable. Just try it: call Method(null); and you will get a runtime ArgumentNullException, exactly where you throw it in the code. You don’t get a compiler error, because null is a valid value for reference types.

For value types it’s a different story. If you have a parameter of type int, it cannot be null. In fact, if (i == null) won’t even be accepted by the compiler.