Question

I have a method that uses the params keyword, like so:

private void ParamsMethod(params string[] args)
{
    // Etc...
}

Then, I call the method using various combinations of arguments:

                            // Within the method, args is...
ParamsMethod();             // - a string array with no elements
ParamsMethod(null);         // - null (Why is this?)
ParamsMethod((string)null); // - a string array with one element: null
ParamsMethod(null, null);   // - a string array with two elements: null and null
ParamsMethod("s1");         // - a string array with one element: "s1"
ParamsMethod("s1", "s2");   // - a string array with two elements: "s1" and "s2"

I understand all of the cases, except for the second one. Can someone explain why ParamsMethod(null) causes args to be null, instead of an array with one null element?

Answer 1

A params parameter is only meant to provide a convenient way of specifying values – you can still pass an array reference directly.

Now, null is convertible to either string[] or string, so both interpretations are valid – it’s up to the spec which is preferred. The spec states in section 10.6.1.4 that:

• The argument given for a parameter array can be a single expression that is implicitly convertible to the parameter array type. In this case, the parameter array acts precisely like a value parameter.

• Alternatively, […]

In other words, the compiler checks to see whether the argument is valid as the “normal” parameter type first, and only builds an array if it absolutely has to.