C# And Accepting Parameters

Have you ever written a function that looked similar to the following – Passing in an array of a value?

public void UpdateList(string[] list) {
    //...etc...
}

It’s not that uncommon of a method, but did you know you can take it a little further?

The params Keyword

The params keyword is an interesting modifier to an argument for a method. It allows a user to pass individual values instead of using the array itself.

public void ProcessList(params string[] args) {
    //...etc...
}

//old way still works
ProcessList(new string[] { "a", "b", "c" });

//new way works as well
ProcessList("a", "b", "c");

You’ll notice you can still pass in an array like before, but now you can add each parameter one at a time.

The params keyword has to be the last parameter in your method, but other than that, there isn’t much more to it. You treat the argument just like you would any other array.

More Arguments Using ‘Pure Evil’

The params keyword gives you a little more flexibility when working with arguments, but have you ever heard about the __argslist keyword?

If not, it’s not that big of a deal — You really shouldn’t use it that much.

public static void CallArgList(__arglist) {
    ArgIterator args = new ArgIterator(__arglist);
    int total = args.GetRemainingCount();
    for (int i = 0; i < total; i++) {
        object value = TypedReference.ToObject(args.GetNextArg());
        Console.WriteLine(
            "Arg #{0}: {1} ({2})",
            i,
            value,
            value.GetType()
            );
    }
}

//then used like...
CallArgList(
    __arglist(
        "Jim",
        1,
        false,
        (new StringBuilder()).Append("howdy")
        ));

Now, that is some strange looking code but even stranger, it works! We’re now able to pass in values into our method without any constraints.

I don’t really recommend using this keyword. Writing a similar method using an object array (using params) found no real difference in the speed of execution. Given that it isn’t a commonly used feature there is a fair chance that other developers that stumble across it will have to hit Google before they can go any further.