C# and variable number of parameters

Question

I tried the following code:

class Program: ProgParent
    {

        public int Max(params int[] op)
        {
            return 0;
        }

        public int Max(int i, params int[] op)
        {
            return 1;
        }

        public int Max(int i, int j, params int[] op)
        {
            return 2;
        }

        public static void Main(string[] args)
        {
            System.Console.WriteLine((new Program()).Max(5, 6, 7, 8));
            System.Console.ReadKey();
        }
    }

It executes, and uses the most specific function available. But the compiler gives no warning or error about this. Why?

Your code doesn't compile - the first two method signatures are identical. After removing one of them it's fine though. Why wouldn't it be? — Jon Skeet
– Jon Skeet, Commented Mar 31, 2009 at 7:05
Yes, that was the original version, I just wanted to add a new method here — Dutow
– Dutow, Commented Mar 31, 2009 at 7:16
your code does just fine, after I remove the base class (class Program instead of class Program: ProgParent). I don't know where your problem? — Deltax76
– Deltax76, Commented Mar 31, 2009 at 13:04

user · Accepted Answer · 2012-08-10 23:28:57Z

The C# language spec says:

When performing overload resolution, a method with a parameter array may be applicable either in its normal form [i.e. passing an array] or its expanded form [i.e. passing a variable number of parameters]. The expanded form of a method is available only if the normal form of the method is not available and only if a method with the same signature as the expanded form is not already declared in the same type"

In a (slightly simplified) nutshell: If overload resolution is ambiguous, the compiler chooses the non-params overload.

I guess the reasons for that decision (instead of making code like yours illegal) include:

If your method has the signature: void fn(params object[] p), you want to have some way to call the "normal form" (by passing an object[]). So the compiler has to handle ambiguous cases anyway.
Creating a temporary array is a lot more expensive than a method call, so you might want to create non-params overloads with 1,2,3 parameters that behave the same but are more efficient. (like e.g. String.Format)

Marc Gravell · Accepted Answer · 2009-03-31 07:08:21Z

2

Ignoring the build errors (which I'm putting down to typos) - what warning would you expect or want? It is finding a matching overload and using it...

Strictly speaking, I can call different overloads - by passing arrays, but yes, the usage isn't entirely clear.

Without the multiple overloads with params, this pattern is used quite heavily in things like string.Concat etc (which underpins + for strings under the bonnet).

answered Mar 31, 2009 at 7:08

Marc Gravell

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