Method Parameter Arrays

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∟Method Parameter Arrays

This section describes method parameter arrays, which can be used as the last parameter in method definition to take an undefined number of parameters.

A parameter array is a parameter by which an undefined number of parameters can be passed into the called method from the caller. A parameter array must be an array type with the "params" modifier. The caller can supply an array, one value, two values or any number of values for a parameter array. A parameter array must be the last parameter of a method.

The following program shows the differences between a normal array parameter and a special parameter array:

// ParameterArray.cs
// Copyright (c) 2008 HerongYang.com. All Rights Reserved.

using System;
class ParameterArray {
   static void Main() {
      int i1, i2;
      int[] a = new int[2];

      Console.WriteLine("Testing array parameter...");
      i1 = 11;
      i2 = 12;
      a[0] = 11;
      a[1] = 12;
      AParameter(a);
      // AParameter(i1, i2); //not allowed
      Console.WriteLine("After calling: a[0] = {0}",a[0]);
      Console.WriteLine("After calling: a[1] = {0}",a[1]);

      Console.WriteLine("Testing parameter array...");
      PArray(a);
      PArray(i1,i2);
      Console.WriteLine("After calling: a[0] = {0}",a[0]);
      Console.WriteLine("After calling: a[1] = {0}",a[1]);
      Console.WriteLine("After calling: i1 = {0}",i1);
      Console.WriteLine("After calling: i2 = {0}",i2);
   }
   static void AParameter(int[] a) {
      Console.WriteLine("In method: a[0] = {0}",a[0]);
      Console.WriteLine("In method: a[1] = {0}",a[1]);
      a[0] = 21;
      a[1] = 22;
   }
   static void PArray(params int[] a) {
      Console.WriteLine("In method:  a[0] = {0}",a[0]);
      Console.WriteLine("In method:  a[1] = {0}",a[1]);
      a[0] = 31;
      a[1] = 32;
   }
}

Output:

Testing array parameter...
In method: a[0] = 11
In method: a[1] = 12
After calling: a[0] = 21
After calling: a[1] = 22
Testing parameter array...
In method:  a[0] = 21
In method:  a[1] = 22
In method:  a[0] = 11
In method:  a[1] = 12
After calling: a[0] = 31
After calling: a[1] = 32
After calling: i1 = 11
After calling: i2 = 12

Note that if you are calling parameter array with an array, it will be used as a reference parameter. If you are calling parameter array with a list of elements, and the elements are value based types, they will be used a value parameters.