C# 0 The Complete Reference

readonly

 fields are 

allowed.

N

OTE

N

OTE

Although similar, 

readonly

 fields are not the same as 

const

 fields, which are described in the 

following section.

Here is an example that creates a 

readonly

 field:

// Demonstrate readonly.

using System;

class MyClass {

  public static readonly int SIZE = 10;

}

class DemoReadOnly {

  static void Main() {

    int[] source = new int[MyClass.SIZE];

    int[] target = new int[MyClass.SIZE];

    // Give source some values.

    for(int i=0; i < MyClass.SIZE; i++)

      source[i] = i;

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    foreach(int i in source)

      Console.Write(i + " ");

    Console.WriteLine();

    // Reverse copy source into target.

    for(int i = MyClass.SIZE-1, j = 0; i > 0; i--, j++)

      target[j] = source[i];

    foreach(int i in target)

      Console.Write(i + " ");

    Console.WriteLine();

//  MyClass.SIZE = 100; // Error!!! can’t change

  }

}

Here, 

MyClass.SIZE

 is initialized to 10. After that, it can be used, but not changed. To prove 

this, try removing the comment symbol from before the last line and then compiling the 

program. As you will see, an error will result.