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The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

You can assign a value to a readonly field only in the following contexts:

  • When the variable is initialized in the declaration, for example:
    public readonly int y = 5;
  • For an instance field, in the instance constructors of the class that contains the field declaration, or for a static field, in the static constructor of the class that contains the field declaration. These are also the only contexts in which it is valid to pass a readonly field as an out or ref parameter.


// cs_readonly_keyword.cs
// Readonly fields
using System;
public class ReadOnlyTest 
   class MyClass 
      public int x;
      public readonly int y = 25; // Initialize a readonly field
      public readonly int z;

      public MyClass() 
         z = 24;   // Initialize a readonly instance field

      public MyClass(int p1, int p2, int p3) 
         x = p1; 
         y = p2; 
         z = p3;

   public static void Main() 
      MyClass p1= new MyClass(11, 21, 32);   // OK
      Console.WriteLine("p1: x={0}, y={1}, z={2}" , p1.x, p1.y, p1.z);   
      MyClass p2 = new MyClass();
      p2.x = 55;   // OK
      Console.WriteLine("p2: x={0}, y={1}, z={2}" , p2.x, p2.y, p2.z);


p1: x=11, y=21, z=32
p2: x=55, y=25, z=24

In the preceding example, if you use a statement like this:

p2.y = 66;        // Error

you will get the compiler error message:

The left-hand side of an assignment must be an l-value

which is the same error you get when you attempt to assign a value to a constant.

Note   The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants as in the following example:
public static readonly uint l1 = (uint) DateTime.Now.Ticks;

See Also

C# Keywords | Modifiers