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readonly (C# Reference)

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.

In this example, the value of the field year cannot be changed in the method ChangeYear, even though it is assigned a value in the class constructor:

class Age
{
    readonly int _year;
    Age(int year)
    {
        _year = year;
    }
    void ChangeYear()
    {
        //_year = 1967; // Compile error if uncommented.
    }
}

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.

NoteNote:

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 timeStamp = (uint)DateTime.Now.Ticks;
public class ReadOnlyTest
{
   class SampleClass
   {
      public int x;
      // Initialize a readonly field 
      public readonly int y = 25;
      public readonly int z;

      public SampleClass()
      {
         // Initialize a readonly instance field
         z = 24;
      }

      public SampleClass(int p1, int p2, int p3)
      {
         x = p1;
         y = p2;
         z = p3;
      }
   }

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

For more information, see the following section in the C# Language Specification:

  • 10.4.2 Readonly fields

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