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.
Example
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.
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 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.
C# Language Specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.