The following table lists the difference between Static, Readonly, and constant in C#.
| static | readonly | const |
|---|---|---|
Declared using the static keyword.
|
Declared using the readonly keyword.
|
Declred using the const keyword.
By default a const is static that cannot be changed.
|
| Classes, constructors, methods, variables, properties, event and operators can be static. The struct, indexers, enum, destructors, or finalizers cannot be static. | Only the class level fields can be readonly. The local variables of methods cannot be readonly. | Only the class level fields or variables can be constant. |
| Static members can only be accessed within the static methods. The non-static methods cannot access static members. |
Readonly fields can be initialized at declaration or in the constructor. Therefore, readonly variables are used for the run-time constants. |
The constant fields must be initialized at the time of declaration. Therefore, const variables are used for compile-time constants. |
Value of the static members can be modified using ClassName.StaticMemberName.
|
Readonly variable cannot be modified at run-time. It can only be initialized or changed in the constructor. | Constant variables cannot be modified after declaration. |
Static members can be accessed using ClassName.StaticMemberName, but cannot be accessed using object.
|
Readonly members can be accessed using object, but not ClassName.ReadOnlyVariableName.
|
Const members can be accessed using ClassName.ConstVariableName, but cannot be accessed using object.
|
The following example demonstrates the difference between static, readonly, and const variables.
Example: static vs readonly vs const
public class Program
{
public static void Main()
{
MyClass mc = new MyClass(50);
mc.ChangeVal(45);
mc.Display();
Console.WriteLine("MyClass.constvar = {0}", MyClass.constvar);
Console.WriteLine("MyClass.staticvar = {0}", MyClass.staticvar);
}
}
public class MyClass
{
public readonly int readonlyvar1 = 10, readonlyvar2;
public const int constvar = 20;
public static int staticvar = 0;
public MyClass(int i)
{
readonlyvar2 = i; // valid
//z = i; //compile-time error
staticvar = i; // valid
}
public void ChangeVal(int val)
{
//x = val;
//z = i; //compile-time error
staticvar = val; // valid
}
public void Display()
{
Console.WriteLine(staticvar);
Console.WriteLine(readonlyvar1);
Console.WriteLine(constvar);
}
}
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