Constant and ReadOnly keyword are used to make a field constant which value cannot be modified.
Constant
Constant
fields or local variables must be assigned a value at the time of
declaration and after that they cannot be modified. By default constant
are static, hence you cannot define a constant type as static.public const int X = 10;
A const field is a compile-time constant. A constant field or local variable can be initialized with a constant expression which must be fully evaluated at compile time.
void Calculate(int Z)
{
const int X = 10, X1 = 50;
const int Y = X + X1; //no error, since its evaluated a compile time
const int Y1 = X + Z; //gives error, since its evaluated at run time
}
You can apply const keyword to built-in value types (byte, short, int, long, char, float, double, decimal, bool), enum, a string literal, or a reference type which can be assigned with a value null.
const MyClass obj1 = null;//no error, since its evaluated a compile time
const MyClass obj2 = new MyClass();//gives error, since its evaluated at run time
Constants can be marked as public, private, protected, internal, or protected internal access modifiers.
Use the const modifier when you sure that the value a field or local variable would not be changed.
ReadOnly
A
readonly field can be initialized either at the time of declaration or
with in the constructor of same class. Therefore, readonly fields can be
used for run-time constants.class MyClass {
readonly int X = 10; // initialized at the time of declaration
readonly int X1;
public MyClass(int x1)
{
X1 = x1; // initialized at run time
}
}
Explicitly, you can specify a readonly field as static since, like constant by default it is not static. Readonly keyword can be apply to value type and reference type (which initialized by using the new keyword) both. Also, delegate and event could not be readonly.
Use the readonly modifier when you want to make a field constant at run time.
