What is the difference between const and readonly in C#?

Question

What is the difference between const and readonly in C#?

When would you use one over the other?

Answer 1

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const keyword and must be initialized as they are declared.

public class MyClass
{
    public const double PI1 = 3.14159;
}

A readonly member is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor, as well being able to be initialized as they are declared.

public class MyClass1
{
     public readonly double PI2 = 3.14159;

     //or

     public readonly double PI3;

     public MyClass2()
     {
         PI3 = 3.14159;
     }
}

const

• They can not be declared as static (they are implicitly static)
• The value of constant is evaluated at compile time
• constants are initialized at declaration only

readonly

• They can be either instance-level or static
• The value is evaluated at run time
• readonly can be initialized in declaration or by code in the constructor