difference between Constant And Readonly

What is the difference between constant and readonly?

'const':
Can't be static.
Value is evaluated at compile time.
Initiailized at declaration only.

'readonly':
Can be either instance-level or static.
Value is evaluated at run time.
Can be initialized in declaration or by code in the constructor.

You can't use the static keyword with a const because it would be redundant. You can think of const as a subset of static.

Consts are static, not instance members.

public class Constant
{
public const int C = 12345;
}

------------
public class ReadOnly
{
private readonly int r = 1; //variable initialization upon declaration is possible for readonly variables

public ReadOnly()
{
r = 2; //assignment to a readonly variable is possible in the constructor
}

public int R
{
get { return r; }
set { r = value; } //this line won't compile because r is declared a readonly
}
}