static readonly field

In short, the difference is that static readonly field can be modified by the containing class, but const field can never be modified and must be initialized where it is declared. A static readonly field can be changed by the containing class using static constructor as shown below.
class Program
{
// Initialize the static readonly field
// to an initial value of 100
public static readonly int Number=100;
static Program()
{
//Value changed to 200 in the static constructor
Number = 200;
}
}