What is the difference between const and static read-only member?

A const field must be initialized at the place where it is declared as shown in the example below.
class Program
{
      public const int Number = 100;
}
It is a compile time error to declare a const without a value. The code below will generate a compiler error stating "A const field requires a value to be provided"
class Program
{
      public const int Number;
}
It is a compile time error to change the value of a constant. The following code will generate a compiler error stating "The left-hand side of an assignment must be a variable, property or indexer"
class Program
{
     public const int Number = 100;
     static void Main()
     {
           Number = 200;
     }
}



It is not mandatory to initialize a static readonly field where it is declared. You can declare a static readonly field without an initial value and can later initialize the static field in a static constructor as shown below.
class Program
{
     public static readonly int Number;
     static Program()
     {
           Number = 100;
     }
}


Once a static readonly field is initialized, the value cannot be changed. The code below will generate a compiler error stating "A static readonly field cannot be assigned to (except in a static constructor or a variable initializer)"
class Program
{
       public static readonly int Number;
       static Program()
       {
             Number = 100;
       }
       static void Main()
       {
              Number = 200;
        }
}


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;
          }
}