How to use readonly variable

You can assign a value to a readonly field in two way:

1)  When the variable is initialized in the declaration
public readonly int Data1 = 25;

2) For an instance field, in the instance constructors of the class that contains the field declaration, or for a static field, in the static constructor of the class that contains the field declaration.

public class Test
   {
      public readonly int Data1 = 25; 
      public readonly int Data2;

      public Test()
      {
         Data2 = 24;  
      }

      public Test(int arg1)
      { 
        Data2 = arg1;
      }
   }

hope this help.