Difference between ReadOnly and Constant in C#

If you are using constant value you have intitalize that timeitself,but readonly filed u can initialize that timeitself or else in constructor.

Constant value is assigned in Compile time
readonly value is assigned in Run time

using System;

namespace difference
{

 class read
 {
  public readonly int d = DateTime.Now.Hour; 
  public const int c = 10;
  
  public read(int x)
  {
   d = x;   
   
  }
  public read(int x,int y)
  {
   d = 12;   
  }


 
  }
 class Class1
 {
    
  static void Main(string[] args)
  {
   read obj = new read(DateTime.Now.Hour);   
   Console.WriteLine(obj.d);
   Console.ReadLine();
  }
 }
}