Constant: The value of a constant variable is set at 
compile time and can't be reset again.

Readonly: Can be set in a constructor or can be reset again.

constant value set on initialization it not may be change or
reset when that readonly can be change in running time.

For Both Constants and Read only values, we can set values 
for them only once. 

In the case of Constants, we set the value at the compile 
time itself. After that we cannot change it

Example:

class Test
    {
        const double pi = 3.14;
        .......................
        .......................

    }

In the case of Read only data, we can set the value in the 
constructor.

Example:

class Test
    {
        private double _bonus;

        public Test(double bonus)
        {
            _bonus = bonus;
        }

        public double Bonus
        {
            get { return _bonus; }
        }

    }

Test a(5.25);
Test b(6.75);

In this case, at the time object creation, we can set the 
value for the bonus. That is, we can decide the value at 
the runtime. So a reaonly variable is also known as runtime 
constants.


So, the basic difference is, in the case of contstant, the 
value will be same across the objects of that class, but it 
may differ in the case of readonly variables.