const vs. readonly

const

• A constant member is defined at compile time and cannot be changed at runtime. 
• Constants are declared as a field, using the const keyword and must be initialized as they are declared.
• Constants must be a value type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, orbool), an enumeration, a string literal, or a reference to null.(Since classes or structures are initialized at run time with the new keyword, and not at compile time, you can't set a constant to a class or structure.).
•  It cant be static.

Example:- 

public class MyClass { public const double PI = 3.14159; }



ReadOnly

• A read only member is like a constant in that it represents an unchanging value.
• They are declared as a field, using the readonly keyword and can be initialized at runtime,in a constructor as well being able to be initialized as they are declared
• They can be declared as static

Example:-

public class MyClass { public readonly double PI = 3.14159; }
public class MyClass { public readonly double PI; public MyClass() { PI = 3.14159; } }