const Vs readonly

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.

public class MyClass
{
    public const double PI1 = 3.14159;
}

A readonly member is like a constant in that it represents an unchanging value. The difference is that a readonly member can be initialized at runtime, in a constructor as well being able to be initialized as they are declared.

public class MyClass1
{
     public readonly double PI2 = 3.14159;

     //or

     public readonly double PI3;

     public MyClass2()
     {
         PI3 = 3.14159;
     }
}

const Vs readonly
const,

• They Can not be static
• The value of constant is evaluated at compile time
• constants are initiailized at declaration only

readonly,

• They Can be either instance-level or static
• The value is evaluated at run time
• readonly Can be initialized in declaration or by code in the constructor 

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Additionally want to know this: Garbage Collection