Const vs Static vs Readonly

Const vs Static vs Readonly

A variable declared as const (read: constant) must be assigned a value at declaration, and this value may not then change at a later time. The value in a const variable is what's called a "compile-time" value, and is immutable (which means it does not change over the life of the program). Only primitive or "built-in" C# types (e.g. int, string, double) are allowed to be declared const.You want to use const when you have a variable whose value will not change, ever, during the time your application is being used. Further, any variable declared as const will also, implicitly, be declared static.

public const string ConnectionString = "YourConnectionString";  

A static member (variable, method, etc) belongs to the type of an object rather than to an instance of that type.

A readonly field is one where assignment to that field can only occur as part of the declaration of the class or in a constructor. This means that a readonly variable can have different value at runtime. But once it is evaluated(at runtime), it will  not change for the lifetime.

public class TestClass  

{

    public readonly string ConnectionString = "TestConnection";

    public TestClass()

    {// This shows the ‘ConnectionString’ can have different values depending upon where it is instantiated from.

        ConnectionString = "DifferentConnection";

    }

    public void TestMethod ()

    {

        ConnectionString = "NewConnection";//Will not compile

    }

}

Const vs. Static Readonly

So, What exactly is a static readonly :- It cannot be changed outside of its declaration or containing class's constructor (due to readonly). It is part of the type, not part of an instance of that type (due to static).

1.        a const variable is not a reference to anything; it is literal value "burned" into the code (using a constant is the true definition of hard coding a value). A static readonly variable is a reference, and consequently a lookup is performed any time this variable is accessed

2.       But there's another, more subtle difference that we should be aware of. If a const variable exists in Assembly A and is used in Assembly B, when Assembly A gets recompiled with a new value for the const variable Assembly B will still have the previous value until it is also recompiled.

1.   If you know the value will never, ever, ever change for any reason, use const. 2.   If you're unsure of whether or not the value will change, but you don't want other classes or code to be able to change it, use readonly. 3.   If you need a field to be a property of a type, and not a property of an instance of that type, use static. 4.   A const value is also implicitly static.

Constants: 1. Constants can be assigned values only at the time of declaration 2. Constant variables have to be accessed using "Classname.VariableName" 3. Constants are known at compile time

Read Only: 1. Read only variables can be assigned values either at runtime(i.e. from user) or at the time of instance initialization via constructor 2. Read only variables have to be accessed using the "InstanceName.VariableName" 3. Read only variables are known at run time.