What is the difference between String.Empty and "" (empty string)?

what is the difference between String.Empty and "", and are they interchangable

string.Empty is a read-only field whereas "" is a compile time constant. Places where they behave differently are:

Default Parameter value in C# 4.0 or higher

void SomeMethod(int ID, string value = string.Empty)
// Error: Default parameter value for 'value' must be a compile-time constant
{
    //... implementation
}

Case expression in switch statement

string str = "";
switch(str)
{
    case string.Empty: // Error: A constant value is expected. 
        break;

    case "":
        break;

}

Attribute arguments

[Example(String.Empty)]
// Error: An attribute argument must be a constant expression, typeof expression 
//        or array creation expression of an attribute parameter type

In .NET prior to version 2.0, "" creates an object while string.Empty creates no object^ref, which makes string.Empty more efficient.

In version 2.0 and later of .NET, all occurrences of "" refer to the same string literal, which means "" is equivalent to .Empty, but still not as fast as .Length == 0.

.Length == 0 is the fastest option, but .Empty makes for slightly cleaner code.

See the .NET specification for more information.