Design Time Expression Evaluation - Immediate Window

Immediate Window is a built-in Visual Studio tool that people often forget about. Mostly because it shows only at debug time and usage is not obvious as a watch window. The Immediate window is used at debug and design time to debug and evaluate expressions, execute statements, print variable values, and so forth. Yes, you can use Immediate window at design time. Let’s say you write a method:

private static int Sum(int a, int b)
{
    return a + b;
}

Then you can type in Immediate window:

?Sum(10,12)

and below would be result:

22

You can even debug, while you are in design mode. More information about debugging at design time in MSDN.