Check if instance is of a type

The different answers here have two different meanings.

If you want to check whether an instance is of an exact type then

if (c.GetType() == typeof(TForm))

is the way to go.

If you want to know whether c is an instance of TForm or a subclass then use is/as:

if (c is TForm)

or

TForm form = c as TForm;
if (form != null)

It's worth being clear in your mind about which of these behaviour you actually want.

if(c is TFrom)
{
   // Do Stuff
}

or if you plan on using c as a TForm, use the following example:

var tForm = c as TForm;
if(tForm != null)
{
   // c is of type TForm
}

The second example only needs to check to see if c is of type TForm once. Whereis if you check if see if c is of type TForm then cast it, the CLR undergoes an extra check. Here is a reference.

Edit: Stolen from Jon Skeet

If you want to make sure c is of TForm and not any class inheriting from TForm, then use

if(c.GetType() == typeof(TForm))
{
   // Do stuff cause c is of type TForm and nothing else
}