typeof is applied to a name of a type or generic type parameter known at compile time (given as identifier, not as string). GetType is called on an object at runtime. In both cases the result is an object of the type System.Type containing meta-information on a type.
Example where compile-time and run-time types are equal
string s = "hello";
Type t1 = typeof(string);
Type t2 = s.GetType();
t1 == t2 ==> true
Example where compile-time and run-time types are different
object obj = "hello";
Type t1 = typeof(object); // ==> object
Type t2 = obj.GetType(); // ==> string!
t1 == t2 ==> false
i.e., the compile time type (static type) of the variable obj is not the same as the runtime type of the object referenced by obj.
Testing types
If, however, you only want to know whether mycontrol is a TextBox then you can simply test
if (mycontrol is TextBox)
Note that this is not completely equivalent to
if (mycontrol.GetType() == typeof(TextBox))
because mycontrol could have a type that is derived from TextBox. In that case the first comparison yields true and the second false! The first and easier variant is OK in most cases, since a control derived from TextBox inherits everything that TextBox has, probably adds more to it and is therefore assignment compatible to TextBox.
public class MySpecializedTextBox : TextBox
{
}
MySpecializedTextBox specialized = new MySpecializedTextBox();
if (specialized is TextBox) ==> true
if (specialized.GetType() == typeof(TextBox)) ==> false
Casting
If you have the following test followed by a cast and T is nullable ...
if (obj is T) {
T x = (T)obj; // The casting tests, whether obj is T again!
...
}
... you can change it to ...
T x = obj as T;
if (x != null) {
...
}
Testing whether a value is of a given type and casting (which involves this same test again) can both be time consuming for long inheritance chains. Using the as operator followed by a test for null is more performing.
Starting with C# 7.0 you can simplify the code by using pattern matching:
if (obj is T t) {
// t is a variable of type T having a non-null value.
...
}
Btw.: this works for value types as well. Very handy for testing and unboxing. Note that you cannot test for nullable value types:
if (o is int? ni) ===> does NOT compile!
This is because either the value is null or it is an int. This works for int? o as well as for object o = new Nullable<int>(x);:
if (o is int i) ===> OK!
I like it, because it eliminates the need to access the Nullable<T>.Value property.