When and where to use GetType() or typeof()?
typeof
is an operator to obtain a type known at compile-time (or at least a generic type parameter). The operand of typeof
is always the name of a type or type parameter - never an expression with a value (e.g. a variable). See the C# language specification for more details.
GetType()
is a method you call on individual objects, to get the execution-time type of the object.
Note that unless you only want exactly instances of TextBox
(rather than instances of subclasses) you'd usually use:
if (myControl is TextBox)
{
// Whatever
}
Or
TextBox tb = myControl as TextBox;
if (tb != null)
{
// Use tb
}
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.