Type Checking: typeof, GetType, or is?

Question

I\'ve seen many people use the following code:

Type t = typeof(obj1);
if (t == typeof(int))
    // Some code here

But I know you could also

Answer 1

All are different.

• typeof takes a type name (which you specify at compile time).
• GetType gets the runtime type of an instance.
• is returns true if an instance is in the inheritance tree.

Example

class Animal { } 
class Dog : Animal { }

void PrintTypes(Animal a) { 
    Console.WriteLine(a.GetType() == typeof(Animal)); // false 
    Console.WriteLine(a is Animal);                   // true 
    Console.WriteLine(a.GetType() == typeof(Dog));    // true
    Console.WriteLine(a is Dog);                      // true 
}

Dog spot = new Dog(); 
PrintTypes(spot);

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What about typeof(T)? Is it also resolved at compile time?

Yes. T is always what the type of the expression is. Remember, a generic method is basically a whole bunch of methods with the appropriate type. Example:

string Foo(T parameter) { return typeof(T).Name; }

Animal probably_a_dog = new Dog();
Dog    definitely_a_dog = new Dog();

Foo(probably_a_dog); // this calls Foo and returns "Animal"
Foo(probably_a_dog); // this is exactly the same as above
Foo(probably_a_dog); // !!! This will not compile. The parameter expects a Dog, you cannot pass in an Animal.

Foo(definitely_a_dog); // this calls Foo and returns "Dog"
Foo(definitely_a_dog); // this is exactly the same as above.
Foo(definitely_a_dog); // this calls Foo and returns "Animal". 
Foo((Animal)definitely_a_dog); // this does the same as above, returns "Animal"