How to Check If a Type Is a Subtype or the Type of an Object

How do I check if a type is a subtype OR the type of an object?

Apparently, no.

Here's the options:

• Use Type.IsSubclassOf
• Use Type.IsAssignableFrom
• is and as

Type.IsSubclassOf

As you've already found out, this will not work if the two types are the same, here's a sample LINQPad program that demonstrates:

void Main()
{
    typeof(Derived).IsSubclassOf(typeof(Base)).Dump();
    typeof(Base).IsSubclassOf(typeof(Base)).Dump();
}

public class Base { }
public class Derived : Base { }

Output:

True
False

Which indicates that Derived is a subclass of Base, but that Baseis (obviously) not a subclass of itself.

Type.IsAssignableFrom

Now, this will answer your particular question, but it will also give you false positives. As Eric Lippert has pointed out in the comments, while the method will indeed return True for the two above questions, it will also return True for these, which you probably don't want:

void Main()
{
    typeof(Base).IsAssignableFrom(typeof(Derived)).Dump();
    typeof(Base).IsAssignableFrom(typeof(Base)).Dump();
    typeof(int[]).IsAssignableFrom(typeof(uint[])).Dump();
}

public class Base { }
public class Derived : Base { }

Here you get the following output:

True
True
True

The last True there would indicate, if the method only answered the question asked, that uint[] inherits from int[] or that they're the same type, which clearly is not the case.

So IsAssignableFrom is not entirely correct either.

is and as

The "problem" with is and as in the context of your question is that they will require you to operate on the objects and write one of the types directly in code, and not work with Type objects.

In other words, this won't compile:

SubClass is BaseClass
^--+---^
   |
   +-- need object reference here

nor will this:

typeof(SubClass) is typeof(BaseClass)
                    ^-------+-------^
                            |
                            +-- need type name here, not Type object

nor will this:

typeof(SubClass) is BaseClass
^------+-------^
       |
       +-- this returns a Type object, And "System.Type" does not
           inherit from BaseClass

Conclusion

While the above methods might fit your needs, the only correct answer to your question (as I see it) is that you will need an extra check:

typeof(Derived).IsSubclassOf(typeof(Base)) || typeof(Derived) == typeof(Base);

which of course makes more sense in a method:

public bool IsSameOrSubclass(Type potentialBase, Type potentialDescendant)
{
    return potentialDescendant.IsSubclassOf(potentialBase)
           || potentialDescendant == potentialBase;
}

Check if an object is a type or a subtype

Just use the is operator.

if (!(myObject is MyClass))

If possible, limit the argument type so the compiler will enforce that rule for you;

public MyMethod(MyClass myObject)

How can I check if a type is a subtype of a type in Python?

Maybe issubclass?

>>> class A(object): pass
>>> class B(A): pass
>>> class C(object): pass
>>> issubclass(A, A)
True
>>> issubclass(B, A)
True
>>> issubclass(C, A)
False

Typescript - Determine object subtype from union type by checking the presence of some properties

Probably the safest way to do this is with a custom type-guard

function isFirstType(item: FirstType | SecondType): item is FirstType {
  return (item as FirstType).a === "first"; //or however you want to differentiate
}

then

const renderItem = (item: FirstType | SecondType) => {
  if (isFirstType(item)) {
    // item is narrowed to FirstType
  } else {
    // item is narrowed to SecondType
  }
}

How to check if slice interface elements have the same dynamic type?

You can't use the equality operator == on the interface values. Even if the dynamic types are the same, the comparison may return false if they have fields with different values. Or it panics if B, C and D aren't comparable to begin with.

Instead you can use reflection and use == on reflect.Type. This solution doesn't require you to update the code if you add more types that implement A.

func dynamicTypesEq(args []A) bool {
    var a reflect.Type
    for _, d := range args {
        t := reflect.TypeOf(d)
        if a == nil {
            a = t
            continue
        }
        if a != t {
            return false
        }

    }
    return true
}

Calling the function with some example slices:

func main() {
    a := []A{&B{}, &B{}, &B{}}
    fmt.Println(dynamicTypesEq(a)) // true

    b := []A{&C{}, &B{}, &B{}}
    fmt.Println(dynamicTypesEq(b)) // false


    c := []A{&D{}, &D{}, &B{}}
    fmt.Println(dynamicTypesEq(c)) // false
}

Note that this function reports false in case the input has *B and B. Clearly, a pointer type is not the same as the base type.

Playground: https://go.dev/play/p/QOCvSyxGPRU

How do I check if an object's type is a particular subclass in C++?

You really shouldn't. If your program needs to know what class an object is, that usually indicates a design flaw. See if you can get the behavior you want using virtual functions. Also, more information about what you are trying to do would help.

I am assuming you have a situation like this:

class Base;
class A : public Base {...};
class B : public Base {...};

void foo(Base *p)
{
  if(/* p is A */) /* do X */
  else /* do Y */
}

If this is what you have, then try to do something like this:

class Base
{
  virtual void bar() = 0;
};

class A : public Base
{
  void bar() {/* do X */}
};

class B : public Base
{
  void bar() {/* do Y */}
};

void foo(Base *p)
{
  p->bar();
}

Edit: Since the debate about this answer still goes on after so many years, I thought I should throw in some references. If you have a pointer or reference to a base class, and your code needs to know the derived class of the object, then it violates Liskov substitution principle. Uncle Bob calls this an "anathema to Object Oriented Design".

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