[c#] How do I check if a type is a subtype OR the type of an object?

To check if a type is a subclass of another type in C#, it's easy:

typeof (SubClass).IsSubclassOf(typeof (BaseClass)); // returns true

However, this will fail:

typeof (BaseClass).IsSubclassOf(typeof (BaseClass)); // returns false

Is there any way to check whether a type is either a subclass OR of the base class itself, without using an OR operator or using an extension method?

You should try using Type.IsAssignableFrom instead.

I'm posting this answer with the hope of someone sharing with me if and why it would be a bad idea. In my application, I have a property of Type that I want to check to be sure it is typeof(A) or typeof(B), where B is any class derived from A. So my code:

public class A
{
}

public class B : A
{
}

public class MyClass
{
    private Type _helperType;
    public Type HelperType
    {
        get { return _helperType; }
        set 
        {
            var testInstance = (A)Activator.CreateInstance(value);
            if (testInstance==null)
                throw new InvalidCastException("HelperType must be derived from A");
            _helperType = value;
        }
    }
}

I feel like I might be a bit naive here so any feedback would be welcome.

typeof(BaseClass).IsAssignableFrom(unknownType);

If you're trying to do it in a Xamarin Forms PCL project, the above solutions using IsAssignableFrom gives an error:

Error: 'Type' does not contain a definition for 'IsAssignableFrom' and no extension method 'IsAssignableFrom' accepting a first argument of type 'Type' could be found (are you missing a using directive or an assembly reference?)

because IsAssignableFrom asks for a TypeInfo object. You can use the GetTypeInfo() method from System.Reflection:

typeof(BaseClass).GetTypeInfo().IsAssignableFrom(typeof(unknownType).GetTypeInfo())