CODEWITHSUNDEEP
c#subclasstypeof
Popinjay
Popinjay

Reputation: 73

How can I tell if one object is derived from a particular class

I was asked for more info: The original problem is below

I saw the answer I was pointed to above. I did look before posting. That is how I found .IsSubclassOf which I didn't know existed (I am VERY new to C#).

What I don't understand is why I am getting an error when I try to use the command.

I need to be able to tell if some object O is a sub-class of some other class C. It is okay if I specify the type of class C is directly (such as ListBox) but what class O is will vary, which is why I need to check. O might be one of many objects derived from ListBox or it might be an object derived from TextBox or it might be something else. I need to be able to act on those cases where a method is being passed objects that are derived from or sub-classes of ListBox or whatever class O may or may not be derived/sub-classed from

The example I give below would work fine if "st" could work in last statement. But it doesn't work. It gives me an error. I am told that I can not use a variable there--I have to use a Type. But the variable IS a type. I declare it as a type above. I can't name the type specifically because I will not know the type of the object. I have to get it with GetType.

Original problem/post:

I need to be able to tell if an instance of an object is derived from a particular class.

This is just some example code to test the type of and "is a IsSubclassOf" methods, but they do not seem to work for me.

ListBox tb = new ListBox();
NoVerticalScrollCheckedListBox nvlb = new NoVerticalScrollCheckedListBox();
Type st = nvlb.GetType();
Type uType = tb.GetType();
bool b = typeof(st).IsSubclassOf(uType);
Console.Write("BOOL: " + b);

I get the error that I am trying to use variable st as a type, but st is a type. I can put in the actual class name such as "NoVerticalScrollCheckedListBox" but that will not give me the information I want because what I want to be able to discover is if a particular object instance is derived from another class, not whether or not the class is derived from that other class abstractly.

I would like to make a method, for example, that would treat all objects derived from a ListBox in a particular way, regardless of whether it is one of several existing sub-types or of a new sub-type that may come along.

Upvotes: 3

Views: 2140

Answers (2)

Greg
Greg

Reputation: 1116

You cannot pass an instance of the type you are comparing. 

public class Base{}    
public class Child : Base{}
public class ChildOfChild : Child{}
public class Another{}

//...

bool b1 = typeof(Child).IsSubclassOf(typeof(Base)); //true
bool b2 = typeof(ChildOfChild).IsSubclassOf(typeof(Base)); //true
bool b3 = typeof(Another).IsSubclassOf(typeof(Base)); //false

If you want to check the current instance you can use as instead of is.

Child c = new Child();
//...

Base b = c as Base;
if(b != null)
{
    //...
}

Upvotes: 1

Ry-
Ry-

Reputation: 225281

Using reflection for common tasks is a good indicator that there’s a better way. A promising first step seems like it could be:

bool b = nvlb is ListBox;

Depending on how you’re using this information, though, even this might not be necessary. You should describe the purpose of your method in more detail.

Upvotes: 8

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