CODEWITHSUNDEEP
c#castingtypes
Eric Anastas
Eric Anastas

Reputation: 22213

How to tell if an instance is of a certain Type or any derived types

I'm trying to write a validation to check that an Object instance can be cast to a variable Type. I have a Type instance for the type of object they need to provide. But the Type can vary. This is basically what I want to do.

        Object obj = new object();
        Type typ = typeof(string); //just a sample, really typ is a variable

        if(obj is typ) //this is wrong "is" does not work like this
        {
            //do something
        }

The type object itself has the IsSubClassOf, and IsInstanceOfType methods. But what I really want to check is if obj is either an instance of typ or any class derived from typ.

Seems like a simple question, but I can't seem to figure it out.

Upvotes: 16

Views: 12558

Answers (3)

Gishu
Gishu

Reputation: 136613

If you are working with Instances, you should be going for Type.IsInstanceOfType

(Returns) true if the current Type is in the inheritance hierarchy of the object represented by o, or if the current Type is an interface that o supports. false if neither of these conditions is the case, or if o is nullNothingnullptra null reference (Nothing in Visual Basic), or if the current Type is an open generic type (that is, ContainsGenericParameters returns true). -- MSDN

        Base b = new Base();
        Derived d = new Derived();
        if (typeof(Base).IsInstanceOfType(b)) 
            Console.WriteLine("b can come in.");    // will be printed
        if (typeof(Base).IsInstanceOfType(d)) 
            Console.WriteLine("d can come in.");    // will be printed

If you are working with Type objects, then you should look at Type.IsAssignableFrom

(Returns) true if c and the current Type represent the same type, or if the current Type is in the inheritance hierarchy of c, or if the current Type is an interface that c implements, or if c is a generic type parameter and the current Type represents one of the constraints of c. false if none of these conditions are true, or if c is nullNothingnullptra null reference (Nothing in Visual Basic). -- MSDN

Upvotes: 6

Samuel
Samuel

Reputation: 38346

I think you need to restate your conditions, because if obj is an instance of Derived, it will also be an instance of Base. And typ.IsIstanceOfType(obj) will return true.

class Base { }
class Derived : Base { }

object obj = new Derived();
Type typ = typeof(Base);

type.IsInstanceOfType(obj); // = true
type.IsAssignableFrom(obj.GetType()); // = true

Upvotes: 7

Hadi Eskandari
Hadi Eskandari

Reputation: 26354

How about this:


    MyObject myObject = new MyObject();
    Type type = myObject.GetType();

    if(typeof(YourBaseObject).IsAssignableFrom(type))
    {  
       //Do your casting.
       YourBaseObject baseobject = (YourBaseObject)myObject;
    }

This tells you if that object can be casted to that certain type.

Upvotes: 26

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