CODEWITHSUNDEEP
c#interfacepolymorphismderived-class
Tizz
Tizz

Reputation: 840

How to check if an object is derived from an interface?

I have some classes that are derived from an interface, and I would like to be able to check in code to see if an object that is passed in is derived from that interface, but im not sure exactly the method calls ...

interface IFile
{
}

class CreateFile : IFile
{
    string filename;
}

class DeleteFile : IFile
{
    string filename;
}

// Input here can be a string or a file
void OperateOnFileString( object obj )
{
    Type theType = obj.GetType();

    // Trying to avoid this ...
    // if(theType is CreateFile || theType is DeleteFile)

    // I dont know exactly what to check for here
    if( theType is IFile ) // its not, its 'CreateFile', or 'DeleteFile'
        print("Its an IFile interface");
    else
        print("Error: Its NOT a IFile interface");
}

In actuality I have hundreds of derived classes from that interface and i'm trying to avoid having to check for each type, and having to add the check when I create another class from that type.

Upvotes: 2

Views: 6207

Answers (6)

McGarnagle
McGarnagle

Reputation: 102753

You could use BaseType:

if (type.BaseType is file)

Since file is an interface, use Type.GetInterfaces to check the underlying interfaces of type:

if (type.GetInterfaces().Any(i => i.Equals(typeof(file))

Or probably a bit faster, use Type.GetInterface:

if (type.GetInterface(typeof(file).FullName) != null)

(This searches the interfaces of type and of any inherited classes or interfaces.)

Upvotes: 2

Brian Liang
Brian Liang

Reputation: 7774

  1. is operator works or you can do:

    if (someInstance is IExampleInterface) { ... }

  2. or

    if(typeof(IExampleInterface).IsAssignableFrom(type)) {
 ...
}

Upvotes: 6

DotNetUser
DotNetUser

Reputation: 6612

you can create an extension method like below

    /// <summary>
    /// Return true if two types or equal, or this type inherits from (or implements) the specified Type.
    /// Necessary since Type.IsSubclassOf returns false if they're the same type.
    /// </summary>
    public static bool IsSameOrSubclassOf(this Type t, Type other)
    {
        if (t == other)
        {
            return true;
        }
        if (other.IsInterface)
        {
            return t.GetInterface(other.Name) != null;
        }
        return t.IsSubclassOf(other);
    }

    and use it like below

    Type t = typeof(derivedFileType);
    if(t.IsSameOrSubclassOf(typeof(file)))
    { }

Upvotes: 1

Aghilas Yakoub
Aghilas Yakoub

Reputation: 28970

If( yourObject is InterfaceTest)
{
   return true;
}

Upvotes: 2

Jon
Jon

Reputation: 437376

You are passing the wrong argument to is. The correct is

if (obj is file) {
    // ...
}

However it would be even better if you had an overload of the method that directly accepts a file parameter. In fact it's quite unclear how one that accepts an object could efficiently use it.

Upvotes: 3

SLaks
SLaks

Reputation: 887453

is is exactly correct.
However, you need to check the instance itself.

obj.GetType() returns an instance of the System.Type class that describes the object's actual class.

You can just write if (obj is IFile).

Upvotes: 8

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