C# How to determine if a type implements a given interface?

Question

I want the following to return true:

var isIt = IsDisposable(typeof(TextWriter));

where:

bool IsIDisposable(Type t){
    ??? 
    // I tried:
    //return t.IsSubclassOf(typeof(IDisposable)); //returns false
    // the other t.IsXXX methods don't fit the requirement as far as I can tell
}

Use reflection. Does this SO article help? http://stackoverflow.com/questions/1519530/using-reflection-to-find-interfaces-implemented — Ripside, Feb 28 '13 at 15:11
@Anthony I don't have an instance of a type, but a type. Can't use `is` on that (I mean I can, but it won't work as expected) — Cristian Diaconescu, Feb 28 '13 at 15:12
Of course. I deleted my comment as it became more evident you were actually working with the type. — Anthony Pegram, Feb 28 '13 at 15:13
@RvdK Same comment as for Anthony. I don't work on an instance of some type, but on a `Type` — Cristian Diaconescu, Feb 28 '13 at 15:16
Possible duplicate of [How to determine if a type implements an interface with C# reflection](http://stackoverflow.com/questions/4963160/how-to-determine-if-a-type-implements-an-interface-with-c-sharp-reflection) — user247702, Nov 30 '16 at 09:20

Tim Schmelter · Accepted Answer · 2013-02-28T15:17:36.890

15

You can use IsAssignableFrom

bool IsDisposable = typeof(IDisposable).IsAssignableFrom(typeof(TextWriter));

DEMO

edited Feb 28 '13 at 15:17

answered Feb 28 '13 at 15:11

Tim Schmelter

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no you can't. `IsAssignableFrom` works *the other way around*: BaseType.IsAssignableFrom(Derived) returns true. – Cristian Diaconescu Feb 28 '13 at 15:15
Ok, that works :) Actually, it's conceivably faster than `GetInterfaces()`.Upvoted – Cristian Diaconescu Feb 28 '13 at 15:20
I had made a demo on ideone too, to prove it doesn't work :) (before reading your updated answer) – Cristian Diaconescu Feb 28 '13 at 15:27
@Christi: Yes, i've updated my answer after one minute but before you've posted your comment. I find the correct order in `IsAssignableFrom` hard to remember. – Tim Schmelter Feb 28 '13 at 15:30
The seemingly backwards semantics of `IsAssignableFrom()` made me miss it when searching for the solution on my own. It would be better, IMO, if there was a `Type.IsAssignableTo(Type)` method instead (or in addition to...) – Cristian Diaconescu Feb 28 '13 at 16:00

Cristian Diaconescu · Answer 2 · 2013-02-28T15:26:37.283

6

I found it: Type.GetInterfaces() is what I need:

bool IsIDisposable(Type t){
    return t.GetInterfaces().Contains(typeof(IDisposable));
}

From the documentation, Type.GetInterfaces() returns:

Type: System.Type[]
An array of Type objects representing all the interfaces implemented or inherited by the current Type.

edited Feb 28 '13 at 15:26

answered Feb 28 '13 at 15:11

Cristian Diaconescu

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Will this work even if you implemented say `IFoo` which in-turn implements `IDisposable`? – Earlz Feb 28 '13 at 15:21

C# How to determine if a type implements a given interface?

2 Answers2