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C# List<T>.Find

Oliver · 2012-09-19T11:18:27

Hey folks, Reasonably new to C# here, and was wondering if you could point out what I'm doing wrong here : class ObjectManager { public ObjectInstance GetObject<ObjectInstance>( int objectId ) { if( m_objects.ContainsKey(typeof(ObjectInsance) ) { List<EngineObject> objectList = m_objects[typeof(ObjectInstance)]; // This doesn't compile EngineObject obj = objectList.Find( i => i.ID == objectId ); } } ... Dictionary<Type, List<EngineObject>> m_objects; } I'm trying to return an EngineObject instance given an ID. EngineObject contains the ID property, but compiling generates the following error : Error 2 'System.Collections.Generic.List<Engine.Objects.EngineObject>' does not contain a definition for 'Find' and no extension method 'Find' accepting a first argument of type 'System.Collections.Generic.List<Engine.Objects.EngineObject>' could be found (are you missing a using directive or an assembly reference?) C:\Code\Outskirts\Engine\Managers\ObjectManager.cs 64 Any ideas what I'm doing wrong here?

General and Gameplay Programming Programming

Started by Capoeirista September 17, 2012 12:16 PM

33 comments, last by Anthracis 11 years, 7 months ago

Capoeirista

3,266

Author

September 18, 2012 01:03 PM

Yes, what I meant with the "First" LINQ extension was actually the method CALLED First:

var y = myList.First(x => x.ID == id);

or

var y = myList.FirstOrDefault(x => x.ID == id);

This is getting weird... Have you done a clean solution and then a rebuild solution?

This just gets stranger :



EngineObject elObjectO = objectList.First( x => x.ID == objectId );

Works perfectly, as does :



EngineObject elObjectO = objectList.FirstOrDefault( x => x.ID == objectId );

Just not :



EngineObject elObjectO = objectList.Find( x => x.ID == objectId );

I thought it might be something to do with EngineObject being abstract... but no.

And yeah, I've cleaned my solution and rebuilt... I also reboot my PC

-- Olly
http://toronto.ubisoft.com/

http://farcry.ubi.com

http://tomclancy-thedivision.ubi.com

Capoeirista

3,266

Author

September 18, 2012 01:10 PM

Soo... I tried pasting the code you have supplied in a new XNA project in Visual Studio 2010. Compiles fine. Then I did the same in VS2008. No errors there either. It seems like the errors is somewhere else in the code. What compiler are you using? If you don't mind, maybe you can zip the project and attach it to a post here so we can take a look

I'd be happy to link to upload my engine project, but don't have a premium membership. I'm using VS2010.

-- Olly
http://toronto.ubisoft.com/

http://farcry.ubi.com

http://tomclancy-thedivision.ubi.com

Xai

1,851

September 18, 2012 01:45 PM

The "Find" function is a member of the List<T> class, the other functions are LINQ statements. These have different signatures (Find takes a Predicate<T>, where the LINQ functions take Func<T, bool>) and are implemented in completely different libraries / namespaces. This must be the root of the problem, but I don't have a compiler handy to investigate. When in doubt and using LINQ, hover over the function to see which signature it is trying to use and then find examples to match that (for instance look to an OLDER, .NET 2.0 example for using Find before LINQ instead of a newer 3.5 or 4.0 example).

I'm also not sure if the problem will be in some assumption the Predicate version makes about your class "EngineObject" or if it will be a simple issue with the syntax having to be slightly different. Good Luck.

Capoeirista

3,266

Author

September 18, 2012 02:00 PM

The "Find" function is a member of the List<T> class, the other functions are LINQ statements. These have different signatures (Find takes a Predicate<T>, where the LINQ functions take Func<T, bool>) and are implemented in completely different libraries / namespaces. This must be the root of the problem, but I don't have a compiler handy to investigate. When in doubt and using LINQ, hover over the function to see which signature it is trying to use and then find examples to match that (for instance look to an OLDER, .NET 2.0 example for using Find before LINQ instead of a newer 3.5 or 4.0 example).

I'm also not sure if the problem will be in some assumption the Predicate version makes about your class "EngineObject" or if it will be a simple issue with the syntax having to be slightly different. Good Luck.

Thanks for the advice - I had a look at the Predicate that the Find operation, and sure enough it's



Predicate<EngineObject> match

I'll keep looking

-- Olly
http://toronto.ubisoft.com/

http://farcry.ubi.com

http://tomclancy-thedivision.ubi.com

Jontan

1,502

September 18, 2012 03:10 PM

Oops, sorry.. didn't know you had to have premium membership to do that

Anyway... Here's a random wild guess: what happens if you write it like this:
[source lang="csharp"]Predicate<EngineObject> pred = i => i.ID == objectId;

EngineObject obj = objectList.Find( pred );[/source]
Is the errors still with the Find() function, or is it with the predicate?

This surely is weird.. My last advice is to create a new project, isolate your ObjectManager and EngineObject, copy those to the new project and try compiling. If it works, add stuff until it stops working. If it still doesn't work it should be small enough to post the code in code tags here.

Capoeirista

3,266

Author

September 18, 2012 03:43 PM

This surely is weird.. My last advice is to create a new project, isolate your ObjectManager and EngineObject, copy those to the new project and try compiling. If it works, add stuff until it stops working. If it still doesn't work it should be small enough to post the code in code tags here.

Just tried this out :



Predicate<EngineObject> pred = i => i.ID == objectId;

objectList.Find( pred );

aaaand the same error code... this is getting ridiculous

-- Olly
http://toronto.ubisoft.com/

http://farcry.ubi.com

http://tomclancy-thedivision.ubi.com

CyberRascal

208

September 18, 2012 03:58 PM

Why don't you rar it and use say http://www.speedyshare.com/ to host it so we can check it out?

Capoeirista

3,266

Author

September 18, 2012 04:23 PM

Why don't you rar it and use say http://www.speedyshare.com/ to host it so we can check it out?

Done http://www.2shared.com/file/GWlPZyxm/Engine.html

-- Olly
http://toronto.ubisoft.com/

http://farcry.ubi.com

http://tomclancy-thedivision.ubi.com

arthursouza

1,622

September 18, 2012 06:19 PM

Could you link me the other stuff you have reference on the project? Like JigLibX

#Woops, nevermind, downloading already. Gonna take a look at it right now.

arthursouza

1,622

September 18, 2012 06:27 PM

Weird, I just tried



		/// <summary>

		/// Returns an Object of the supplied type, given it's id

		/// </summary>

		public T GetObject<T>(int objectId)

		{

			if (m_objectInstances.ContainsKey(typeof(T)))

			{

				List<EngineObject> objectList = m_objectInstances[typeof(T)];

				objectList.Find(delegate(EngineObject obj) { return obj.ID == objectId; });

			}

			return default(T);

		}

And it compiled perfectly. You just gotta change the signature a bit, since you are receiving a generic T, any type at all, and you actually need a T that is an EngineObject, unless I'm not understanding anything. So it would be:



		public T GetObject<T>(int objectId) where T: EngineObject

		{

			if (m_objectInstances.ContainsKey(typeof(T)))

			{

				List<EngineObject> objectList = m_objectInstances[typeof(T)];

				return  (T) objectList.Find(delegate(EngineObject obj) { return obj.ID == objectId; });

			}

			return default(T);

		}



	    public T GetObject<T>(int objectId) where T: EngineObject

	    {

		    if (m_objectInstances.ContainsKey(typeof(T)))

		    {

			    List<EngineObject> objectList = m_objectInstances[typeof(T)];

			    return  (T) objectList.Find(obj => obj.ID == objectId);

		    }

		    return default(T);

	    }

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