.NET: Unable to cast object to interface it implem

问题:

I have a class (TabControlH60) that both inherits from a base class (UserControl) and implements an interface (IFrameworkClient). I instantiate the object using the .NET Activator class. With the returned instance, I can cast to the UserControl base class, but not to the interface. The exception I get is below the code snipet. How do I cast to the interface?

object obj = Activator.CreateInstance(objType);
Type[] interfaces = obj.GetType().GetInterfaces(); // contains IFrameworkClient

m_Client = (UserControl)obj;                 // base class cast works
IFrameworkClient fc = (IFrameworkClient)obj; // interface cast fails

// Note: The (IFrameworkClient)obj cast works fine in the debugger Watch window.
{"Unable to cast object of type 'FPG.H60.AFF.TabControlH60' to type 
    'FPG.AFF.Interfaces.IFrameworkClient'."}

回答1:

I hat the same problems with a library of mine providing "plugin"-functionality... I got it finally working...

Here was my problem: I had one main assembly using plugins, one assembly with the plugin (Plugin.dll) AND (important) another assembly providing the plugin-functionality (Library.dll).

The Plugin.dll referenced the main assembly (in order to be able to extend it) and the Library.dll with the plugin-func. - it's binaries got to a directory "./Plugins" relative to the main assembly.

The main assembly also referenced the plugin-func. assembly in order to use the "PluginManager" is wrote. This "PluginManager" gets a path and loads all *.dll files via reflection in order to analyze if there is a "IPlugin"-interface (which comes from Library.dll too).

Everytime I called the PluginManager to load the plugins it could not cast them to "IPlugin" although they implemented it.

I nearly got mad - but then I found out the whole problem. By compiling the plugin there was not only the "Plugin.dll" but the "Library.dll" written to the "./Plugins" directory. By accidentally loading the "Library.dll" every time with my PluginManager I now had two types of "IPlugin" - one in the actual "Library.dll" that is used from the main assembly and one that was loaded through my PluginManager - and those were incompatible!

Attention - if you just do not load "./Plugins/Library.dll" you nevertheless encounter the problem - because if you load "Plugin.dll" which references "Library.dll" then it just uses the one in the same directory... TILT...!! My PluginManager now just deletes "Library.dll" where it find it.

The clue is: Be sure that you do not access two assemblies in different contexts!

回答2:

The most likely cause here is that IFrameworkClient is from a different assembly in the two cases, and is thus a different .NET type. Even if it is the same code, it can be a different type.

Check the AssemblyQualifiedName. Note also that if you are loading this assembly with reflection you can get a different type even with the same AssemblyQualifiedName, thanks to the load-context.

回答3:

Define IFrameworkClient Interface in independent namespace (must be have namespace) of independent project (class library).Then add refrence of the class library to Control project and main project

回答4:

Something tells me your sample code is leaving some stuff out...

class Program
{
    static void Main(string[] args)
    {
        var type = typeof(MyClass);
        object obj = Activator.CreateInstance(type);
        Type[] interfaces = obj.GetType().GetInterfaces();

        var m_Client = (UserControl)obj;          
        IFrameworkClient fc = (IFrameworkClient)obj;
    }
}

public interface IFrameworkClient { }

public class UserControl { }

public class MyClass : UserControl, IFrameworkClient { }

This compiles and runs.

I'm betting that the DLL containing the definition of IFrameworkClient hasn't yet been loaded before you try to cast. This can happen when you're using Activator.CreateInstance.

Try inserting var forceLoad = typeof(IFrameworkClient); before the cast.

回答5:

When the Interface is in a different assembly and i get my class dynamically at run-time in a different assembly, interface casting will be failed like your sample (C# knows our interface as a different type than which one the class inherited from that).

This is my simple and useful technique in this cases:

When I'm sure my Class has inherited from the mentioned Interface (eq. IFrameworkClient), so i write one magic line of code like this:

dynamic fc = obj as IFrameworkClient ?? (dynamic) obj;

By this technique you can:

• Write your codes after this line of code for fc at design time base on Interface members info and vs editor intelligences system.
• Prevent any interface casting error at run-time

Notes:

• You need C# v4 to use dynamic type
• Usually i don't like to use dynamic types in my codes but it can help us in some cases like this

回答6:

If the class FPG.H60.AFF.TabControlH60 actually does implement IFrameworkClient there should be no reason this would fail. The only thing I can think of that causes this exception is if the assembly that contains IFrameworkClient is strongly named and the Tab Control object happens to reference a different version of the containing assembly or your are using a different interface with the name IFrameworkClient.

回答7:

In my case I had to add a build event to copy the needed DLL since I was creating instances and assigning to interface types at run time. Otherwise the DLL loaded might not be the most up-to-date DLL, and therefore may not cast to the interface.

The reason I used build events in this case (instead of adding the DLL as a reference) is that the architecture is such that the main application should only reference the interface types, and everything else should be loaded dynamically.

TLDR; In the case of loading types dynamically from another DLL, make sure you copy the most recent version of that DLL to the bin directory using build events, otherwise casting may not work when it appears that it should.

回答8:

The cast isn't working because you're trying to cast from type object to the interface. If you replace the interface cast line with:

IFrameworkClient fc = (IFrameworkClient)m_Client;

It will work.

Alternately, I'm mildly certain that you could do the cast from the object to the interface with the as operator.

See this article for more information: http://blogs.msdn.com/ericlippert/archive/2009/10/08/what-s-the-difference-between-as-and-cast-operators.aspx

One more piece of the puzzle. Interfaces do not derive from object: http://blogs.msdn.com/ericlippert/archive/2009/08/06/not-everything-derives-from-object.aspx