Downcast not working. Am I doing it wrong?

Question

2.00/5 (2 votes)

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I am loading a class from an assembly and trying to instantiate and instance.

Here's what I have

C#

public interface IActivity

C#

public abstract class BaseActivity:IActivity

C#

public class MyActivity:BaseActivity //I don't really call stuff MyXxx

And to load it:

C#

Type type = validIActivityTypes.Single(); //I've already performed my checks for this
ConstructorInfo constructor = type.GetConstructor(Type.EmptyTypes);
if (constructor == null)
   return result;
IActivity activity = (IActivity)constructor.Invoke(null);

At this point, the 'type' is 'MyActivity' but...

Boom:

Quote:
A first chance exception of type 'System.InvalidCastException' occurred in ServerControls.dll

Additional information: Unable to cast object of type 'WorkFlowEngine.Activities.MyActivity' to type 'WorkFlowEngine.Interfaces.IActivity'.

What am I doing wrong?

Thanks

Edit: Just tested type.GetInterfaces():

>type.GetInterfaces() | {System.Type[1]}
>[0] | {Name = "IActivity" FullName = "WorkFlowEngine.Interfaces.IActivity"}

Posted 1-May-15 1:20am

Andy Lanng

Updated 1-May-15 1:31am

v4

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Comments

BillWoodruff 1-May-15 7:33am

I'm puzzled by what you are doing with 'result: where is that declared, what is it ?

Andy Lanng 1-May-15 8:52am

result is just the return of the method. At this stage it's a string[0].
If the method was void, that line would just be 'return;'

Alan N 1-May-15 8:11am

Have you declared WorkFlowEngine.Interfaces.IActivity in more than one assembly? Although if you have, there should be a compiler warning on the line where the cast occurs.

Andy Lanng 1-May-15 8:53am

Nope - my namespaces are very clean. + I don't often use interfaces unless I need to do this sort of casting.

Richard Deeming 1-May-15 8:12am

Just tried your code in LINQPad, and it works fine. Are you sure you don't have two different versions of the interface?

Andy Lanng 1-May-15 8:57am

hmm. I am loading the assembly from a path but I do this earlier on:
var iActivityTypes = types.Where(t => t.GetInterfaces().Select(i => i.GUID).Contains(typeof(IActivity).GUID));
Note I have to compare the GUID because the t.GetInterfaces != typeof(IActivity). Is this because the loaded assembly is not the same dll (but is the same version) as the referenced dll?

Andy Lanng 1-May-15 8:58am

Gah! it appears that the dll has to be the same one as is references. Not even the same module version will do. It has to be the same exact file.

I'm sure this hasn't been the case in the past >_<

Richard Deeming 1-May-15 9:29am

The behaviour is different for standard .NET interfaces, which must be the same type, and embedded COM interop interfaces:
Type Equivalence and Embedded Interop Types[^]

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Andy Lanng · Accepted Answer · 2015-05-01T03:34:00

Ok - i was doing it wrong:

My activity type lived in the same assembly as the IActivity interface.

When you load an assembly, all related assemblies will also be loaded into the AppDomain unless it has already been loaded into the AppDomain .

If you load an assembly through reflection then it will still be loaded and will not be the same as the referenced assembly.

So essentially I did have two versions of the IActivity loaded :(

The solution is simple. I have moved my interface declarations into a difference assembly (project) which is referenced by both the app and the loaded dll. This means that the loaded dll will use the pre-loaded interface assembly instead of it's own copy.

Thus the issue (highlighted by Richard Deeming at 1 sec ago) is solved

Thanks for looking into this, guys ^_^

Downcast not working. Am I doing it wrong?

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Downcast not working. Am I doing it wrong?

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