Object does not match target type in DesignData

Question

I'm throwing this out in case someone has encountered this before.

When creating DesignData for use within the WPF designer, I get one of two errors:

Object does not match target type.
at System.Reflection.RuntimeMethodInfo.CheckConsistency(Object target)
(SNIP)
at Microsoft.Expression.DesignModel.InstanceBuilders.ClrObjectInstanceBuilder. UpdateProperty(IInstanceBuilderContext context, ViewNode viewNode, IProperty propertyKey, DocumentNode valueNode)

The other one is a little more informative:

The value "_.di0.MyProjectLol.MyType" is not of type "MyProjectLol.MyType" and cannot be used in this generic collection.
at System.ThrowHelper.ThrowWrongValueTypeArgumentException(Object value, Type targetType)
(SNIP)
at Microsoft.Expression.DesignModel.InstanceBuilders.ClrObjectInstanceBuilder. InstantiateChildren(IInstanceBuilderContext context, ViewNode viewNode, DocumentCompositeNode compositeNode, Boolean isNewInstance)

When debugging, I can see that there is a dynamic assembly that is loaded with proxy-ish types that look like mine but, obviously, are not. This assembly is called Blend_RuntimeGeneratedTypeAssembly(Guid goes here). When attempting to load types in this assembly, it throws a type load exception for a number of them. So, some types get proxied, some types are left as God and I intended, and when they mix unnatural acts occur.

For example, the type "Foo" might get a proxy created, but no other types do (TypeLoadExceptions). Then the designer tries to hand one of my real types the proxy (helped by the fact that xaml serialization likes to cast collections to IList, thus shitting on type safety) and you get one of the above exceptions.

I have spent half a week trying to fix this. I've tried a hundred different things, but I can't figure out exactly what is causing it to fail. Suggestions are welcome, TIA.

Answer 1

Solution in two parts:

1) Ensure VS is fully updated. At this point, it means installing the Silverlight 4 tools for Visual Studio 2010. They include the latest updates to the WPF designer. If you're reading this off in the distant future, ignore this one.

2) Hit the properties for your design data files. Clear out "Custom Tool", and set the "Build Action" to either "DesignData" or "DesignTimeDataWithDesignTimeCreatableTypes".

DesignData means that your types cannot be deserialized from xaml directly (due to dependencies or the such), so the designer attempts to create mocks for those types and presents the mocks to your design surface.

DesignTimeDataWithDesignTimeCreatableTypes means that the designer will load your assemblies and deserialize the xaml directly into your types without creating mocks.