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WPF–Using the XmlnsDefinitionAttribute causes issues when loading a WPF form in an Addin

Yesterday a colleague asked me for help. She was building an addin for ESRI ArcGis and she wanted to load a WPF form and show some related data. The problem was that when loading the WPF form some of the DLL’s were searched in the Addin folder but for some of the DLL’s, .NET was looking inside the bin folder of the main application. Easy solution would be to just copy over all addin assemblies to the bin folder of ArcGis. Of course this was not what we want as it removes the advantages of the whole addin concept.

So the big question is; why are some DLL’s loaded from the correct location and others not?

Let’s have a look at the WPF form first:

In this form the services DLL was loaded correctly whereas the caliburn.micro dll(still my favorite MVVM framework Smile) was not. The difference is that the services DLL namespace is constructed using the clr-namespace syntax whereas the Caliburn.Micro namespace is constructed using an URL.

Where does this URL come from?

WPF defines a CLR attribute that is consumed by XAML processors in order to map multiple CLR namespaces to a single XAML namespace. This attribute, XmlnsDefinitionAttribute, is placed at the assembly level in the source code that produces the assembly. The WPF assembly source code uses this attribute to map the various common namespaces, such as System.Windows and System.Windows.Controls, to the http://schemas.microsoft.com/winfx/2006/xaml/presentation namespace. This prevents you from loading a lot of namespaces yourself as they can be grouped using one XAML namespace alias.

Unfortunately it was exactly this functionality that caused the Addin loader to look for assemblies at the wrong location.

How did we fix it?

By relying on the CLR behavior that the runtime first will check if an assembly is already loaded before it will try to search for it on the file system, we were able to solve the issue. Therefore inside the bootstrap logic of the addin we added the following line to explicitly load the Caliburn.Micro assembly into the appdomain:

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