I have built a NuGet package for my Simple.OData.Client with support for Xamarin Android and iOS. This library uses Microsoft.Bcl.Async. Tests using Android and iOS simulator went OK, but when I install this new NuGet package into another project, it fails to compile Droid/Touch projects with the following message:
C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(720,2): error : Exception while loading assemblies: System.IO.FileNotFoundException: Could not load assembly 'Microsoft.Threading.Tasks.Extensions, Version=1.0.12.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono for Android profile?
C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(720,2): error : File name: 'Microsoft.Threading.Tasks.Extensions.dll'
Of course there is no Microsoft.Bcl.Async pack (yet) for Mono platforms, on the other hand since the library tests pass on these platforms, I guess this more of a challenge to make Visual Studio happy rather than do some code modification.
Has anyone managed to use on Mono PCLs dependent on Microsoft.Bcl.Async?
Generally, if the code in your package depends on another NuGet package, you need to declare a package dependency so that anyone installing your package will get the one you depend on too. That should take care of issues like this.
However, in this case it is a lot more complicated. Since Microsoft.Bcl.Async doesn't support Mono platforms, you can't declare that package dependency (or if you did it would always fail to install). Unfortunately the license for Microsoft.Bcl.Async restricts it to "Windows platforms". I'm not a lawyer but I think that means you shouldn't distribute Microsoft.Threading.Tasks.Extensions.dll as part of your package. Finally, there's no official PCL support for Mono platforms yet, so if you do come up with a solution that works now it may change when official PCL support comes out.
If you want to attempt getting this to work, what you basically need to do is create your own DLLs with the same names as are in Microsoft.Bcl and Microsoft.Bcl.Async for use on Mono platforms. In those assemblies, put type forwards for the types that Mono already implements (which should probably be the types in the System.* DLLs), and create an implementation of your own for the other ones (the AsyncBridge source code is useful for this). This is what I did for one of my BUILD 2012 PCL demos. The code for that is here. AsyncFacades and AsyncBridge are the relevant projects.
For those stumbling into this issue when getting the Simple.oData.Client from nuget, just install the following packages from nuget:
Microsoft Bcl Async Portable Microsoft Bcl Async
I believe I've found the problem.
Earlier I had the following line in Simple.OData.Client nuspec file:
<file src="Simple.OData.Client.Core\bin\Release\Microsoft.Threading.Tasks.Extensions.dll" target="lib\portable-net403+win8+sl5+wp8+MonoAndroid16+MonoTouch40\Microsoft.Threading.Tasks.Extensions.dll" />
This is not how it should be because Microsoft.Bcl.Async should have taken care of copying Tasks.Extensions.dll. I reported this to Microsoft and expected this to be fixed in the RTM version of Microsoft.Bcl.Async. Apparently it was not, so all NuGet packages dependent on Microsoft.Bcl.Async should still copy this file. Then it will work.
for who has the same issue I think you can downgrade System.Threading.Tasks.Extensions 4.4.0
来源：https://stackoverflow.com/questions/16389315/microsoft-threading-tasks-extensions-in-xamarin-mono-projects