Visual Studio 2008 locks custom MSBuild Task assemblies

Question

I'm developing a custom MSBuild task that builds an ORM layer, and using it in a project. I'm being hampered by Visual Studio's behaviour of holding onto MSBuild task DLLs and not letting go.

I'd like to organize my solution like this;

My Solution
 |
 +- (1) ORM Layer Custom Task Project
 |  |
 |  +- BuildOrmLayerTask.cs     // here's my task
 |  
 +- (2) Business Logic Project  // and here's the project that uses it.
    |
    +- <UsingTask TaskName="BuildOrmLayerTask" AssemblyFile="$(TaskAssembly)" />

However, when project (2) builds, it locks onto the assembly from project (1). So now I can't build project (1) again without closing the solution and re-opening it.

Is there any way I can organize things so that the custom build task is not kept locked by Visual Studio?

Answer 1

(Edit: Sayed Ibrahim Hashimi, who literally wrote the book on msbuild, suggests the AppDomainIsolatedTask class for a better approach)

I've managed to solve this one myself...

Found this forum post from Dan Moseley, one of the MSBuild developers from Microsoft:

Hi there,

Unfortunately this is because MSBuild loads task assemblies in the primary appdomain. The CLR does not allow assemblies to unload from an appdomain as this allows important optimizations on their part.

The only workarounds I suggest is to call out tomsbuild.exe to build the projects that use the task. To do this, create MSBuild.exe <> as an external tool in VS.

Dan
developer on msbuild
DanMoseley - MSFT

So, it seems that to stop the locks, you must spawn out a new MSBuild.exe process. It can't be the one that runs inside Visual Studio, because when MSBuild runs, it loads the tasks into Visual Studio's primary app domain, and that can never be unloaded.

• create a new MSBuild project (a .csproj or similar) which overrides the 'Build' Target and performs your custom actions, eg;

  <!-- fragment of Prebuild.csproj -->   
  <Target Name="Build">   
       <BuildOrmLayerTask Repository="$(Repository)" />   
  </Target>
  

• Add it to visual studio if you want, but use Configuration Manager to make sure it is notbuilt in any configuration. Just let VS take care of source control and suchlike, not building.

• Edit the .csproj file of the project that depends on Prebuild.csproj. Add a BeforeBuild target which invokes MSBuild using the Exec task. This will start a new process, and when that process ends, the file locks are released. Example;

  <PropertyGroup>   
       <PrebuildProject>$(SolutionDir)Prebuild\Prebuild.csproj</PrebuildProject>   
  </PropertyGroup>   
  <Target Name="BeforeBuild">   
       <Exec Command="msbuild.exe &quot;$(PrebuildProject)&quot;" />   
  </Target>
  

Now, when you build the dependent project, it executes MSBuild in a new process before running the compile.

Answer 2

Can you edit the project files and include the following property declaration

<PropertyGroup>
    <GenerateResourceNeverLockTypeAssemblies>true</GenerateResourceNeverLockTypeAssemblies>
</PropertyGroup>

Let me know if that works for you.

Answer 3

As I mentioned in a comment directed at @Al-Muhandis, it seems possible to create a wrapper around the custom task so that the wrapper gets locked but not the custom task DLL. I have taken an initial shot at doing so with the isolated-task project. It may be buggy, and it only works with VS2008 for now. Pull requests welcome.

The idea for the project was based on the observation that tasks deriving from MarshalByRefObject (using, perhaps, AppDomainIsolatedTask) appear to be loaded into the main application domain for reflection purposes, but a new application domain is created to execute the task. Since loading into the main application domain still seems to lock the DLL, it was useful to create a DLL with a task derived from AppDomainIsolatedTask that loads the custom task DLLs. That way, the wrapper DLL gets locked, but because it executes in its own app domain, the custom task DLLs are unloaded when the wrapper task's executing domain is unloaded. This procedure avoids keeping a lock on the custom task DLLs after the build is complete.