Visual studio one project with several dlls as output?

You could create one project for each plugin and group all projects in a solution.

If you don’t want to have one project per plugin, you could create a custom build with MSBuild using CSC task

How to generate a dll for each plugin file

1. In a project you add all plugins files

2. Edit the project file to specify which class will generate a plugin library :

   <ItemGroup>
     <Compile Include="Class1.cs">
       <Plugin>true</Plugin>
     </Compile>
     <Compile Include="Class2.cs" />
     <Compile Include="Class3.cs">
       <Plugin>true</Plugin>
     </Compile>
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>
   

3. Add a new target in your project file to generate the plugins library

   <Target Name="BuildPlugins">
     <CSC Condition="%(Compile.Plugin) == 'true'"
          Sources="%(Compile.FullPath)"
          TargetType="library"
          OutputAssembly="$(OutputPath)%(Compile.FileName).dll"
          EmitDebugInformation="true" />
   </Target>
   

4. If you want to create the plugins library after each build, add an after build target :

   <Target Name="AfterBuild" DependsOnTargets="BuildPlugins">
   </Target>