Run a target exactly once

If you're building a .NET project for multiple frameworks using .NET SDK multitargeting, generally each build is a complete build of all relevant targets. However, in some cases, you want a certain target to only run once, for example, a target that increments a version.

Set up the targets

The solution shown here runs the MyBeforeBuildTarget once and once only regardless of whether a single framework is being built, or if multiple frameworks are being built, without a change in the target itself.

1. Add Condition expressions that use the Multitarget property to determine whether to build multiple frameworks, or just one.

   <PropertyGroup>
    <TargetFrameworks Condition=" '$(Multitarget)' == 'true' ">net7.0;net8.0</TargetFrameworks>
    <TargetFramework  Condition=" '$(Multitarget)' != 'true' ">net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
   </PropertyGroup>
   

2. Add three targets to the project, yours that you want to run just once (MyBeforeBuildTarget), the other two as shown here:

   <Project Sdk="Microsoft.NET.Sdk">
   
   <PropertyGroup>
       <TargetFrameworks Condition=" '$(Multitarget)' == 'true' ">net7.0;net8.0</TargetFrameworks>
       <TargetFramework  Condition=" '$(Multitarget)' != 'true' ">net8.0</TargetFramework>
       <ImplicitUsings>enable</ImplicitUsings>
       <Nullable>enable</Nullable>
       <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
   </PropertyGroup>
   
   <Target Name="MyBeforeBuildTarget">
       <Warning Text="MyBeforeBuildTarget" />
   </Target>
   
   <Target Name="BuildMyBeforeBuildTargetBeforeOuterBuild"
           DependsOnTargets="MyBeforeBuildTarget"
           BeforeTargets="DispatchToInnerBuilds"
           />
   
   <Target Name="BuildMyBeforeBuildTargetBeforeInnerBuild"
           BeforeTargets="BeforeBuild">
      <MSBuild Projects="$(MSBuildProjectFullPath)"
            Targets="MyBeforeBuildTarget"
            RemoveProperties="TargetFramework" />
   </Target>
   
   </Project>
   

This method relies on the way that multitargeting works in .NET SDK projects. The outer build sets TargetFrameworks but the inner builds set TargetFramework for each desired target. Therefore, to build once for the inner build, remove the TargetFramework property. This invokes the outer build with the desired target instead of running it in each of the inner builds.

Build the project

1. From the command line, build without multitargeting:

   dotnet msbuild -nologo -tl:false -bl -clp:nosummary -p:Multitarget=false

   ExtendMaybeMultitargeted.csproj(12,5): warning : MyBeforeBuildThing
   ExtendMaybeMultitargeted -> ExtendMaybeMultitargeted\bin\Debug\net8.0\ExtendMaybeMultitargeted.dll
   

2. Now try building with multitargeting:

   dotnet msbuild -nologo -tl:false -bl -clp:nosummary -p:Multitarget=true

   ExtendMaybeMultitargeted\ExtendMaybeMultitargeted.csproj(12,5): warning : MyBeforeBuildThing
     ExtendMaybeMultitargeted -> ExtendMaybeMultitargeted\bin\Debug\net8.0\ExtendMaybeMultitargeted.dll
     ExtendMaybeMultitargeted -> ExtendMaybeMultitargeted\bin\Debug\net7.0\ExtendMaybeMultitargeted.dll
   

As you can see in the output, the warning was only emitted once regardless of whether multitargeting was used or not.

The solution shown here works because the target you want to run once, MyBeforeBuildTarget, is tied to a different target usingBeforeTargets, but regardless of whether multitargeting is true or false, the target it's attached to is always one that runs once. The outer build is a single target, DispatchToInnerBuilds, that runs the inner build for all the different frameworks specified in TargetFrameworks. DispatchToInnerBuilds itself only runs once for a multitargeted build, but for a single targeted build, it doesn't run at all. For a single targeted build, you set BeforeTargets to BeforeBuild as usual to run your target, but the Condition makes sure that it's only attached to BeforeBuild in the single-targeted case.

Related content

.NET SDK multitargeting