How to get the process id in msbuild target - msbuild

Question

How can I get the proc-id of the msbuild process inside a target? I want to read it and write it out?
Alternatively, does MSBuild has some kind of session id that we can capture?
I want to run a T4 task only once per build run and thinking of a conditional way to build it. Otherwise it runs multiple times
<TransformOnBuild>false</TransformOnBuild>
<TransformOnBuild Condition="'$(SomeCondition)' == 'true'">false</TransformOnBuild>

Answer 1

If you want to run something once you can wrap it in a target, MSBuild does not execute same target twice no matter how many times it was called. If you want a simple mutex you can use the usual lock file technique using built-in WriteLinesToFile and Delete tasks with InitialTargets. If you want actual process id, i.e. the MSBuild node (of many) that is executing your task, you can write an inline task, see example below.
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<UsingTask TaskName="Process" TaskFactory="CodeTaskFactory" AssemblyName="Microsoft.Build.Tasks.v12.0">
<ParameterGroup>
<Id ParameterType="System.Int32" Output="true" />
</ParameterGroup>
<Task>
<Code>Id = System.Diagnostics.Process.GetCurrentProcess().Id;</Code>
</Task>
</UsingTask>
<Target Name="Foo">
<Process>
<Output PropertyName="Id" TaskParameter="Id" />
</Process>
<Message Text="Process $(Id)" />
</Target>
</Project>