MSBuild will not execute targets in parallel - msbuild

Question

I am trying to understand why MSBuild does not execute targets in parallel. Given a simple test.proj:
<Project>
<ItemGroup>
<_Project Include="$(MSBuildThisFileFullPath)"
Targets="_T1" />
<_Project Include="$(MSBuildThisFileFullPath)"
Targets="_T2" />
</ItemGroup>
<Target Name="Build">
<MSBuild BuildInParallel="true"
Projects="#(_Project)"
Targets="%(_Project.Targets)" />
</Target>
<Target Name="_T1">
<Exec Command="ping -n 3 127.0.0.1" />
</Target>
<Target Name="_T2">
<Exec Command="ping -n 3 127.0.0.1" />
</Target>
</Project>
and command:
msbuild /m /t:Build test.proj
_T1 and _T2 targets are executed sequentially. I must be missing something really simple, but I cannot figure out what it is.

Answer 1

You are invoking batching.
The %(_Project.Targets) value to the Targets parameter is creating batches based on distinct values of the Targets metadata. There is a batch for _T1 and a batch for _T2.
The MSBuild task is being invoked twice in sequence, once for each batch.
See "MSBuild batching".
Try the following instead to pass multiple projects to one invocation of the MSBuild task:
<MSBuild BuildInParallel="true"
Projects="#(_Project)"
Targets="_T1" />
Update
The code above is not correct for the example scenario in the question. The MSBuild task recognizes that the two items in #(_Project) are duplicates and executes the project once. If two different project files were used they wouldn't be duplicates and both would be run.
The items in #(_Project) will also not be duplicates if they have different metadata. There are special Properties and AdditionalProperties metadata that can be used with the MSBuild task. See "Pass properties to projects".
In the example code below:
I added different Properties metadata to each project (and removed the Targets metadata).
I added a Ping target. The Ping target runs different targets based on the passed property of PingDependsOn.
<Project>
<ItemGroup>
<_Project Include="$(MSBuildThisFileFullPath)">
<Properties>PingDependsOn=_T1</Properties>
</_Project>
<_Project Include="$(MSBuildThisFileFullPath)">
<Properties>PingDependsOn=_T2</Properties>
</_Project>
</ItemGroup>
<Target Name="Build">
<MSBuild BuildInParallel="true"
Projects="#(_Project)"
Targets="Ping" />
</Target>
<Target Name="Ping" DependsOnTargets="$(PingDependsOn)" />
<Target Name="_T1">
<Exec Command="ping -n 3 127.0.0.1" />
</Target>
<Target Name="_T2">
<Exec Command="ping -n 3 127.0.0.1" />
</Target>
</Project>
The project can be run as
dotnet msbuild parallel.proj /v:n
or
msbuild parallel.proj /m
(The switches are different because the dotnet tool runs the embedded MSBuild as /v:m /m and the standalone MSBuild runs as /v:n /m:1. )
A slightly edited example of the resulting output shows interleaved lines of text from targets _T1 and _T2:
1>Project "parallel.proj" on node 1 (default targets).
1>Project "parallel.proj" (1) is building "parallel.proj" (1:2) on
node 1 (Ping target(s)).
1>_T1:
ping -n 3 127.0.0.1
1>Project "parallel.proj" (1) is building "parallel.proj" (1:3) on
node 2 (Ping target(s)).
1>_T2:
ping -n 3 127.0.0.1
1>_T1:
Pinging 127.0.0.1 with 32 bytes of data:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
1>_T2:
Pinging 127.0.0.1 with 32 bytes of data:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
1>_T1:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
1>_T2:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
1>_T1:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Packets: Sent = 3, Received = 3, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
1>Done Building Project "parallel.proj" (Ping target(s)).
1>_T2:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Packets: Sent = 3, Received = 3, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
1>Done Building Project "parallel.proj" (Ping target(s)).
1>Done Building Project "parallel.proj" (default targets).
Build succeeded.
0 Warning(s)
0 Error(s)
Time Elapsed 00:00:02.13