Make sure that MSBuild.dll is hostable

[ladipro]

Part of .NET 6.0 user story #5866

To load MSBuild directly into the CLI process we'll need to assembly-load MSBuild.dll and invoke its Main method via reflection. This issue is tracking the effort to:

• Verify that the above is doable, i.e. the command line is always passed as Main parameters.
• Introduce a hosting interface or another entry-point if Main is not enough.

This work is scoped only to the .NET Core version of MSBuild.

Acceptance criteria: A simple .NET application is able to load MSBuild.dll by path and run it with arbitrary command line arguments with full fidelity. Such a host can be a drop-in replacement for "dotnet build".

[ladipro]

Attaching a simple program to host MSBuild.dll. I have verified that command line arguments are passed as Main parameters and invoking Main is enough. The host must declare the same run-time dependencies as MSBuild.dll, the easy way to do it is placing its executable in the SDK directory alongside MSBuild.dll and passing --additional-deps MSBuild.deps.json to dotnet exec.

This command line is equivalent to running dotnet build <dotnet build args> (replace hard-coded SDK path as needed):

dotnet exec --additional-deps "\Program Files\dotnet\sdk\5.0.201\MSBuild.deps.json" "\Program Files\dotnet\sdk\5.0.201\build-inproc.dll"  -maxcpucount -verbosity:m -restore -consoleloggerparameters:Summary "-distributedlogger:Microsoft.DotNet.Tools.MSBuild.MSBuildLogger,C:\Program Files\dotnet\sdk\5.0.201\dotnet.dll*Microsoft.DotNet.Tools.MSBuild.MSBuildForwardingLogger,C:\Program Files\dotnet\sdk\5.0.201\dotnet.dll" <dotnet build args>

build-inproc.zip

[ladipro]

One additional thing that dotnet build does is set the DOTNET_HOST_PATH environment variable. This is required for e.g. the Roslyn build tasks: https://github.com/dotnet/roslyn/blob/451fa62b3b4071cabba1c0a5c471e6b180f43bab/src/Compilers/Shared/RuntimeHostInfo.cs#L65

[ladipro]

I can successfully build large solutions with the test host and the role of dotnet build is understood. Closing.