specify project file of a solution using msbuild

MSBuild actually works through the use of projects not the solution. The solution is only used to parse it into a temporary project file in MSBuild internally. You should be able to just build the project of interest directly through MSBuild by executing the following command.

"msbuild testproject /p:Configuration=Release /p:Platform=x86"

There is one major issue I know you could run into using the project directly instead of the solution: if you use the solution to express dependencies between the projects, instead of adding the references to the project and letting the build system work out the dependencies automatically.

If you are enforcing a build order using the sln file, I recommend working those dependencies directly into the proj files and removing them from the sln. This will allow you to invoke any proj file from MSBuild directly and the projects will all build independently without any additional work. You really should treat the sln file as a group of projects to make working in Visual Studio easier and not as a build input.

msbuild test.sln /t:project /p:Configuration="Release" /p:Platform="x86" /p:BuildProjectReferences=false

Notice that what is assigned to /t is the project name in the solution, it can be different from the project file name.

Also, as stated in How to: Build specific targets in solutions by using MSBuild.exe:

If the project name contains any of the characters %, $, @, ;, ., (, ), or ', replace them with an _ in the specified target name.

You can also build multiple projects at once:

msbuild test.sln /t:project;project2 /p:Configuration="Release" /p:Platform="x86" /p:BuildProjectReferences=false

To rebuild or clean, change /t:project to /t:project:clean or /t:project:rebuild