Had the same problem here.
Our cmake generated solution created configurations for Win32 and x64 platforms while the external C# project had x86 and x64 platforms. After cmake generated the solution, all projects (internal and external) are set to use platform Win32. (Check the solutions Configuration Manager.)
To force cmake to include the external projects as x86, specify the PLATFORM parameter when calling include_external_msproject.
include_external_msproject(TestApp "/TestApp/TestApp.csproj" PLATFORM x86)
(And of course ensure that you just do this for the x86 version...)
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