Vs: fix CSharp custom command by introducing inline MSBuild <Targets>s

The custom command implementation is based on the Microsoft support article:

https://docs.microsoft.com/en-us/visualstudio/msbuild/how-to-extend-the-visual-studio-build-process

Fixes: #16960

Tests/RunCMake/CSharpCustomCommand/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.3)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/CSharpCustomCommand/CommandWithOutput-check.cmake

@@ -0,0 +1,21 @@
+if(checkLevel EQUAL 0)
+  message("checking generation (${srcName} does not exist)")
+  if(EXISTS "${generatedFileName}")
+    set(RunCMake_TEST_FAILED "file \"${generatedFileName}\" should not exist")
+  endif()
+elseif(checkLevel EQUAL 1)
+  message("checking build 1 (generate ${srcName})")
+  if(NOT "${actual_stdout}" MATCHES "${commandComment}")
+    set(RunCMake_TEST_FAILED "command not executed")
+  endif()
+elseif(checkLevel EQUAL 2)
+  message("checking build 2 (no change in ${srcName}.in)")
+  if("${actual_stdout}" MATCHES "${commandComment}")
+    set(RunCMake_TEST_FAILED "command executed")
+  endif()
+elseif(checkLevel EQUAL 3)
+  message("checking build 3 (update ${srcName})")
+  if(NOT "${actual_stdout}" MATCHES "${commandComment}")
+    set(RunCMake_TEST_FAILED "command not executed")
+  endif()
+endif()

Tests/RunCMake/CSharpCustomCommand/CommandWithOutput.cmake

@@ -0,0 +1,13 @@
+enable_language(CSharp)
+
+add_executable(CSharpCustomCommand dummy.cs)
+
+add_custom_command(OUTPUT ${generatedFileName}
+  COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    ${inputFileName} ${generatedFileName}
+  MAIN_DEPENDENCY ${inputFileName}
+  COMMENT "${commandComment}")
+
+target_sources(CSharpCustomCommand PRIVATE
+  ${inputFileName}
+  ${generatedFileName})

Tests/RunCMake/CSharpCustomCommand/RunCMakeTest.cmake

@@ -0,0 +1,34 @@
+include(RunCMake)
+
+# Use a single build tree for a few tests without cleaning.
+set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CommandWithOutput-build)
+set(RunCMake_TEST_NO_CLEAN 1)
+file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+file(MAKE_DIRECTORY "${RunCMake_TEST_BINARY_DIR}")
+set(RunCMake-check-file CommandWithOutput-check.cmake)
+
+set(srcName "test.cs")
+set(srcFileName "${CMAKE_CURRENT_LIST_DIR}/${srcName}.in")
+set(inputFileName "${RunCMake_TEST_BINARY_DIR}/${srcName}.in")
+set(generatedFileName "${RunCMake_TEST_BINARY_DIR}/${srcName}")
+set(commandComment "Generating ${srcName}")
+
+# copy the input file to build dir to avoid changing files in cmake
+# source tree.
+file(COPY "${srcFileName}" DESTINATION "${RunCMake_TEST_BINARY_DIR}")
+
+set(RunCMake_TEST_OPTIONS ${RunCMake_TEST_OPTIONS}
+  "-DinputFileName=${inputFileName}"
+  "-DgeneratedFileName=${generatedFileName}"
+  "-DcommandComment=${commandComment}")
+
+set(checkLevel 0)
+run_cmake(CommandWithOutput)
+set(checkLevel 1)
+run_cmake_command(CommandWithOutput-build1 ${CMAKE_COMMAND} --build . --config Debug)
+set(checkLevel 2)
+run_cmake_command(CommandWithOutput-build2 ${CMAKE_COMMAND} --build . --config Debug)
+# change file content to trigger custom command with next build
+file(APPEND ${inputFileName} "\n")
+set(checkLevel 3)
+run_cmake_command(CommandWithOutput-build3 ${CMAKE_COMMAND} --build . --config Debug)

Tests/RunCMake/CSharpCustomCommand/test.cs.in

@@ -0,0 +1,8 @@
+class TestCs
+{
+  public static int Main(string[] args)
+  {
+    System.Console.WriteLine("Test C#");
+    return 0;
+  }
+}