VS: Add DOTNET_SDK property to generate SDK-style C# projects

Changes in cmVisualStudio10TargetGenerator::Generate to write .Net
SDK-style project for VS generators VS 19 and above. Also adds
documentation and tests.

Issue: #20227

Help/manual/cmake-properties.7.rst

@@ -191,6 +191,7 @@ Properties on Targets
    /prop_tgt/DEPLOYMENT_REMOTE_DIRECTORY
    /prop_tgt/DEPRECATION
    /prop_tgt/DISABLE_PRECOMPILE_HEADERS
+   /prop_tgt/DOTNET_SDK
    /prop_tgt/DOTNET_TARGET_FRAMEWORK
    /prop_tgt/DOTNET_TARGET_FRAMEWORK_VERSION
    /prop_tgt/EchoString

Help/manual/cmake-variables.7.rst

@@ -49,6 +49,7 @@ Variables that Provide Information
    /variable/CMAKE_DEBUG_TARGET_PROPERTIES
    /variable/CMAKE_DIRECTORY_LABELS
    /variable/CMAKE_DL_LIBS
+   /variable/CMAKE_DOTNET_SDK
    /variable/CMAKE_DOTNET_TARGET_FRAMEWORK
    /variable/CMAKE_DOTNET_TARGET_FRAMEWORK_VERSION
    /variable/CMAKE_EDIT_COMMAND

Help/prop_tgt/DOTNET_SDK.rst

@@ -0,0 +1,25 @@
+DOTNET_SDK
+----------
+
+.. versionadded:: 3.23
+
+Specify the .NET SDK for C# projects.  For example: ``Microsoft.NET.Sdk``.
+
+This property tells :ref:`Visual Studio Generators` for VS 2019 and
+above to generate a .NET SDK-style project using the specified SDK.
+The property is meaningful only to these generators, and only in C#
+targets.  It is ignored for C++ projects, even if they are managed
+(e.g. using :prop_tgt:`COMMON_LANGUAGE_RUNTIME`).
+
+This property must be a non-empty string to generate .NET SDK-style projects.
+CMake does not perform any validations for the value of the property.
+
+This property may be initialized for all targets using the
+:variable:`CMAKE_DOTNET_SDK` variable.
+
+.. note::
+
+  The :ref:`Visual Studio Generators` in this version of CMake have not
+  yet learned to support :command:`add_custom_command` in .NET SDK-style
+  projects.  It is currently an error to attach a custom command to a
+  target with the ``DOTNET_SDK`` property set.

Help/release/dev/vs-csharp-dotnet-sdk.rst

@@ -0,0 +1,9 @@
+vs-csharp-dotnet-sdk
+--------------------
+
+* The :ref:`Visual Studio Generators` for VS 2019 and above learned to
+  support .NET SDK-style project files (``.csproj``) for C# projects.
+  See the :prop_tgt:`DOTNET_SDK` target property and corresponding
+  :variable:`CMAKE_DOTNET_SDK` variable.
+  However, this version of CMake does not yet support using
+  :command:`add_custom_command` in .NET SDK-style projects.

Help/variable/CMAKE_DOTNET_SDK.rst

@@ -0,0 +1,9 @@
+CMAKE_DOTNET_SDK
+----------------
+
+.. versionadded:: 3.23
+
+Default value for :prop_tgt:`DOTNET_SDK` property of targets.
+
+This variable is used to initialize the :prop_tgt:`DOTNET_SDK`
+property on all targets. See that target property for additional information.