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PROJECT_IS_TOP_LEVEL
--------------------
.. versionadded:: 3.21
A boolean variable indicating whether the most recently called
:command:`project` command in the current scope or above was in the top
level ``CMakeLists.txt`` file.
Some modules should only be included as part of the top level
``CMakeLists.txt`` file to not cause unintended side effects in the build
tree, and this variable can be used to conditionally execute such code. For
example, consider the :module:`CTest` module, which creates targets and
options:
.. code-block:: cmake
project(MyProject)
...
if(PROJECT_IS_TOP_LEVEL)
include(CTest)
endif()
The variable value will be true in:
* the top-level directory of the project
* the top-level directory of an external project added by
:module:`ExternalProject`
* a directory added by :command:`add_subdirectory` that does not also contain
a :command:`project` call
* a directory added by :command:`FetchContent_MakeAvailable`,
if the fetched content does not contain a :command:`project` call
The variable value will be false in:
* a directory added by :command:`add_subdirectory` that also contains
a :command:`project` call
* a directory added by :command:`FetchContent_MakeAvailable`,
if the fetched content contains a :command:`project` call
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