code: def test_simple_cs(lang):
      source = """// A Hello World! program in C#.
  Console.WriteLine("Hello World!");
  """
      md = """```{lang}
  {source}
  ```
  """.format(
          lang=lang, source=source
      )
      nb = jupytext.reads(md, "md")
      assert nb.metadata["jupytext"]["main_language"] == "csharp"
      assert len(nb.cells) == 1
      assert nb.cells[0].cell_type == "code"
  
      cs = jupytext.writes(nb, "cs")
      assert source in cs
      if lang != "csharp":
          assert cs.startswith(f'// + language="{lang}"')
  
      md2 = jupytext.writes(nb, "md")
      compare(md2, md)
  
imports: import pytest
  import jupytext
  from jupytext.compare import compare
label_desc: Use image node_id 1 for calling a global function with example usage: test_simple_cs(lang) without return types