public static Task<bool> SetSingleEditWorkspaceAsync( 
   Project project,
   MapMember mapMember
)

{
  // ApplicationOptions.EditingOptions.EnableEditingFromEditTab is true
  // ApplicationOptions.EditingOptions.IsSingleWorkspaceEditSession is true

  // find a layer
  var mm = MapView.Active.Map.GetLayersAsFlattenedList().FirstOrDefault(l => l.Name == "Roads");
  if (mm == null)
    return;

  var project = Project.Current;
  var success = await project.SetSingleEditWorkspaceAsync(mm);

  // if success = true then an edit session was started
  // and project.IsEditingEnabled will be true
}

{
  // ApplicationOptions.EditingOptions.EnableEditingFromEditTab is true
  // ApplicationOptions.EditingOptions.IsSingleWorkspaceEditSession is true

  var project = Project.Current;

  // check if already editing
  if (project.IsEditingEnabled)
  {
    // save or discard any edits
    if (project.HasEdits)
    {
      var res = MessageBox.Show("Save edits?", "Save Edits?", System.Windows.MessageBoxButton.YesNoCancel);
      if (res == System.Windows.MessageBoxResult.Cancel)
        return;
      else if (res == System.Windows.MessageBoxResult.No)
        await project.DiscardEditsAsync();
      else
        await project.SaveEditsAsync();
    }
    // close the edit session
    await project.SetIsEditingEnabledAsync(false);
  }

  // find a layer
  var mm = MapView.Active.Map.GetLayersAsFlattenedList().FirstOrDefault(l => l.Name == "Roads");
  if (mm == null)
    return;

  // start the edit session on the workspace attached to the layer
  var success = await project.SetSingleEditWorkspaceAsync(mm);

  // if success = true then an edit session was started
  // and project.IsEditingEnabled will be true
}