Document lock holder management
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
The running document table (RDT) maintains a count of open documents and any edit locks they have. You can place an edit lock on a document in the RDT when it is programmatically edited in the background without the user seeing an open document in a document window. This functionality is often used by designers that modify multiple files through a graphical user interface.
Document lock holder scenarios
File "a" has a dependence on file "b"
Consider a situation where you implement a standard editor "A" for file type "a", and each file of type "a" has a reference to (or dependence on) a file of type "b". A standard editor "B" exists for files of type "b". When editor "A" opens file "a" it retrieves the reference to the corresponding file "b". File "b" is not displayed, but editor "A" can modify it. Editor "A" obtains a reference to the document data of file "b" from the FindAndLockDocument method and also maintains an edit lock on file "b". After editor "A" is done modifying file "b" you can decrement the edit lock count on file "b" by calling the UnlockDocument method. You can omit this step if you had called the FindAndLockDocument method with the parameter dwRDTLockType set to _VSRDTFLAGS.RDT_NoLock.
File "b" is opened by a different editor
In the event that the file "b" is already opened by editor "B" when editor "A" tries to open it, there are two separate scenarios to handle:
If file "b" is open in a compatible editor, you must have editor "A" register a document edit lock on file "b" using the RegisterDocumentLockHolder method. After your editor "A" is done modifying file "b", un-register the document edit lock using the UnregisterDocumentLockHolder method.
If file "b" is open in an incompatible way, you can either let the attempted opening of file "b" by editor "A" fail, or you can let the view associated with editor "A" partially open and display an appropriate error message. The error message should instruct the user to close file "b" in the incompatible editor and then re-open file "a" using editor "A". You can also implement the Visual Studio SDK method QueryCloseRunningDocument to prompt the user to close file "b" that is open in the incompatible editor. If the user closes file "b", the opening of file "a" in editor "A" continues normally.
Additional document edit lock considerations
You get different behavior if editor "A" is the only editor that has a document edit lock on file "b" than you would if editor "B" also holds a document edit lock on file "b". In Visual Studio, Class Designer is an example of a visual designer that does not hold an edit lock on the associated code file. That is, if the user has a class diagram open in design view and the associated code file open simultaneously, and if the user modifies the code file but does not save the changes, the changes are also lost to the class diagram (.cd) file. If the Class Designer has the only document edit lock on the code file, the user is not asked to save the changes when closing the code file. The IDE asks the user to save the changes only after the user closes the Class Designer. The saved changes are reflected in both files. If both the Class Designer and the code file editor held document edit locks on the code file, then the user is prompted to save when closing either the code file or the form. At that point the saved changes are reflected in both the form and the code file. For more information on class diagrams, see Work with class diagrams (Class Designer).
Note that if you need to place an edit lock on a document for a non-editor, you must implement the IVsDocumentLockHolder interface.
Many times a UI designer that modifies code files programmatically makes changes to more than one file. In such cases the SaveItemsViaDlg method handles the saving of one or more documents by means of the Do you want to save changes to the following items? dialog box.