RDT_ReadLock Usage

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_VSRDTFLAGS is a flag that provides logic for locking a document in the Running Document Table (RDT), which is the list of all the documents that are currently open in the Visual Studio IDE. This flag determines when documents are opened, and whether a document is visible in the user interface or held invisibly in memory.

Generally, you would use _VSRDTFLAGS when one of the following is true:

  • When you want to open a document invisibly and read-only, but it is not yet established which IVsHierarchy should own it.

  • When you want the user to be prompted to save a document that was invisibly opened before the user displayed it in the UI and then attempted to close it.

How to Manage Visible and Invisible Documents

When a user opens a document in the UI, an IVsHierarchy owner for the document must be established and an _VSRDTFLAGS flag must be set. If no IVsHierarchy owner can be established, then the document will not be saved when the user clicks Save All or closes the IDE. This means if a document is open invisibly where it is modified in memory, and the user is prompted to save the document on shutdown or saved if Save All is chosen, then an RDT_ReadLock cannot be used. Instead, you must use an RDT_EditLock and register a IVsDocumentLockHolder when an __VSREGDOCLOCKHOLDER flag.

RDT_EditLock and Document Modification

The previous flag mentioned indicates that the invisible opening of the document will yield its RDT_EditLock when the document is opened by the user into a visible DocumentWindow. When this occurs, the user is presented with a Save prompt when the visible DocumentWindow is closed. Microsoft.VisualStudio.Package.Automation.OAProject.CodeModel implementations that use the IVsInvisibleEditorManager service initially work when only an RDT_ReadLock is taken (i.e., when the document is opened invisibly to parse information). Later, if the document must be modified, then the lock is upgraded to a weak RDT_EditLock. If the user then opens the document in a visible DocumentWindow, the CodeModel's weak RDT_EditLock is released.

If the user then closes the DocumentWindow and chooses No when prompted to save the open document, then the CodeModel implementation disposes of all information in the document and reopens the document from disk invisibly the next time more information is required for the document. The subtlety of this behavior is an instance where the user opens the DocumentWindow of the invisible open document, modifies it, closes it, and then chooses No when prompted to save the document. In this case, if the document has an RDT_ReadLock, then the document will not actually be closed and the modified document will stay open invisibly in memory, even though the user chose not to save the document.

If the invisible opening of the document uses a weak RDT_EditLock, then it yields its lock when the user opens the document visibly and no other locks are held. When the user closes the DocumentWindow and chooses No when prompted to save the document, then the document must be closed from memory. This means the invisible client must listen for RDT events to track this occurrence. The next time the document is required, the document must be reopened.