Which value do I have to return in COleDropTarget::OnDrop() when handling multiple files?

Question

I made my MFC application a drop target by deriving the class CDropTarget from COleDropTarget and overriding all necessary functions. Everything works as expected. However, the return value of OnDrop() confuses me. Its description reads:

Nonzero if the drop is successful; otherwise 0.

I don't understand what "successful" means if multiple files are dropped on my application. For example, consider the following implementation:

BOOL CDropTarget::OnDrop(CWnd* pWnd, COleDataObject* pDataObj, DROPEFFECT tDropEffect, CPoint tPoint)
{
    // I left out declaration/definition of hDrop and path for reasons of clarity.
    [...]

    UINT numHandledFiles = 0;

    // Determine the number of dropped files.
    UINT numDroppedFiles = DragQueryFile(hDrop, 0xFFFFFFFF, NULL, 0);

    // Iterate over all dropped files.
    for (UINT n = 0; n < numDroppedFiles; n++)
    {
        // Get the path of the current file from the HDROP structure.
        if (DragQueryFile(hDrop, n, path, PATH_MAX) > 0)
        {
            // Try to handle each dropped file in my function handleFile().
            // It returns true if a file could be handled and false otherwise.
            // (The latter happens if a file with the wrong type was dropped.)
            if (handleFile(path))
                numHandledFiles++;
        }
    }

    return ?  // See description below.
}

Now assume that my function handleFile() can only handle .png files and that multiple files with different file types are dropped on my application at once.

How do I replace return ? in the above code correctly? I see two options:

return numHandledFiles > 0;                 // At least one file could be handled.

And:

return numHandledFiles == numDroppedFiles;  // All files could be handled.

I tried both, but when dropping files from Windows Explorer or Total Commander on my application, I don't notice any difference at all. What effect does the return value have?

Answer 1

When reading MFC documentation leaves you puzzled you should turn to the Windows SDK documentation, as recommended in the link you provided: "For more information, see IDropTarget::Drop in the Windows SDK.":

On return, must contain one of the DROPEFFECT flags, which indicates what the result of the drop operation would be.

Note that IDropTarget::Drop more closely resembles COleDropTarget::OnDropEx, which you should be implementing instead of COleDropTarget::OnDrop. There is no strict rule for the case you describe. However, the DROPEFFECT should match application behavior (i.e. accept or reject).