IDropTarget::DragOver method (oleidl.h)

Article
11/19/2022

Provides target feedback to the user and communicates the drop's effect to the DoDragDrop function so it can communicate the effect of the drop back to the source.

Syntax

HRESULT DragOver(
  [in]      DWORD  grfKeyState,
  [in]      POINTL pt,
  [in, out] DWORD  *pdwEffect
);

Parameters

[in] grfKeyState

The current state of the keyboard modifier keys on the keyboard. Valid values can be a combination of any of the flags MK_CONTROL, MK_SHIFT, MK_ALT, MK_BUTTON, MK_LBUTTON, MK_MBUTTON, and MK_RBUTTON.

[in] pt

A POINTL structure containing the current cursor coordinates in screen coordinates.

[in, out] pdwEffect

On input, pointer to the value of the pdwEffect parameter of the DoDragDrop function. On return, must contain one of the DROPEFFECT flags, which indicates what the result of the drop operation would be.

Return value

This method returns S_OK on success. Other possible values include the following.

Return code	Description
E_UNEXPECTED	An unexpected error has occurred.
E_INVALIDARG	The pdwEffect value is not valid.
E_OUTOFMEMORY	There was insufficient memory available for this operation.

Remarks

You do not call DragOver directly. The DoDragDrop function calls this method each time the user moves the mouse across a given target window. DoDragDrop exits the loop if the drag-and-drop operation is canceled, if the user drags the mouse out of the target window, or if the drop is completed.

In implementing IDropTarget::DragOver, you must provide features similar to those in IDropTarget::DragEnter. You must determine the effect of dropping the data on the target by examining the FORMATETC defining the data object's formats and medium, along with the state of the modifier keys. The mouse position may also play a role in determining the effect of a drop. The following modifier keys affect the result of the drop.

Key Combination	User-Visible Feedback	Drop Effect
CTRL + SHIFT	=	DROPEFFECT_LINK
CTRL	+	DROPEFFECT_COPY
No keys or SHIFT	None	DROPEFFECT_MOVE

You communicate the effect of the drop back to the source through DoDragDrop in pdwEffect. The DoDragDrop function then calls IDropSource::GiveFeedback so the source application can display the appropriate visual feedback to the user.

On entry to IDropTarget::DragOver, the pdwEffect parameter must be set to the allowed effects passed to the pdwOkEffect parameter of the DoDragDrop function. The IDropTarget::DragOver method must be able to choose one of these effects or disable the drop.

Upon return, pdwEffect is set to one of the DROPEFFECT flags. This value is then passed to the pdwEffect parameter of DoDragDrop. Reasonable values are DROPEFFECT_COPY to copy the dragged data to the target, DROPEFFECT_LINK to create a link to the source data, or DROPEFFECT_MOVE to allow the dragged data to be permanently moved from the source application to the target.

You may also wish to provide appropriate visual feedback in the target window. There may be some target feedback already displayed from a previous call to IDropTarget::DragOver or from the initial IDropTarget::DragEnter. If this feedback is no longer appropriate, you should remove it.

For efficiency reasons, a data object is not passed in IDropTarget::DragOver. The data object passed in the most recent call to IDropTarget::DragEnter is available and can be used.

When IDropTarget::DragOver has completed its operation, the DoDragDrop function calls IDropSource::GiveFeedback so the source application can display the appropriate visual feedback to the user.

Notes to Implementers

This function is called frequently during the DoDragDrop loop so it makes sense to optimize your implementation of the DragOver method as much as possible.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	oleidl.h