This function must be called before the DrgDrag function is called.
The caller can define a default operation for the objects represented by the DRAGINFO structure by modifying the usOperation field. If the usOperation field is modified, the new value will be sent to the target as the operation whenever a DO_DEFAULT operation would normally be sent. The caller should not modify any other part of the DRAGINFO structure. The DRAGITEM structures associated with the DRAGINFO structure should only be altered with DrgSetDragitem or by using a pointer obtained with DrgQueryDragitemPtr.