This message occurs when the user double-clicks the middle mouse button.
| Parameter | Description |
| wParam | Contains a value that indicates whether various virtual keys are down. It can be any combination of the following values: | ||
| Value | Meaning | ||
| MK_CONTROL | Set if CONTROL key is down. | ||
| MK_LBUTTON | Set if left button is down. | ||
| MK_MBUTTON | Set if middle button is down. | ||
| MK_RBUTTON | Set if right button is down. | ||
| MK_SHIFT | Set if SHIFT key is down. | ||
| lParam | Contains the x- and y-coordinates of the cursor. The x-coordinate is in the low-order word; the y-coordinate is in the high-order word. These coordinates are always relative to the upper-left corner of the window. | ||
Only windows whose window class has CS_DBLCLKS style can receive double-click messages. Windows generates a double-click message when the user presses, releases, and then presses a mouse button again within the system's double-click time limit. Double-clicking actually generates four messages: a down-click message, an up-click message, the double-click message, and another up-click message.