您可以使用HitTest来获取鼠标所在的单元格。(它不依赖于MouseDown-Event,您只需要相对于DataGrid的鼠标位置)
例子:
Private Sub dg_MouseDown(sender As Object, e As MouseEventArgs) Handles dg.MouseDown
Dim htinfo As HitTestInfo = dg.HitTest(new Point(e.X, e.Y))
If htinfo.Type = HitTestType.Cell Then
Dim clickedCell As DataGridCell = dg.Item(htinfo.Row, htinfo.Column)
End If
End Sub
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