private void MainForm_DragEnter(object sender, DragEventArgs e) { string[] dataArgs = (string[])e.Data.GetData(DataFormats.FileDrop, false); string fileName = dataArgs[0]; bool isDropped = e.Data.GetDataPresent(DataFormats.FileDrop) ? true : false; if (isDropped && FileStream.CheckFileTypeIsCsv(fileName)) { e.Effect = DragDropEffects.All; } else { e.Effect = DragDropEffects.None; } }