The DataGridView.CellContextMenuStripNeeded event occurs only when the System.Windows.Forms.DataGridView control DataGridView.DataSource property is set or its DataGridView.VirtualMode property is true.
When you handle the DataGridView.CellContextMenuStripNeeded event, the shortcut menu that you specify in the handler is shown whenever the user right-clicks a cell. This is useful when you want to display a shortcut menu determined by the current state or value of a cell.
The DataGridView.CellContextMenuStripNeeded event also occurs whenever the value of the DataGridViewCell.ContextMenuStrip property is retrieved, either programmatically or when the user right-clicks a cell.
You can use the DataGridViewCellEventArgs.ColumnIndex and DataGridViewCellEventArgs.RowIndex properties to determine the state or value of a cell, and use this information to change or modify the DataGridViewCellContextMenuStripNeededEventArgs.ContextMenuStrip property. This property is initialized with the value of the cell DataGridViewCell.ContextMenuStrip property, which the event value overrides.
Handle the DataGridView.CellContextMenuStripNeeded event when working with large amounts of data to avoid the performance penalties of setting the cell DataGridViewCell.ContextMenuStrip value for multiple cells. For more information, see Best Practices for Scaling the Windows Forms DataGridView Control.
You can also specify shortcut menus for individual rows rather than individual cells by setting the row DataGridViewRow.ContextMenuStrip property or handling the DataGridView.RowContextMenuStripNeeded event. The cell DataGridViewCell.ContextMenuStrip property setting overrides the row DataGridViewRow.ContextMenuStrip property setting, and the DataGridView.CellContextMenuStripNeeded event overrides both the DataGridView.RowContextMenuStripNeeded event and the row DataGridViewRow.ContextMenuStrip property setting. However, you can specify null for a cell shortcut menu to prevent a row shortcut menu from being overridden.
For more information about handling events, see Consuming Events.