Occurs after the measuring the cells, so that the user can combine adjacent cells. In order to merge the cells you should call the method Merge of the DataGridMergingCellsEventArgs passed in.

'Declaration
 
Public Event MergingCells As System.EventHandler(Of DataGridMergingCellsEventArgs)

public event System.EventHandler<DataGridMergingCellsEventArgs> MergingCells