Sets or returns the outline level of the column, as a value between 0 and 7. The outline level is used to control grouping in Excel. For example, to group several columns, set their outline levels to 1; to group a subset of columns within the group, set their outline levels to 2.

public int OutlineLevel{ get; set; }
Public Property OutlineLevel() As Integer
          int groupLevel = colProps.OutlineLevel;
          colProps.OutlineLevel = 4;
          Dim groupLevel As Integer = colProps.OutlineLevel
          colProps.OutlineLevel = 4