ExcelLineBaseOutlineLevel Property

public int OutlineLevel { get; set; }

Public Property OutlineLevel As Integer
	Get
	Set

Property Value

Allows you to create hierarchical groups. A number of consecutive objects (rows or columns) with the same value of this parameter belong to the same group. The default value is zero, grouping isn't performed.

If you want outline group to be collapsed, you need to set three properties:

OutlineLevel property needs to be set to correct outline level.

All rows in the collapsed group Hidden property needs to be set to true.

The row where collapsed (plus) sign is located Collapsed property needs to be set to true.

Reference