property Items.CellRadioGroup([Item as Variant], [ColIndex as Variant]) as Long

Retrieves or sets a value indicating the radio group where the cell is contained.

TypeDescription
Item as Variant A long expression that indicates the item's handle.
ColIndex as Variant A long expression that indicates the column's index, a string expression that indicates the column's caption or the column's key.
Long A long value that identifies the cell's radio group.

Use the CellRadioGroup property to add or remove a radio button from a group. In a radio group only one radio button can be checked. A radio cell cannot be contained by two different radio groups. Use the CellHasRadioButton property to add a radio button to a cell. When the cell's state is changed the control fires the CellStateChanged event. The CellState property specifies the cell's state. By default, when a cell of radio type is created  the radio cell is not grouped to any of existent radio groups. 

The following VB sample sets the radio type for all cells in the first column, and group all of them in the same radio group ( 1234 ):

Dim h As Variant
Gantt1.BeginUpdate
With Gantt1.Items
For Each h In Gantt1.Items
    .CellHasRadioButton(h, 0) = True
    .CellRadioGroup(h, 0) = 1234
Next
End With
Gantt1.EndUpdate

or

Private Sub Gantt1_AddItem(ByVal Item As EXGANTTLibCtl.HITEM)
    Gantt1.Items.CellHasRadioButton(Item, 0) = True
    Gantt1.Items.CellRadioGroup(Item, 0) = 1234
End Sub

To find out the radio cell that is checked in the radio group 1234 you have to call: MsgBox Gantt1.Items.CellCaption(, Gantt1.Items.CellChecked(1234)) 

The following sample group all cells of the first column into a radio group, and display the cell's checked on the radio group when the state of a radio group has been changed:

Private Sub Gantt1_AddItem(ByVal Item As EXGANTTLibCtl.HITEM)
    Gantt1.Items.CellHasRadioButton(Item, 0) = True
    Gantt1.Items.CellRadioGroup(Item, 0) = 1234 ' The 1234 is arbirary and it represents the identifier for the radio group
End Sub

Private Sub Gantt1_CellStateChanged(ByVal Item As EXGANTTLibCtl.HITEM, ByVal ColIndex As Long)
    Debug.Print "In the 1234 radio group the """ & Gantt1.Items.CellCaption(, Gantt1.Items.CellChecked(1234)) & """ is checked."
End Sub

The following VB sample assigns a radio button to the focused cell:

With Gantt1.Items
    .CellHasRadioButton(.FocusItem, 0) = True
    .CellRadioGroup(.FocusItem, 0) = 1234
End With

The following C++ sample assigns a radio button to the focused cell:

#include "Items.h"
CItems items = m_gantt.GetItems();
items.SetCellHasRadioButton( COleVariant( items.GetFocusItem() ), COleVariant( (long)0 ), TRUE );
items.SetCellRadioGroup( COleVariant( items.GetFocusItem() ), COleVariant( (long)0 ), 1234 );

The following VB.NET sample assigns a radio button to the focused cell:

With AxGantt1.Items
    .CellHasRadioButton(.FocusItem, 0) = True
    .CellRadioGroup(.FocusItem, 0) = 1234
End With

The following C# sample assigns a radio button to the focused cell:

axGantt1.Items.set_CellHasRadioButton(axGantt1.Items.FocusItem, 0, true);
axGantt1.Items.set_CellRadioGroup(axGantt1.Items.FocusItem, 0, 1234);

The following VFP sample assigns a radio button to the focused cell:

with thisform.Gantt1.Items
	.DefaultItem = .FocusItem
	.CellHasRadioButton(0,0) = .t.
	.CellRadioGroup(0,0) = 1234
endwith

Note: A cell is the intersection of an item with a column. All properties that has an Item and a ColIndex parameters are referring to a cell. The Item parameter represents the handle of an item, and the ColIndex parameter indicates an index ( a numerical value, see Column.Index property ) of a column , the column's caption ( a string value, see Column.Caption property ), or a handle to a cell. Here's few hints how to use properties with Item and ColIndex parameters:

Gantt1.Items.CellBold(, Gantt1.Items.ItemCell(Gantt1.Items(0), 0)) = True
Gantt1.Items.CellBold(Gantt1.Items(0), 0) = True
Gantt1.Items.CellBold(Gantt1.Items(0), "ColumnName") = True