Grouping list before added to the gridview

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C#

WinForms

Gridview

DevExpress

I have this code that added items to list

private void GetParts(bool AllObjects)
{
    ModelObjectEnumerator ImportModel = null;
    List<Beam> parts = new List<Beam>();
    if (AllObjects)
    {
        ImportModel = MyModel.GetModelObjectSelector().GetAllObjectsWithType(ModelObject.ModelObjectEnum.BEAM);
    }
    else
    {
        TSMUI.ModelObjectSelector GetSelectedObjects = new TSMUI.ModelObjectSelector();
        ImportModel = GetSelectedObjects.GetSelectedObjects();
    }

    while (ImportModel.MoveNext())
    {
        Beam ThisBeam = ImportModel.Current as Beam;

        if (ThisBeam != null)
        {
            parts.Add(ThisBeam);

        }
    }

After that I'm supposed to iterate through all items to added to the GridView like this

for (int i = 0; i < parts.Count; i++)
{ 
        DataRow row = dt.NewRow();

        var partMark = string.Empty;
        parts[i].GetReportProperty("PART_POS", ref partMark);
        row[0] = partMark;
        row[1] = parts[i].Profile.ProfileString;
        int num = 0;
        row[2] = parts[i].GetReportProperty("MODEL_TOTAL", ref num);
        double length = 0;
        parts[i].GetReportProperty("LENGTH", ref length);
        row[3] = length;
        dt.Rows.Add(row);
}

but before I do that I want to group items using (partMark ,length ) and Sum(num ).

How can I do that?Thanks in advance.

What I have tried:

I try to use GroupBy technique but unfortunately I did not succeed