How to create weighted summaries in the WinForms GridGroupingControl?

void TableDescriptor_QueryCustomSummary(object sender, Syncfusion.Windows.Forms.Grid.Grouping.GridQueryCustomSummaryEventArgs e)
{
    //Check whether the summary type is custom or not.
    if (e.SummaryDescriptor.SummaryType == SummaryType.Custom)
    {
        //Method to calculate weighted summaries.
        e.SummaryDescriptor.CreateSummaryMethod = new CreateSummaryDelegate(Weightedsummaries.CreateSummaryMethod);
    }
    e.Handled = true;
}

private GridSummaryColumnDescriptor GetWeightedSummaryColumnDescriptor(string sourceCol, string weightCol)
{
    GridSummaryColumnDescriptor wgtSumCol = new GridSummaryColumnDescriptor();
    wgtSumCol.Name = string.Format("{0}_{1}", sourceCol, weightCol); //Special name following the convention above.
    wgtSumCol.DataMember = sourceCol; //The column this summary is applied to.
    wgtSumCol.DisplayColumn = sourceCol; //Where this summary is displayed.
    wgtSumCol.Format = "{WeightedAverage:#.##}"; //What is displayed in the summary.
    wgtSumCol.SummaryType = SummaryType.Custom; //Marks this as a CustomSummary.
    wgtSumCol.Appearance.AnySummaryCell.HorizontalAlignment = GridHorizontalAlignment.Right;
    wgtSumCol.MaxLength = 6;
    return wgtSumCol;
}

Private Sub TableDescriptor_QueryCustomSummary(ByVal sender As Object, ByVal e As Syncfusion.Windows.Forms.Grid.Grouping.GridQueryCustomSummaryEventArgs)

    'Check whether the summary type is custom or not.
    If e.SummaryDescriptor.SummaryType = SummaryType.Custom Then

        'Method to calculate weighted summaries.
        e.SummaryDescriptor.CreateSummaryMethod = New CreateSummaryDelegate(Weightedsummaries.CreateSummaryMethod)
    End If
    e.Handled = True
End Sub

Private Function GetWeightedSummaryColumnDescriptor(ByVal sourceCol As String, ByVal weightCol As String) As GridSummaryColumnDescriptor
    Dim wgtSumCol As New GridSummaryColumnDescriptor()
    wgtSumCol.Name = String.Format("{0}_{1}", sourceCol, weightCol) 'Special name following the convention above.
    wgtSumCol.DataMember = sourceCol 'The column this summary is applied to.
    wgtSumCol.DisplayColumn = sourceCol 'Where this summary is displayed.
    wgtSumCol.Format = "{WeightedAverage:#.##}" 'What is displayed in the summary.
    wgtSumCol.SummaryType = SummaryType.Custom 'Marks this as a CustomSummary.
    wgtSumCol.Appearance.AnySummaryCell.HorizontalAlignment = GridHorizontalAlignment.Right
    wgtSumCol.MaxLength = 6
    Return wgtSumCol
End Function