Cross-Column Custom Aggregation under non-numeric column

[Chris]

Hi,

Could you clarify how to implement custom aggregation under any column group across multiple columns in a grid view that is bound to a collection of business objects?

I have tried to follow this sample but I get the following error:

No generic method 'OnTarget' on type 'Blah.SummaryGridCounter' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

Here are the parts of my code:
XAML:

        <telerik:RadGridView x:Name="summaryView"
                              ItemsSource="{Binding Data}">

            <telerik:RadGridView.Columns>
                <telerik:GridViewDataColumn Header="Area" DataMemberBinding="{Binding AreaName}" IsVisible="False">
                    <telerik:GridViewDataColumn.AggregateFunctions>
                        <telerik:CountFunction Caption="Total Indicators:" />
                        <local:SummaryGridCountOnTargetFunction Caption="Total On Target:"/>
                    </telerik:GridViewDataColumn.AggregateFunctions>
                </telerik:GridViewDataColumn>

etc

<telerik:RadGridView.GroupDescriptors>
    <telerik:GroupDescriptor Member="AreaName" SortDirection="Ascending"   />
</telerik:RadGridView.GroupDescriptors>

Business Object (bound to grid as List<SummaryItem> instance)

public class SummaryItem
{
    public string AreaName { get; set; }
    public string IndicatorName { get; set; }
    public List<SomeComplexType> Metrics { get; set; }
}

Custom functions (ignore the "return null" bit):

    public class SummaryGridCountOnTargetFunction : EnumerableSelectorAggregateFunction
    {
        protected override string AggregateMethodName
        {
            get
            {
                return "OnTarget";
            }
        }

        protected override Type ExtensionMethodsType
        {
            get
            {
                return typeof(SummaryGridCounter);
            }
        }

    }

    public static class SummaryGridCounter
    {
        public static object OnTarget<TSource>(IEnumerable<TSource> source, Func<TSource, object> selector)
        {
            return null;
        }
    }
}

I have read in other posts that the signature of SummaryGridCounter may need to change but I have tried various signatures including double, int, SummaryItem, string etc and have had no luck.

I'm grouping on a hidden string column, but want to display numeric aggregate data (and maybe text too with a seperate function) in the group header for the column based on the contents of the SummaryItem object to which the rows are bound.

Thanks for any help,
Chris.