SumFunction causing bind error when using Converter

[Brian Graves]

Here's my code:

 

GridViewDataColumn colPI = new GridViewDataColumn();

 

colPI.DataType = columnType;

colPI.Header =

"PositionCalc";

 

colPI.DataFormatString = "{0:F2}";

colPI.IsVisible =

true;

 

colPI.UniqueName =

"PositionCalc";

 

colPI.DataMemberBinding =

new Binding("PositionCalc");

 

colPI.DataMemberBinding.Converter =

new CalculatingConverter();

 

colPI.DataMemberBinding.ConverterParameter =

"Position*Impressions";

 

 

var sumFunction = new SumFunction();

 

sumFunction.FunctionName =

"SumPosition";

 

sumFunction.SourceField =

"PositionCalc";

 

sumFunction.ResultFormatString =

"{0:F2}";

 

colPI.AggregateFunctions.Add(sumFunction);

_dg.Columns.Add(colPI);

If I comment out the 5 lines of code above my Add(), I don't get errors when I assign my data to _dg.ItemSource and my grid displays correctly (with the Footer summary for this column is missing).  However, when I try to add an aggregate function and my SourceField is set this column using the Converter, I get the following error during runtime:
Invalid property or field - 'PositionCalc' for type: Data

If I set my SourceField to a different column, it works fine.  Why can't I add an aggregate function for my Converter column?  Is there a different approach I can take?

[Brian Graves]

Found a solution here:
http://www.telerik.com/community/forums/silverlight/gridview/how-can-i-mimic-excel-s-sumproduct-for-my-footer.aspx