How to get field unique values of a ArcGIS Server dynamic map service layer field.

[SuiHuang]

Hi Everybody:

    I am making a Silverlight application using a ArcGIS Server Dynamic map service. One of the feature I want to implement is to let the application get all the unique values of a field provided by a layer of the map service.

Inputs from user:
1. the field name
2. the layer id
Output: the list of unique values of the specified field

This feature is for some layer with millions of records but only few possible values.

The performance will be poor if I use QueryTask class to get all the records and aggregate the result in the client side C# code (big data transimission through network and non-trivial computation in client side). Is there anyway to provide this feature with better performance? such as setup the map service in a certain way to allow the aggregation happen on server side?

Thank you.

[SanajyJadhav]

To fix this issue, we used WCF code.In that code, it was simple DISTINCT query that returned unique values of the concerned field to the Silverlight client.These values can then be bound to the Silverlight control.

Hope this helps.

[wangzhifang · Accepted solution]

If you have relative few records, you may want to retrieve them once and using LINQ to get the unique value on one field.
If you have millions of records and want to get its unique value, it's better to do this job on server side rather than get all of them to the client side.
On server side, you could using a GP services or a SOE operation to easily achieve your goal.

[MarkCederholm]

As of 3.0, you can use GenerateRendererTask to get a list of unique field values.

List<string> FieldList = new List<string>() { sFieldName };
ColorRamp colorRamp = new ColorRamp();
colorRamp.Algorithm = Algorithm.CIELabAlgorithm;
colorRamp.From = Colors.Black;
colorRamp.To = Colors.White;
ObservableCollection<ColorRamp> ColorRamps = new ObservableCollection<ColorRamp>() { colorRamp };
UniqueValueDefinition uvDef = new UniqueValueDefinition();
uvDef.Fields = FieldList;
uvDef.ColorRamps = ColorRamps;
GenerateRendererParameters grParam = new GenerateRendererParameters();
grParam.ClassificationDefinition = uvDef;
if (sWhereClause != null)
 grParam.Where = sWhereClause;
GenerateRendererTask grt = new GenerateRendererTask(sUrl);
grt.ExecuteCompleted += new EventHandler<GenerateRendererResultEventArgs>(grt_ExecuteCompleted);
grt.Failed += new EventHandler<TaskFailedEventArgs>(grt_Failed);
grt.ExecuteAsync(grParam, userToken);

[snip]

GenerateRendererResult grResult = e.GenerateRendererResult;
UniqueValueRenderer uvRenderer = (UniqueValueRenderer)grResult.Renderer;
List<string> UniqueValues = new List<string>();
foreach (UniqueValueInfo uvInfo in uvRenderer.Infos)
{
 if (uvInfo.Value == null)
  continue;
 UniqueValues.Add(uvInfo.Value.ToString());
}