It's hard to tell where the problem could be without knowing how the ClassBreaksRenderer was created. It might be that the Attribute of ClassBreaksRenderer does not match an OutField in your FeatureLayer or the symbol does not match the geometry type of the layer. http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#FeatureLayerRenderingThe equivalent code-behind of the XAML in this sample is as follows:
private void FeatureLayer_Initialized(object sender, System.EventArgs e)
{
FeatureLayer layer = sender as FeatureLayer;
ClassBreaksRenderer classBreaksRenderer = new ClassBreaksRenderer()
{
Attribute = "POP07_SQMI"
};
classBreaksRenderer.Classes.Add(
new ClassBreakInfo()
{
MinimumValue = 0d,
MaximumValue = 50d,
Symbol = this.LayoutRoot.Resources["LowFillSymbol"] as Symbol
}
);
classBreaksRenderer.Classes.Add(
new ClassBreakInfo()
{
MinimumValue = 50d,
MaximumValue = 200d,
Symbol = this.LayoutRoot.Resources["MediumFillSymbol"] as Symbol
}
);
classBreaksRenderer.Classes.Add(
new ClassBreakInfo()
{
MinimumValue = 200d,
MaximumValue = 5000d,
Symbol = this.LayoutRoot.Resources["HighFillSymbol"] as Symbol
}
);
layer.Renderer = classBreaksRenderer;
}