How to retrieve the per-feature value rendered when IGeoFeatureLayer.DisplayAnnotation is true (ArcObjects with C#)?

Question

In another area of my application I'd like to display the per-feature label value rendered when Labeling is turned on in ArcMap, i.e. when IGeoFeatureLayer.DisplayAnnotation becomes true.

I have a list of items of type IFeature from say myFeatLayer. I am able to retrieve the layer-level labeling properties, including text formatting, with:

IGeoFeatureLayer geoLayer = (IGeoFeatureLayer)myFeatLayer;
IAnnotateLayerPropertiesCollection anno = pGeoFeatureLayer.AnnotationProperties;
IElementCollection iecp;
IElementCollection iecup;
IAnnotateLayerProperties props;
anno.QueryItem(0, out props, out iecp, out iecup);
ILabelEngineLayerProperties leprops = props as ILabelEngineLayerProperties;
ITextSymbol text = leprops.Symbol;

What I don't know is necessarily how to query an IFeature, and with what (leprops.Expression?), to retrieve the actual value of what will be rendered near each feature by the labeling engine.

Thanks in advance.

Answer 1

The basic workflow for doing this is:

1. Get the AnnotationExpressionEngine from ILabelEngineLayerProperties via get_ExpressionParser
2. Set the label expression via SetExpression for simple expressions of SetCode for complex expressions on the parser. This will hand back the actual parser. Simplicity of expression is determined by ILabelEngineLayerProperties::get_IsExpressionSimple.
3. Use the FindLabel method on the parser to get the label text.

Answer 2

This code is a follow-up on Craig's post. It is not real clear to me where the preCode comes from. This should probably be a comment, but we can't put code in a comment.

public static void ListLabels(IGeoFeatureLayer gfLayer)
{
    var parsers = GetParsers(gfLayer);

    IFeature feat;
    var fCur = gfLayer.FeatureClass.Search(null, false);
    while ((feat = fCur.NextFeature()) != null)
    {
        foreach (KeyValuePair<string,IAnnotationExpressionParser> kvp in parsers)
        {
            string label = kvp.Value.FindLabel(feat);
            Debug.Print("class: {0} oid: {1} label: {2}",kvp.Key, feat.OID, label);
        }
    }
    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(fCur);
}

public static Dictionary<string,IAnnotationExpressionParser> GetParsers(IGeoFeatureLayer gfLayer)
{
    var parsers = new Dictionary<string,IAnnotationExpressionParser>();
    for (int i = 0; i < gfLayer.AnnotationProperties.Count; i++)
    {
        IElementCollection placedElements;
        IElementCollection unplacedElements;
        IAnnotateLayerProperties alProps;

        gfLayer.AnnotationProperties.QueryItem(
            i, out alProps, out placedElements, out unplacedElements);
        var leProps = alProps as ILabelEngineLayerProperties2;
        string preCode = ""; // where to get this?
        var aeParser = leProps.ExpressionParser.SetExpression(preCode, leProps.Expression);
        parsers.Add(alProps.Class, aeParser);
    }
    return parsers;
}