Is there a faster way to get unique values for a given FeatureClass in ArcObject?

Question

I'm trying to count rows by unique values for a given FeatureClass. I'm using DataStatistics to get the unique values for a specific column for a given FeatureClass.

This seems to be (very) slow with big shapefiles (> 200k of features) - is there a faster way to determine unique values?

The second step of the process (using QueryFilter in combination with FeatureClass.FeatureCount()) seems fast enough although I would be glad if im pointed to a better solution.

public static Dictionary<string, int> CountUniquesStatistic(ITable table, string fldName)
{
    Dictionary<string, int> uniqueValuesDictionary = new Dictionary<string, int>(StringComparer.CurrentCultureIgnoreCase);

    DataStatistics dataStatistics = new DataStatistics();
    dataStatistics.Field = fldName;
    dataStatistics.Cursor = table.Search(null, false);

    IEnumerator uniqueValues = dataStatistics.UniqueValues;

    while (uniqueValues.MoveNext())
    {
        object current = uniqueValues.Current;
        uniqueValuesDictionary[current.ToString()] = 0;
    }

    foreach (string key in uniqueValuesDictionary.Keys.ToList())
    {
        IQueryFilter queryFilter = new QueryFilter();
        queryFilter.WhereClause = string.Format("{0} = '{1}'", fldName, key);

        uniqueValuesDictionary[key] = table.RowCount(queryFilter);
    }

    return uniqueValuesDictionary;
}

Answer 1

I haven't tested this code, but I'd be very interested in how it compares performance-wise with DataStatistics. Maybe use recycling=true when you call Search to for the cursor for datastatistics for a fair comparison. Cast the IFeatureclass to ITable when calling this. I'm also curious how this would compare to python.

public static Dictionary<string, int> CountUniques(ITable table, string fldName)
{
    int idx = table.Fields.FindField(fldName);
    if(idx == -1)
    {
        throw new Exception(string.Format(
            "field {0} not found in {1}",
            fldName,((IDataset)table).Name));
    }
    IQueryFilter qf = new QueryFilterClass();
    qf.AddField(fldName);

    var outDict = new Dictionary<string,int>(StringComparer.CurrentCultureIgnoreCase);
    ICursor cur = null;
    IRow row = null;
    try
    {
        cur = table.Search(qf, true);
        while ((row = cur.NextRow()) != null)
        {
            string key = row.get_Value(idx) is DBNull ? "<Null>" :
                row.get_Value(idx).ToString();
            if (!outDict.ContainsKey(key))
                outDict[key] = outDict[key] + 1;
            else
                outDict.Add(key, 1);
        }
    }
    catch(Exception ex)
    {
        string msg = row == null ? "error getting value" :
            "error getting value for row " + row.OID.ToString();
        throw new Exception(msg, ex);
    }
    finally
    {
        if(cur != null)
            System.Runtime.InteropServices.Marshal.FinalReleaseComObject(cur);
    }

    return outDict;
}