public string WhereClause {get; set;}

{
    var us_parks = map.GetLayersAsFlattenedList()
          .OfType<FeatureLayer>().First(l => l.Name == "USNationalParks");

    //enumerate the layer's definition queries - if any
    var def_queries = us_parks.DefinitionQueries;
    foreach (var def_qry in def_queries)
    {
        var geom_uri = def_qry.GeometryUri ?? "null";
        var sr_wkid = def_qry.SpatialReference?.Wkid.ToString() ?? "null";
        var geom = def_qry.GetFilterGeometry();
        var geom_type = geom?.GeometryType.ToString() ?? "null";

        System.Diagnostics.Debug.WriteLine($" def_qry.Name: {def_qry.Name}");
        System.Diagnostics.Debug.WriteLine($" def_qry.WhereClause: {def_qry.WhereClause}");
        System.Diagnostics.Debug.WriteLine($" def_qry.GeometryUri: {geom_uri}");
        System.Diagnostics.Debug.WriteLine($" def_qry.SpatialReference: {sr_wkid}");
        System.Diagnostics.Debug.WriteLine($" def_qry.FilterGeometry: {geom_type}");
        System.Diagnostics.Debug.WriteLine("");
    }
}