public string this[Guid pointId, Guid fieldId]
 {
     get
     {
         var key = new PointFieldKey {
             PointId = pointId, FieldId = fieldId
         };
         return(values.ContainsKey(key) ? values[key] : string.Empty);
     }
 }
 public LastValueBucket(IEnumerable <PointFieldValue> values)
 {
     foreach (var value in values)
     {
         var key = new PointFieldKey {
             PointId = value.PointId, FieldId = value.FieldId
         };
         this.values[key] = value.Value;
     }
 }