Form field validation for complex fields. Parse query in field's parameters field value. Query then executed for query results, representing data source of a drop down list field.

gistfile1.cs

private static bool IsValueInFieldDatasource(IEnumerable<ControlResult> fields, string fieldName, string value)
{
	Sitecore.Diagnostics.Assert.IsNotNullOrEmpty(fieldName, "fieldName cannot be null");
	if (string.IsNullOrEmpty(value)) return false;

	bool isValidSelection = false;

	var field = fields.FirstOrDefault(x => string.Compare(x.FieldName, fieldName, StringComparison.OrdinalIgnoreCase) == 0);
  	if (field != null)
	{
		var parameters = Sitecore.Context.Database.Items[field.FieldID].Fields["Parameters"];
		if (parameters.HasValue)
		{
            		IEnumerable<Pair<string, string>> p = ParametersUtil.XmlToPairArray(parameters.Value, true);
			var query = p.FirstOrDefault();
				    
            		bool isQueryValid = query != null && !string.IsNullOrEmpty(query.Part2);
            		if (isQueryValid)
            		{
                		var querySettings = QuerySettings.ParseRange(query.Part2);
                		var dataSource = QueryManager.Select(querySettings);

                		isValidSelection = dataSource.Get(value) != null;
            		}
		}
	}

	return isValidSelection;
}