gistfile1.cs

var existingSetting = _settings.Where(s => s.SettingKey == setting.SettingKey).FirstOrDefault();
if (existingSetting == null)
	throw new ArgumentException("Attempting to update Setting with name '{0}' but the setting is not cached and cannot be updated.");
existingSetting.Value = setting.Value;

var parameters = new UriOperationParameter[] { 
	new UriOperationParameter("userId", _userSession.User.UserID ), 
	new UriOperationParameter("serializedSetting", _serializationService.SerializeAsString<Setting>(existingSetting)) };
var context = _dataContextFactory.Create(MergeOption.AppendOnly);
// Perform WCF Data Services ServiceOperation call.
context.Execute<string>(new Uri("UpdateSetting", UriKind.Relative), "POST", true, parameters);