见：MySqlCommand call function

using (var connection = new sqlConnection("ConnectionString"))
  using (var command = connection.CreateCommand())
  {
    command.CommandType = CommandType.StoredProcedure;
    command.CommandText = "MyFunction";

    sqlParameter returnValue = command.Parameters.Add("@RETURN_VALUE",sqlDbType.Int);
    returnValue.Direction = ParameterDirection.ReturnValue;

    connection.Open();
    command.ExecuteNonQuery();

    return returnValue.Value;
  }