关于存储过程得传出参数的错误

--功能描述：返回当前的用户的昵称
--创建者:SunEy
--日期:2008-03-25
alter Procedure Door_NetWorkDoorPlate_GetUserInfo
@DoorID int,
@UserName nvarchar(50) output
as
begin try
 select @UserName=UserName from DB_NetWorkDoorPlate where DoorPlateID=@DoorID
return @UserName
end try
begin catch
 return @UserName

end catch
此存储过程返回值是string类型的， 

#region 获取用户的昵称
    /// <summary>
    /// 获取用户的昵称
    /// </summary>
    /// <param name="DoorID">ID</param>
    /// <returns></returns>
    public static string GetUserName(int DoorID)
    {
        try
        {
            SqlParameter[] parameters = new SqlParameter[2];
            parameters[0] = new SqlParameter("@DoorID", DoorID);
            parameters[1] = new SqlParameter("@UserName", SqlDbType.NVarChar,50);

          //Size 属性具有无效大小值: 0 如果不指定string类型的传出参数的size就会出现此错误，但是如果是int类型的就不需要，

            parameters[1].Direction = ParameterDirection.Output;

            SqlHelper.ExecuteNonQuery(SqlHelper.DoorConnectionString, CommandType.StoredProcedure, "Door_NetWorkDoorPlate_GetUserInfo", parameters);

            string Temp = null;

            Temp = parameters[1].Value.ToString();

            if (Temp != null)
            {
                return Temp;
            }
            else
            {
                return null;
            }
        }
        catch (System.Exception e)
        {
            throw e;
        }
    }

    #endregion

总结原因:传入参数可以不指定长度,直接给值,但传出的必须指定长度