C# 存储过程的返回日期时间

我有一个疑问：

CREATE PROCEDURE [dbo].[spGetWhenTaskLastRun]   
    @dt datetime out         
AS
BEGIN
    SELECT  
        @dt = whenTaskLastRan
    FROM
        [dbo].[t_whenTaskLastRan]   
END

SqlDataReader reader;

string ConnectionString = SafaConnectionString.ConnectionString;

SqlConnection Connection = new SqlConnection(ConnectionString);

SqlCommand cmd = new SqlCommand("spGetWhenTaskLastRun", Connection);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("@dt", SqlDbType.DateTime);
cmd.Parameters["@dt"].Direction = ParameterDirection.Output;

try
{
    Connection.Open();
    reader = cmd.ExecuteReader();

    if (reader.HasRows)
        return (DateTime)cmd.Parameters["@dt"].Value;
    else
        return DateTime.Now;
}

执行此查询的C#代码：

CREATE PROCEDURE [dbo].[spGetWhenTaskLastRun]   
    @dt datetime out         
AS
BEGIN
    SELECT  
        @dt = whenTaskLastRan
    FROM
        [dbo].[t_whenTaskLastRan]   
END

SqlDataReader reader;

string ConnectionString = SafaConnectionString.ConnectionString;

SqlConnection Connection = new SqlConnection(ConnectionString);

SqlCommand cmd = new SqlCommand("spGetWhenTaskLastRun", Connection);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("@dt", SqlDbType.DateTime);
cmd.Parameters["@dt"].Direction = ParameterDirection.Output;

try
{
    Connection.Open();
    reader = cmd.ExecuteReader();

    if (reader.HasRows)
        return (DateTime)cmd.Parameters["@dt"].Value;
    else
        return DateTime.Now;
}

但是它总是返回

reader.HasRows=false

并且没有记录

---

查询是否出错？

存储过程没有返回结果集；它只分配输出参数值。改用

ExecuteNonQuery

方法。如果没有符合条件的行或列值为

NULL

，则输出参数将为

NULL

cmd.ExecuteNonQuery();
if (cmd.Parameters["@dt"].Value == DBNull.Value)      
    return (DateTime)cmd.Parameters["@dt"].Value;
else
    return DateTime.Now;

---

存储过程未返回结果集；它只分配输出参数值。改用

ExecuteNonQuery

方法。如果没有符合条件的行或列值为

NULL

，则输出参数将为

NULL

cmd.ExecuteNonQuery();
if (cmd.Parameters["@dt"].Value == DBNull.Value)      
    return (DateTime)cmd.Parameters["@dt"].Value;
else
    return DateTime.Now;