如何使用asp.net和命令参数将空值插入Sql Server日期列

I have the following code:

我有以下代码：

SqlDateTime sqldatenull = SqlDateTime.Null; 
sSql = "INSERT INTO mydb.dbo.myTable(FromPeriod) Values (@FromPeriod)";

if (sFromPeriod.Length == 0) {
    cmd.Parameters.Add("@FromPeriod", SqlDbType.DateTime);
    cmd.Parameters["@FromPeriod"].Value = sqldatenull;
} else {
    cmd.Parameters.AddWithValue("@FromPeriod", sTempFromPeriod);
}

cmd.ExecuteNonQuery();
SqlDateT