首先，使用AddWithValue - 它使它很多更清楚：

First off, use AddWithValue - it makes it a lot clearer:

parameters.AddWithValue("@InvoiceNo", 0);
parameters.AddWithValue("@H",DropDownh.SelectedValue.ToString().Replace("'", "''"));
parameters.AddWithValue("@value", Double.Parse(txtvalue.Text.Trim()));
...

然后，不要将DateTime转换为字符串：

Then, don't convert DateTime to string:

parameters.AddWithValue("@InvoiceDatetime", DateTime.Now);

假设您的数据库列是正确的，它应该全部工作正常。

And assuming your database columns are right, it should all work ok.

我认为在你的double变量中有某种方式出现null，如果它那么它就不会转换为datetime。这是我的假设

I think there is null coming somehow in your double variable ,if it so then it will not convert into datetime. It is my assumption only