使用StringBuilder和DataTable，当只有一行时，如何在不中断的情况下返回三列中的多行？

Question

I have a function that returns a list of email addresses from a SQL stored Proc based on an ID called. It is using StringBuilder and returns one column. For most IDs there are 4 or less email addresses and this format is fine. However we are now getting more IDs with 10+ email addresses and this is making the page too long.

The function is:

DataTable dt = DAL.ExecStoredProc(DAL.DatabaseName.DB, "storedProc", param);
StringBuilder sb = new StringBuilder();
sb.Append("<br/><br/>");
sb.Append("<table border='0' cellpadding='3'>");
for (int i = 0; i < dt.Rows.Count; i++)
{
    sb.Append("<tr><td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    sb.Append("</td></tr>");
}
sb.Append("</table>");
return sb.ToString();

I have tried using the following but it breaks when there are too few addresses to return:

DataTable dt = DAL.ExecStoredProc(DAL.DatabaseName.DB, "storedProc", param);
StringBuilder sb = new StringBuilder();
sb.Append("<br/><br/>");
sb.Append("<table border='0' cellpadding='3'>");
for (int i = 0; i < dt.Rows.Count; i++)
{
    sb.Append("<tr><td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    i++;
    sb.Append("</td>");
    sb.Append("<td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    i++;
    sb.Append("</td>");
    sb.Append("<td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    i++;
    sb.Append("</td></tr>");
}
sb.Append("</table>");
return sb.ToString();

Answer 1

Using Linq's Take function, you could replace the following block of code from your first example:

for (int i = 0; i < dt.Rows.Count; i++)
{
    sb.Append("<tr><td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    sb.Append("</td></tr>");
}

with this:

foreach (var row in dt.Rows.OfType<DataRow>().Take(3))
{
    sb.Append("<tr><td>");
    sb.Append(row["EMail"].ToString());
    sb.Append("</td></tr>");
}

Since Take returns up to the specified number of elements from the beginning of the sequence, this block of code will be run anywhere from 0 to 3 times. You'll have 3 addresses displayed at most (even if more are present), and you won't get an IndexOutOfRangeException if you have less than 3.

---

UPDATE: ASP.NET 2.0 Compatible

Since you can't use Linq, this should have the same result:

for (int i = 0; i < (dt.Rows.Count > 3 ? 3 : dt.Rows.Count); i++)
{
    sb.Append("<tr><td>");
    sb.Append(dt.Rows[i]["EMail"].ToString());
    sb.Append("</td></tr>");
}

The expression dt.Rows.Count > 3 ? 3 : dt.Rows.Count dt.Rows.Count > 3 ? 3 : dt.Rows.Count uses the ? operator to cause the for loop to iterate over all of the email addresses unless there are more than 3, in which case it will iterate only 3 times.

Answer 2

Instead of building a html table with a string builder have you considered using an asp.net repeater control . You can bind the DataTable directly to the repeater and then control the html from the html design surface. It would prove to be much more flexible for what you are trying to do. This is assuming you're using asp.net.

Also see my post on creating a custom asp.net control as this will give you the most flexibility as well as encapsulate your custom html logic.

Enjoy!

Answer 3

I think Doug is on the right track, but if you insist on doing it with StringBuilder and a for-loop, try this:

DataTable dt = DAL.ExecStoredProc(DAL.DatabaseName.DB, "storedProc", param);

StringBuilder sb = new StringBuilder();
sb.Append("<br/><br/>");
sb.Append("<table border='0' cellpadding='3'>");

string rowFormat = "<tr><td>{0}</td><td>{1}</td><td>{2}</td></tr>";

for (int i = 0; i < dt.Rows.Count; i+=3)
{
    string[] rowEmails = { String.Empty, String.Empty, String.Empty };

    for (int j = 0; j < 3; j++)
    {
        if (i+j < dt.Rows.Count) rowEmails[j] = dt.Rows[i+j]["Email"].ToString();
    }

    sb.AppendFormat(rowFormat, rowEmails[0], rowEmails[1], rowEmails[2]);
}

sb.Append("</table>");

return sb.ToString();