Depending source for drop down list - asp.net

Question

I have one drop down list in my pages that its source comes of below code. Now I like to put 1 text box adjusted on my drop down list and when I type on that, source of drop down list (DocumentNo) depend on what I type in the text box and when text box is null drop downs list shows all the (DocumentNo) , please help how I have to change my code,
protected void ddlProjectDocument_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
var query = from p in _DataContext.tblDocuments
orderby p.DocumentNo
select p;
int maxs = 0;
foreach (tblDocument v in query)
{
if (v.DocumentNo.Length > maxs)
maxs = v.DocumentNo.Length;
}
foreach (tblDocument vv in query)
{
string doctitle = vv.DocumentNo;
for (int i = vv.DocumentNo.Length; i < maxs; i++)
{
doctitle += " ";
}
doctitle += " | ";
doctitle += vv.TITLE;
// Use HtmlDecode to correctly show the spaces
doctitle = HttpUtility.HtmlDecode(doctitle);
ddlProjectDocument.Items.Add(new ListItem(doctitle, vv.DocId.ToString()));
}
}

Answer 1

First, I would highly recommend storing the result of that query at the beginning of the method into something like a session variable so that you don't have to continually query the database every time you hit this page.
Second, you should use the OnTextChanged event in ASP.NET to solve this problem. Put in the OnTextChanged attribute to point to a method in your code behind that will grab the query result values (now found in your session variable) and will reset what is contained in ddlProjectDocument.Items to anything that matched what was being written by using String.StartsWith():
var newListOfThings = queryResults.Where(q => q.DocumentNo.StartsWith(MyTextBox.Value));
At this point all you need to do is do that same loop that you did at the end of the method above to introduce the correct formatting.