How to create dynamic DropDownList control in Asp.net?
How to retain value across postbacks ?
How to recreate the controls on each postback ?
How to dynamically attach event handlers to DropDownList control in asp.net ?
Add a PreInit event to the Page and add the following snippet in it.
So I have created the following controls :Panel pnlDropDownList;
protected void Page_PreInit(object sender, EventArgs e)
{
//Create a Dynamic Panel
pnlDropDownList = new Panel();
pnlDropDownList.ID = "pnlDropDownList";
pnlDropDownList.BorderWidth = 1;
pnlDropDownList.Width = 300;
this.form1.Controls.Add(pnlDropDownList);
//Create a LinkDynamic Button to Add TextBoxes
LinkButton btnAddDdl = new LinkButton();
btnAddDdl.ID = "btnAddDdl";
btnAddDdl.Text = "Add DropDownList";
btnAddDdl.Click += new System.EventHandler(btnAdd_Click);
this.form1.Controls.Add(btnAddDdl);
//Recreate Controls
RecreateControls("ddlDynamic", "DropDownList");
}
* Dynamic panel pnlDropDownList and added it to the form control on the page
* Dynamic LinkButton btnAddDdl attached a Click event btnAdd_Click to it and added to the form control.
* A function called RecreateControls is being called.
On the Click event of the dynamic LinkButton I have added the following event:
protected void btnAdd_Click(object sender, EventArgs e)
{
int cnt = FindOccurence("ddlDynamic");
CreateDropDownList("ddlDynamic-" + Convert.ToString(cnt + 1));
}
As you will notice 2 function calls are made : The FindOccurence function as the name suggests gets the occurrence of the Dynamic DropDownList in the Request.Form collection. The basic idea is that I have given ID is a common pattern that is all IDs are of the form txtDynamic e.g. txtDynamic-1, txtDynamic-2 and so on.
private int FindOccurence(string substr)
{
string reqstr = Request.Form.ToString();
return ((reqstr.Length - reqstr.Replace(substr, "").Length)
/ substr.Length);
}
CreateDropDownList is used to create dynamic DropDownList. The function accepts ID as parameter .
private void CreateDropDownList(string ID)
{
DropDownList ddl = new DropDownList();
ddl.ID = ID;
ddl.Items.Add(new ListItem("--Select--", ""));
ddl.Items.Add(new ListItem("One", "1"));
ddl.Items.Add(new ListItem("Two", "2"));
ddl.Items.Add(new ListItem("Three", "3"));
ddl.AutoPostBack = true;
ddl.SelectedIndexChanged += new EventHandler(OnSelectedIndexChanged);
pnlDropDownList.Controls.Add(ddl);
Literal lt = new Literal();
lt.Text = "
";pnlDropDownList.Controls.Add(lt);
}
What I do here is that I am creating a new DropDownList and adding Items to it. Once done that I am attaching SelectedIndexChanged Event Handler and setting the AutoPostBack property to true. Finally I am adding it to the panel pnlDropDownList.
The SelectedIndexChanged Event Handler is given below.
protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
DropDownList ddl = (DropDownList)sender;
string ID = ddl.ID;
ClientScript.RegisterClientScriptBlock(this.GetType(), "Alert",
"");
//Place the functionality here
}
The most important function is RecreateControls whose job is to recreate all the DropDownList on each postback.
private void RecreateControls(string ctrlPrefix, string ctrlType)
{
string[] ctrls = Request.Form.ToString().Split('&');
int cnt = FindOccurence(ctrlPrefix);
if (cnt > 0)
{
for (int k = 1; k <= cnt; k++)
{
for (int i = 0; i <>
{
if (ctrls[i].Contains(ctrlPrefix + "-" + k.ToString())
&& !ctrls[i].Contains("EVENTTARGET"))
{
string ctrlID = ctrls[i].Split('=')[0];
if (ctrlType == "DropDownList")
{
CreateDropDownList(ctrlID);
}
break;
}
}
}
}
}
So first find the occurrence of a particular string here ddlDynamic in the Request.Form collection and then loop through each item and keep adding the DropDownList using the CreateDropDownList function described earlier.
One important thing I need to point out which people overlook is always give unique IDs to dynamic controls when you create them.
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