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How to pass selected list value from popup into a textbox using C# webforms?

I have a table called Vendor that has two columns: VendorNum & VendorName.
Is there a way to create a list or a listbox or a listview that when a button is clicked, this vendor list will pop up. Then the selected popup list's row will pass the selected VendorNum into a textbox that's in the parent page..
I only know GridView has the select row option, but is it a proper way to put the gridview into a Listbox so the list can be scrolled up/down all within a fixed box?

You might want to take a look at the ListBox.SelectValue and/or ListBox.SelectedItem properties

Is there a textboxlist control available somewhere?

There are ASP controls such as radiobuttonlist and checkboxlist and you can databind them to a database query. It's great for creating dynamic lists with user interaction. What I'm trying to do is generate a list of textboxes in the same fashion. A list of textboxes that behave the same way.
The object is to have a checkboxlist that is generated via datasource/database. When the user is finished selecting items from this list, they click a button. That list hides (using jquery) and a new list is created based on their selections. However, the new list is now a list of their selections accompanied by an empty textbox. The user fills in the textboxes for each entry and submits again which commits it to a database.
SO:
checkbox - description
checkbox - description
checkbox - description
checkbox - description
Becomes:
Description - Textbox
Description - Textbox
The reason that I'm looking for a list-type control is so that I can ultimately loop through it for submission to the database using linq. Does that make sense? My real question is if there is a control like this yet. I gave the full description in case someone has any other ideas, short of creating a custom control.

There's nothing out of the box that does what you describe no. But you can still loop through controls. I would put your form controls inside of an asp:Panel or a div with runat="server" and use something like the following code to cycle through them as you described.
foreach(Control ctl in myPanel.Controls)
{
//check control type and handle
if (ctl is TextBox)
{
//handle the control and its value here
}
}

asp:ListBox has a property named SelectionMode, which can be set to SelectionMode="Multiple" and allows you to select items you want. This is not exactly what you need but it's a simple solution.

After the selection is made in checkboxlist, make a postback to server. Here create a datatable with two columns (description & text). For every selected item in the checkboxlist add a row to this table and bind it to a gridview control. Here 'text' column will be always empty. Configure the gridview to use template column with a textbox in the itemtemplate

Getting a value in a textbox from a dropdown list

I have a textbox named textbox1 and a dropdownlist. Dropdownlist contains the list of branches and i want that when i select a branch from dropdown i want to get the respective branch code to be generated in a textbox from the Database or from c# coding. How will it be possible ?

Am I missing something, is it more complex that just putting an event handler on the dropdown so that when it's value changes it calls your method that can do whatever it needs to do to generate the string for the textbox?

Skipping passed the "database" portion of your question (and assuming your data isn't bound to the DropDownList)..
One way in ASP.NET, to accomplish this, I believe, is to have your code print DropDownList.SelectedValue's value in the textbox during an event.
Here is a link showing how to bind an event to DropDownList in jQuery

Display ASP.NET GridView inside a selected row in another GridView

I have been given a mockup that I do not know is possible to code in ASP.NET without being a real html and javascript wizard.
I want a GridView that when a row is selected, the selected row expands and below the selected row a panel of additional information is shown, which would also include another small GridView. The idea is this would all be in-line. So if the user selected row 4, then the additional information would appear below row 4 and then after the additional information the parent GridView would continue with row 5.
Ultimately I would want to do a multi-select type of set up, but first I need to figure out if this is even possible. Also, the solution must be 508 Compliant
The one solution I considered was using only one "column". Then I would put all my fields in the ItemTemplate, and my detail panel content in the EditItemTemplate and instead of selecting the row, set it to edit mode. The problem with this solution is I lose the functionality of multiple columns if I throw everything in one huge ItemTemplate.
Any and all suggestions or ideas are appreciated.

What you're describing would be best fulfilled with a ListView control. It takes a little more templating work to set up than a grid view, but you have much more control over it and you can emulate the look of a GridView. You would set your Selected Item template to contain another ListView (or GridView) thats bound to your detailed data.

I've done this using a gridview inside a ListView and using ajaxcontroltoolkit collapsible panel
Your parent list would be a listview that would have 2 table rows for each item, on the first row your parent column, on the second row use colspan and add a gridview wrapped on a collapsible panel

How to add a Textbox to Dynamic GridView

How do I add Textbox to a dynamic Gridview. Do I need to have a customised Templete Field ? Is there any other way ??

Do you mean you want to dynamically generate rows with TextBoxes in a GridView? If so, then there is a good explanation here:
http://geekswithblogs.net/dotNETvinz/archive/2009/06/04/adding-dynamic-rows-in-gridview-with-textboxes.aspx
Here's one if you're just looking to add a control to a single cell. It's a DropDownList, but could be modified to add a TextBox:
http://www.devasp.net/net/articles/display/708.html