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What you are describing fits more inline when using the CheckboxList control. Is there a design reason that you are using drop-down controls and list boxes to allow for multiple selections?<br />
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You can make textboxes invisible the using CSS which will not affect postback that you comment about in the attachment. To do this, add either a CssClass with a &quot;display: none;&quot; or add directly to your control a style property.
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How would you use a checkboxlist if a dropdownbox could have hundreds of items? &nbsp;I found it clogged up the web page, so was seeking an alternative.
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Please see the attached file.<br />
The program should allow users to select one or more options from the dropdown boxes. &nbsp;The selected option gets put into the listbox. &nbsp;When the user presses apply filter the items in the list box get sent to the stored procedure.<br />
To make it work I use textboxes and hidden fields with javascript.<br />
I want to make the textboxes eg txtfilenamecode invisible or at least disabled. &nbsp;However this causes the program to fail. &nbsp;<br />
Any suggestions?<br />
<a href="https://filedb.experts-exchange.com/incoming/2014/01_w02/827509/FileCollection.txt" target="_blank" class="file-inline" title="Code (47 KB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>FileCollection.txt</a>
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FileCollection.txt

Please see the attached file.
The program should allow users to select one or more options from the dropdown boxes.  The selected option gets put into the listbox.  When the user presses apply filter the items in the list box get sent to the stored procedure.
To make it work I use textboxes and hidden fields with javascript.
I want to make the textboxes eg txtfilenamecode invisible or at least disabled.  However this causes the program to fail.  
Any suggestions?

Using dropdown and listboxes to enable user selection

The .NET Framework is not specific to any one programming language; rather, it includes a library of functions that allows developers to rapidly build applications. Several supported languages include C#, VB.NET, C++ or ASP.NET.