"Remembering" the value in a DropDownList

member4953 · April 29th, 2009, 01:50 PM

Imar,

This is one of those questions, the answer to which is likely in the book. If so, if you would kindly direct me to that spot I will figure it out.

I have a DropDownList that is populated with the current value from a field in the database. I want to "remember" that value. If the user selects another value I then display a button to update the database. If, by chance, the user selects the original value I make the button invisible.

My problem is in remembering the original value. I tried a global variable in the class for the page in the Code Behind but that did not work. It seems that the global variable doesn't persist and loses its memory of the original value upon postback. What I have done as an expedient is to put a label on the page that is invisible and put the value in there and then reference it. That seems ham handed, but it does work.

I thought that this was a state engine issue, but the state engine allows a control to remember its value after postback. I want to remember the original value for comparison.

What am I missing?

Thomas

Imar · April 29th, 2009, 03:47 PM

Hi Thomas,

Quote:

What am I missing?

Not much. It's certainly a way that works.

To maintain state you have a few other options as well:

1. Session State
Works acros the session but feels like a bit of overkill for this scenario

2. Cookies
Maintains state between browser sessions but feels like a bit of overkill as well for this scenario

3. Hidden form fields
You can add an <asp:Hidden /> control and write the value to it. Works, but feels slightly clumsy.

4. ViewState
The best option, IMO, is ViewState. You can directly write to and read from ViewState like this:

Code:

 
ViewState["SomeThing"] = "SomeValue";
string whatEver = (string) ViewState["SomeThing"];

However, it's much cleaner to wrap this in a ViewState property:

Code:

 
public string SomeValue
{
  get
  {
    object someValue = ViewState["SomeValue"];
    if (someValue != null)
      return (string)someValue;
    else
      return String.Empty;
  }
  set
  {
    ViewState["SomeValue"] = value;
  }
}

This way, you can access the property like any other, but it's value will be persisted in ViewState. Example:

Code:

if (this.SomeValue == "WhatEver")
{
 
}

You can drop the "this" reference; I just added it here to make it clear it's referring to a property of the Page.

Does this help? Let me know if you want a VB example instead.

Cheers,

Imar