Скачать или смотреть How to Pass Variables Between ASP.NET DropDownList Selections Using ViewState

Learn how to effectively `preserve variable values` in ASP.NET when switching between DropDownList options. This guide will help you implement ViewState to maintain data through postbacks.
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This video is based on the question https://stackoverflow.com/q/63010969/ asked by the user 'dark.vador' ( https://stackoverflow.com/u/5945518/ ) and on the answer https://stackoverflow.com/a/63035662/ provided by the user 'GRFRM' ( https://stackoverflow.com/u/10440330/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Pass Variables Between ASP.NET DropDownList Selections Using ViewState

In web development, especially with ASP.NET, you'll often encounter a challenge: maintaining state across postbacks. This is particularly evident when working with DropDownLists and button clicks. If you've ever wondered how to pass variables from one <asp:ListItem> to another after clicking a button, this guide is for you! Today, we’ll explore a common scenario and provide a straightforward solution using ViewState.

The Problem

Imagine you have a web page (Page.aspx) containing a dropdown list of items. Upon selecting an item and clicking a button, you want to preserve certain values associated with that selection. However, due to auto-refresh on postback, these variables get reset. This situation can be frustrating and may lead to the need for duplicating code if not handled correctly.

Example Scenario

Your dropdown list contains the following items:

Checkfruit

Verify

Testcheck

When you select "Verify," you set an array of values. If you later select "Testcheck," you want to use the values from the previous selection. Unfortunately, the values reset due to the nature of postbacks in ASP.NET.

The Solution: Using ViewState

To tackle this issue, we can use ViewState, which allows you to store values in a special container that persists over postbacks. Here’s how you can implement it step by step.

Step-by-Step Implementation

Declare Your Variables: Ensure you have an array to store your values, like so:

[[See Video to Reveal this Text or Code Snippet]]

Modify Your Button Click Event: Inside your button click event handler, update the logic to save and retrieve values from ViewState.

Here’s the updated code snippet for the test_Click method:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

ViewState["pte"]: This line is responsible for saving your array into the ViewState during the verify case. Now, even when the page refreshes, the values are retained.

Retrieving Values: In the testcheck case, you retrieve the values stored in ViewState. This way, you can operate on the stored data without the risk of losing it to postbacks.

Conclusion

By utilizing ViewState, you can effectively preserve variable values between ASP.NET DropDownList selections through postbacks without duplicating code. This approach not only keeps your data intact but also enhances the maintainability of your code.

If you find yourself in a situation where you need to pass values seamlessly across selections, implementing ViewState is a reliable method to achieve that.

Thanks for reading, and happy coding!