Issue with VB code behind not executing upon button click triggered by JavaScript

A scenario arises when a user is provided with a TDF text file in a report, requiring them to select two filters and fill out two fields before being able to submit. Upon clicking the "download" button without completing these requirements, a friendly error message prompts the user to fill out the necessary fields before proceeding. However, the testing department desires this error message to disappear once the file has been successfully downloaded. The page's response object prevents it from posting back, causing the error message to persist even after the download is complete.

In an attempt to resolve this issue, an additional hidden button has been included, linked to the code responsible for generating the TDF file. A Javascript click event has been attached to the visible button to trigger this hidden button.

 <asp:Button ID="btnSubmit" runat="server" Text="" Style="visibility:hidden;" /><asp:Button ID="btnDownload" runat="server" Text="Download TDF" OnClientClick="document.getElementById('ctl00_MainContent_btnSubmit').click();" />

Despite proper setup and event registration, the backend code fails to execute upon clicking the hidden button. Even though alerts indicate that the button click registers, the expected actions do not take place When attempting to isolate the issue by commenting out TDF generation and downloading, substituting it with a test error message, no changes are observed.

The question remains: why does the code behind fail to run as intended?

javascriptasp.netvb.net

Answer №1

Upon clicking the btnDownload button, I have observed that two postbacks are initiated simultaneously, leading to a competition between them:

  1. The postback triggered by the btnDownload button itself
  2. The postback caused by the btnSubmit.click() function in the OnClientClick event

After testing your code, it seems that the outcome is somewhat unpredictable: at times, the event handler btnSubmit_Click is executed, while other times it is not.

If there was a way to identify in client-side script when the error message becomes irrelevant, it could be cleared directly within the OnClientClick function before the postback occurs:

<asp:Button ID="btnDownload" runat="server" Text="Download TDF" OnClientClick="clearErrorMessage();" />

function clearErrorMessage() {
    if (/* All required fields are filled */) {
        // Clear the error message
    }
}

Alternatively, a two-step approach might be necessary within the event handler of btnDownload in the code-behind: the initial response would involve clearing the error message, followed by adding a startup client script to enforce a second postback. However, this method may not provide an optimal user experience.

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