ScriptManager.RegisterStartupScript() method not working - ASP.NET, C#

My deepest condolences for OP if you still have to work with Webforms. This is how you can solve it in a minimal way while reducing traffic:

Codebehind sample:

public partial class About : Page, IPostBackEventHandler
{
    protected void Page_Init(object sender, EventArgs e)
    {
        // Unless the button is serverside clicking it will reload the page
        // registering the page like this prevents a page reload.
        var scriptManager = ScriptManager.GetCurrent(Page);
        scriptManager?.RegisterAsyncPostBackControl(Page);
    }

    /// <inheritdoc />
    public void RaisePostBackEvent(string eventArgument)
    {
        var javascriptCode = $"alert('server method called and triggered client again. {DateTime.Now.ToString("s")}');";

        // if your key isn't changed this script will only execute once
        ScriptManager.RegisterStartupScript(udpMain, typeof(UpdatePanel), Guid.NewGuid().ToString(), javascriptCode, true);

        // updating the updatepanel will inject your script without reloading anything else
        udpMain.Update();
    }
}

Webforms sample:

<script type="text/javascript">
    function clientFunction(sender, args) {

        alert('client function called');

        <%= Page.ClientScript.GetPostBackEventReference(Page, "callServerFunction") %>

        args.preventDefault();
    }
</script>

<asp:UpdatePanel runat="server" ChildrenAsTriggers="False" UpdateMode="Conditional" ID="udpMain">
    <ContentTemplate>
        <h2><%: Title %>.</h2>
        <h3>Your application description page.</h3>
        <p>Use this area to provide additional information.</p>
        <button onclick="clientFunction(); return false;">raise server function</button>
    </ContentTemplate>
</asp:UpdatePanel>

Try this:

If you Used Update Panels Then You can Use:

ScriptManager.RegisterStartupScript(this, this.GetType(), Guid.NewGuid().ToString(), "javascriptFunction();", true);

Other Wise You can Use

ClientScript.RegisterStartupScript
        (GetType(),Guid.NewGuid().ToString(), "javascriptFunction();",true);

Try this:

Client side:

Use Literal Control
<asp:Literal ID="IMsg" runat="server" Text=""/>

Server side:

string msg = "<script>alert('Change successfully');</script>";
IMsg.Text = msg;

I think it will help you. It works fine in my projects.

Try to remove line containing:

__doPostBack();

in the JavaScript code.

Sorry I am late at the party. Use this overload of RegisterStartupScript method, and set the first parameter the button on which you want to click to register the javascript function. Like:

ScriptManager.RegisterStartupScript(btnSave, this.GetType(), "alert", "alert('msg');", true);

Cheers!

THE REALLY SIMPLE WAY

You can wire up a <button runat="server"></button> to a server event using the onserverclick attribute:

HTML:

<button class="some-class" id="btnSave" onserverclick="uploadDocSave_ServerClick" runat="server">SAVE</button>

C# Handler:

protected void uploadDocSave_ServerClick(Object sender, EventArgs args)
{
    ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "alert", "alert('msg');", true);
}

WITH CLIENT SIDE VALIDATION

If you want to add some sort of JavaScript check before postback, you can do that from the onclick attribute:

HTML:

<button class="some-class" id="btnSave" onclick="return btnSave_clientClick();" onserverclick="uploadDocSave_ServerClick" runat="server">SAVE</button>

JavaScript:

function btnSave_clientClick() {

     // code

     if (some_condition) {

        return true; // allows the control to do a postback

     }
}

C# Handler:

protected void uploadDocSave_ServerClick(Object sender, EventArgs args)
{
    ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "alert", "alert('msg');", true);
}