[c#] How to call window.alert("message"); from C#?

I have my own exception based on some condition and want to raise an alert when control comes in this catch block

catch (ApplicationException ex)
{
    //want to call window.alert function here
}

It's a bit hard to give a definitive answer without a bit more information, but one usual way is to register a startup script:

try
{
  ...
}
catch(ApplicationException ex){
  Page.ClientScript.RegisterStartupScript(this.GetType(),"ErrorAlert","alert('Some text here - maybe ex.Message');",true);
}

You can try this:

Hope it works for you..

`private void validateUserEntry()
{
    // Checks the value of the text.
    if(serverName.Text.Length == 0)
    {
    // Initializes the variables to pass to the MessageBox.Show method.
    string message = "You did not enter a server name. Cancel this operation?";
    string caption = "Error Detected in Input";
    MessageBoxButtons buttons = MessageBoxButtons.YesNo;
    DialogResult result;

    // Displays the MessageBox.
    result = MessageBox.Show(message, caption, buttons);
    if (result == System.Windows.Forms.DialogResult.Yes)
    {
     // Closes the parent form.
        this.Close();
    }
    }
}` 

You can also do this :

 catch (Exception ex)
    {

        ScriptManager.RegisterStartupScript(Page, Page.GetType(), "showError",
           "alert('" + ex.Message + "');", true);

    }

this will show the exeption message in the alert box

if you are using ajax in your page that require script manager Page.ClientScript
will not work, Try this and it would do the work:

ScriptManager.RegisterClientScriptBlock(this, GetType(),
            "alertMessage", @"alert('your Message ')", true);

I'm not sure if I understand but I'm guessing that you're trying to show a MessageBox from ASP.Net?

If so, this code project article might be helpful: Simple MessageBox functionality in ASP.NET

You can use the following extension method from any web page or nested user control:

static class Extensions
{
    public static void ShowAlert(this Control control, string message)
    {
        if (!control.Page.ClientScript.IsClientScriptBlockRegistered("PopupScript"))
        {
            var script = String.Format("<script type='text/javascript' language='javascript'>alert('{0}')</script>", message);
            control.Page.ClientScript.RegisterClientScriptBlock(control.Page.GetType(), "PopupScript", script);
        }
    }
}

like this:

class YourPage : Page
{
    private void YourMethod()
    {
        try
        {
            // do stuff
        }
        catch(Exception ex)
        {
            this.ShowAlert(ex.Message);
        }
    }
}

Simple use this to show the alert message box in code behind.

ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('Record Saved Sucessfully');", true);

Do you mean, a message box?

MessageBox.Show("Error Message", "Error Title", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);

More information here: http://msdn.microsoft.com/en-us/library/system.windows.forms.messagebox(v=VS.100).aspx

MessageBox like others said, or RegisterClientScriptBlock if you want something more arbitrary, but your use case is extremely dubious. Merely displaying exceptions is not something you want to do in production code - you don't want to expose that detail publicly and you do want to record it with proper logging privately.

You should try this.

ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('Sakla Test');", true);