Lost Focus method for asp.net textbox?

So I know everyone has shown the basic client side approach, and that is fine, but I wanted to at least show a solution for handling a specific client side event on the server.

Lets take a look at the code, and go over it piece by piece.

Since ASP.Net TextBox does not expose a server side event for OnBlur, you will have to do it manually. Fortunately this is pretty easy to achieve. Suppose you have this small bit of code in your .aspx page. You want to update a Label control server side whenever the TextBox loses focus.

<asp:Label ID="lblOnBlur" runat="server">On Blur Example</asp:Label><br />
<asp:TextBox ID="tbOnBlur" runat="server" ClientIDMode="Static" /><br />
<asp:Label ID="lblOutput" runat="server" />

ASP.Net has a built in client side function that gets called to trigger postbacks that takes two parameters:

1. Target (the ID of the control causing the event)
2. Argument (optional information you would like to pass to the server)

You could just wireup the event in markup by adding the following attribute and value to your TextBox:

onblur="__doPostBack('tbOnBlur','OnBlur');"

However, the framework has an easy way to generate this script for you server side. In your Page_Init method, simply add a call to GetPostBackEventReference and assign it to the “onblur” attribute for you control like so:

protected void Page_Init(object sender, EventArgs e)
{
    var onBlurScript = Page.ClientScript.GetPostBackEventReference(tbOnBlur, "OnBlur");
    tbOnBlur.Attributes.Add("onblur", onBlurScript);
}

With standard server control events, the event wireup and invocation is handled automagically for you by implementing IPostBackEventHandler. That is a lot of work for a one-off solution, so lets just handle it manually by inspecting the request params.

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        var ctrlName = Request.Params[Page.postEventSourceID];
        var args = Request.Params[Page.postEventArgumentID];

        HandleCustomPostbackEvent(ctrlName, args);
    }
}

private void HandleCustomPostbackEvent(string ctrlName, string args)
{
    //Since this will get called for every postback, we only
    // want to handle a specific combination of control
    // and argument.
    if (ctrlName == tbOnBlur.UniqueID && args == "OnBlur")
    {
        lblOutput.Text = "On Blur Event Handled Server Side!" + DateTime.Now;
    }
}

In the end it isn’t terribly difficult to simulate server side events if you don’t mind digging into the framework a little.

Hope this helps!

Cheers,
Josh