# c# – Difference between RegisterStartupScript and RegisterClientScriptBlock?

## The Question :

141 people think this question is useful

Is the only difference between the RegisterStartupScript and the RegisterClientScriptBlock is that RegisterStartupScript puts the javascript before the closing </form> tag of the page and RegisterClientScriptBlock puts it right after the starting <form> tag of the page?

Also, when would you choose one over the other? I wrote up a quick sample page where I had an issue and I am not sure the exact reason of why it is happening.

### Here is the aspx markup:

<html xmlns="http://www.w3.org/1999/xhtml">
<title></title>
<body>
<form id="form1" runat="server">
<div>
<asp:Label ID="lblDisplayDate" runat="server"
Text="Label" /><br />
<asp:Button ID="btnPostback" runat="server"
Text="Register Startup Script"
onclick="btnPostback_Click" /><br />
<asp:Button ID="btnPostBack2" runat="server"
Text="Register"
onclick="btnPostBack2_Click" />
</div>
</form>
</body>
</html>



### Here is the Code Behind:

protected void Page_Load(object sender, EventArgs e)
{
lblDisplayDate.Text = DateTime.Now.ToString("T");
}

protected void btnPostback_Click(object sender, EventArgs e)
{
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append(@"<script language='javascript'>");
sb.Append(@"var lbl = document.getElementById('lblDisplayDate');");
sb.Append(@"lbl.style.color='red';");
sb.Append(@"</script>");

if(!ClientScript.IsStartupScriptRegistered("JSScript"))
{
ClientScript.RegisterStartupScript(this.GetType(),"JSScript",
sb.ToString());
}
}

protected void btnPostBack2_Click(object sender, EventArgs e)
{
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append(@"<script language='javascript'>");
sb.Append(@"var lbl = document.getElementById('lblDisplayDate');");
sb.Append(@"lbl.style.color='red';");
sb.Append(@"</script>");

if (!ClientScript.IsClientScriptBlockRegistered("JSScriptBlock"))
{
ClientScript.RegisterClientScriptBlock(this.GetType(), "JSScriptBlock",
sb.ToString());
}
}



The problem is when I click the btnPostBack button, it does a postback and changes the label to red, but when I click the btnPostBack2, it does a postback, but the label color does not change to red. Why is this? Is it because the label is not initialized?

I also read that if you are using an UpdatePanel, you need to use ScriptManager.RegisterStartupScript, but if I have a MasterPage, would I use ScriptManagerProxy?

The Question Comments :

## The Answer 1

164 people think this answer is useful

Here’s an old discussion thread where I listed the main differences and the conditions in which you should use each of these methods. I think you may find it useful to go through the discussion.

To explain the differences as relevant to your posted example:

a. When you use RegisterStartupScript, it will render your script after all the elements in the page (right before the form’s end tag). This enables the script to call or reference page elements without the possibility of it not finding them in the Page’s DOM.

Here is the rendered source of the page when you invoke the RegisterStartupScript method:

<html xmlns="http://www.w3.org/1999/xhtml">
<body>
<form name="form1" method="post" action="StartupScript.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="someViewstategibberish" />
</div>
<div> <span id="lblDisplayDate">Label</span>
<br />
<input type="submit" name="btnPostback" value="Register Startup Script" id="btnPostback" />
<br />
<input type="submit" name="btnPostBack2" value="Register" id="btnPostBack2" />
</div>
<div>
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="someViewstategibberish" />
</div>
<!-- Note this part -->
<script language='javascript'>
var lbl = document.getElementById('lblDisplayDate');
lbl.style.color = 'red';
</script>
</form>
<!-- Note this part -->
</body>
</html>



b. When you use RegisterClientScriptBlock, the script is rendered right after the Viewstate tag, but before any of the page elements. Since this is a direct script (not a function that can be called, it will immediately be executed by the browser. But the browser does not find the label in the Page’s DOM at this stage and hence you should receive an “Object not found” error.

Here is the rendered source of the page when you invoke the RegisterClientScriptBlock method:

<html xmlns="http://www.w3.org/1999/xhtml">
<body>
<form name="form1" method="post" action="StartupScript.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="someViewstategibberish" />
</div>
<script language='javascript'>
var lbl = document.getElementById('lblDisplayDate');
// Error is thrown in the next line because lbl is null.
lbl.style.color = 'green';



Therefore, to summarize, you should call the latter method if you intend to render a function definition. You can then render the call to that function using the former method (or add a client side attribute).

For instance, the following function would work:

protected void btnPostBack2_Click(object sender, EventArgs e)
{
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append("<script language='javascript'>function ChangeColor() {");
sb.Append("var lbl = document.getElementById('lblDisplayDate');");
sb.Append("lbl.style.color='green';");
sb.Append("}</script>");

//Render the function definition.
if (!ClientScript.IsClientScriptBlockRegistered("JSScriptBlock"))
{
ClientScript.RegisterClientScriptBlock(this.GetType(), "JSScriptBlock", sb.ToString());
}

//Render the function invocation.
string funcCall = "<script language='javascript'>ChangeColor();</script>";

if (!ClientScript.IsStartupScriptRegistered("JSScript"))
{
ClientScript.RegisterStartupScript(this.GetType(), "JSScript", funcCall);
}
}



## The Answer 2

6 people think this answer is useful

Here’s a simplest example from ASP.NET Community, this gave me a clear understanding on the concept….

what difference does this make?

For an example of this, here is a way to put focus on a text box on a page when the page is loaded into the browser—with Visual Basic using the RegisterStartupScript method:

Page.ClientScript.RegisterStartupScript(Me.GetType(), "Testing", _
"document.forms[0]['TextBox1'].focus();", True)



This works well because the textbox on the page is generated and placed on the page by the time the browser gets down to the bottom of the page and gets to this little bit of JavaScript.

But, if instead it was written like this (using the RegisterClientScriptBlock method):

Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), "Testing", _
"document.forms[0]['TextBox1'].focus();", True)



Focus will not get to the textbox control and a JavaScript error will be generated on the page

The reason for this is that the browser will encounter the JavaScript before the text box is on the page. Therefore, the JavaScript will not be able to find a TextBox1.