document.getElementById() returns null

Hi Friends,

My first post on the forum.

In my custom web part, I need to do "Select All" fucntionality for check boxes . i.e. checking check/unchecking a box would check/uncheck other boxes. This needs to be done on the client side. But every time I check the box , document.getElementById(checkbox.ClientID) comes out to be null.  Any help in this regard is highly appreciated. Please take a look at the code :

 

protected override void CreateChildControls()

{

 

base.CreateChildControls();

csManager =

this.Page.ClientScript;

_chkboxList =

new CheckBoxList();

_chkSelectAll =

new CheckBox();

_chkSelectAll.Text =

"Select All";

 

if (!csManager.IsClientScriptBlockRegistered(typeof(Page), "checkboxes"))

{
//even tried doing

//csManager.RegisterClientScriptBlock(typeof(Page), "checkboxes", jsCheckAll)

 

 

 

 

csManager.RegisterStartupScript(typeof(Page), "checkboxes", jsCheckAll);

}

}

 

protected override void RenderContents(HtmlTextWriter writer)

{

AddAttributesToRender(writer);

writer.Write(

"<table style='BorderStyle:Hidden'>");

 

//Header

writer.Write(

"<tr>");

writer.Write(

"<td>");

_lblHeader.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

btnGetFiles.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

_lblMsg.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

btnMerge.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

_chkboxList.RenderControl(writer);

_chkSelectAll.Attributes.Add(

"OnClick", string.Format("javascript:FinalFunctioChkBox(document.getElementById({0}),{1})", _chkboxList.ClientID, chkboxList.Items.Count));

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

_chkSelectAll.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"<tr>");

writer.Write(

"<td>");

txtMissingFiles.RenderControl(writer);

writer.Write(

"</td>");

writer.Write(

"</tr>");

writer.Write(

"</table>");

 

}

 

Thanks in advance
-Aj