ASP Page_Load元素的值为null

Question

I have a javascript function that gets executed during page_load, its purpose is to process some client workstation values and set the value to an element. The problem is after successfully getting the client-value the element is still null in codebehind but not in ASP-UI.

note : I'm successful at getting a client workstation data thats why i need javascript for it. I just used 'new value' here for example

C#:

protected void Page_Load(object sender, EventArgs e)
{
    //GET CLIENT ADDRESS
    ScriptManager.RegisterStartupScript(this, this.GetType(), "getmac", "jsFunction();", true);
    string newValue = txt.text; //this is null
}

javascript:

function jsFunction() {
var txt = document.getElementById('<%=txt.ClientID %>');
txt.value = 'new value';
}

could please someone help me on this thanks!

Answer 1

try registering the script in Prerender phase

 protected void Page_PreRender(object sender, EventArgs e)
 {
     ScriptManager.RegisterStartupScript(this, this.GetType(), "getmac", "jsFunction();", true);
     string newValue = txt.text;
 }