Why the OuterHTML feature leads to an Error

Question

I have been trying to change the HTML code of a select tag in an HTML code but whenever I do that the

"Object reference not set to an instance of an object"

error occurs.This is the C# code:

public void SetDefaultValue(string ControlID,string Value)
{
    HtmlDocument doc = webBrowser1.Document;
    HtmlElement list = doc.GetElementById(ControlID);
    list.OuterHtml = Value;         
}
    private void Form1_Load(object sender, EventArgs e)
    {            
        htmlLoadWebbrowser1.SetDefaultValue("D8876943-5861-4D62-9249-C5FEF88219FA", "<SELECT id=\"D8876943-5861-4D62-9249-C5FEF88219FA\" multiple size=\"4\" name=\"drop1\"> <OPTION value=\"3\">item 3</OPTION></SELECT>");            
    }

The function is supposed to change the inner code of an HTML page,it receives the ID of the control by "controlID" and generates the inner code by "Value".

I suspect that I need to call it elsewhere,the problem is I do not know where

Thank you.

Vulpes · Answer

Hmm even if you've set the webbrowser control's Url property in the designer I suspect that the Document property will still be null at that point in time because the page hasn't loaded yet. The best thing to do would be to handle the webbrowser's DocumentCompleted event and call your method from there: http://msdn.microsoft.com/en-us/library/system.windows.forms.webbrowser.documentcompleted.aspx If you only want to call this method when a particular page is loaded you can check the WebBrowserDocumentCompletedEventArgs' Url property before calling the method.