HtmlGenericControl gives me an error "Object reference not set to an instance of an object"

Question

I am trying to change the css of the content page which inherits its css from the master page. I tried the below but for some reason I am getting the "Object reference not set to an instance of an object" error.

Below is my cs code:

 protected void Page_Load(object sender, EventArgs e)
 {
    HtmlGenericControl mycontrol = (HtmlGenericControl)this.Page.Master.FindControl("ul1").FindControl("li1") as HtmlGenericControl;

      mycontrol.Attributes.Add("class", "newCSS");    
 }

Find the masterpage content here:

    <div id="menu">
        <ul id="ul1" runat="server">
            <li id="li1" runat="server">
                <asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl ="home.aspx">Home</asp:HyperLink></li>
            <li id="li2" runat="server">
                <asp:HyperLink ID="HyperLink2" runat="server" NavigateUrl ="AboutUs.aspx">About Us</asp:HyperLink></li>
            <li id="li3" runat="server">
                <asp:HyperLink ID="HyperLink3" runat="server" NavigateUrl="~/courses.aspx" >Courses</asp:HyperLink></li>
            <li id="li4" runat="server">
                <asp:HyperLink ID="HyperLink4" runat="server" NavigateUrl="~/News.aspx" >News</asp:HyperLink></li>
            <li id="li5" runat="server">
                <asp:HyperLink ID="HyperLink5" runat="server" NavigateUrl="ContactInfo.aspx" >Contact Us</asp:HyperLink></li>
        </ul>
    </div>

The css as below:

 #menu a:hover
{
    background-color: white;
    color: #bee2f1;
}

 .newCSS
{
    background-color: black;
    color: #fff;

}

I tried giving the .FindControl ID as the HyperLink ID instead of the ul, li Id as shown above but that did not work either.

Edit: Tried the following in the cs page:

protected void Page_Load(object sender, EventArgs e)
{
        HtmlGenericControl mycontrol = (HtmlGenericControl)Master.FindControl("ul1").FindControl("li1"); 
        mycontrol.Attributes.Add("class", "newCSS");    
}

No compiler error but the code does not function.

Have you tried doing the following WebControl mycontrol = (WebControl)Page.FindControl("ul1"); — MethodMan, Dec 28 '12 at 19:18
have you tried moving the runat="server" up as Thousand has suggested — MethodMan, Dec 28 '12 at 19:24
@DJ Yes I did that. Let me just edit it in the question. It was a simple mistake I made while playing around for a solution before posting the question. — A_AR, Dec 28 '12 at 19:26
HtmlGenericControl myControl = (HtmlGenericControl)Master.FindControl("ul1"); should do the trick — MethodMan, Dec 28 '12 at 19:27
Almost all cases of NullReferenceException are the same. Please see "[What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net)" for some hints. — John Saunders, Dec 28 '12 at 19:27
@DJ Ok no errors this time but the css of the menu did not change. The code did not function. — A_AR, Dec 28 '12 at 19:29
update your code with the now working solution at the top of your original question — MethodMan, Dec 28 '12 at 19:31
how come you have this runat="server" on ui1 and not the rest.. be consistent — MethodMan, Dec 28 '12 at 19:48
@John Wonderful post and I now know that the problem is with finding the ul and li ID values. I tried troubleshooting by adding the "if" clause as well for a possible null value but that did not yield either. Any help regarding to this question is most appreciated. — A_AR, Dec 28 '12 at 19:48
@DJ KRAZE Since I was only concentrating on the home page, I did not bother changing the other ones. But now that I have done, there was no change in the outcome :( — A_AR, Dec 28 '12 at 19:52
No need to nest your `FindControl()` calls. Simply look for the `
@andleer I tried that before posting the question here, andleer. I do not get an error but the code does not work. I am puzzled and clueless. — A_AR, Dec 28 '12 at 20:02

score 0 · Answer 1 · answered Dec 28 '12 at 20:03

There is a recursive function I use for situations like this. When you need to find/edit content on the master page from a content page, it's always tricky. This function helped me out a lot. In the example, I am using it to find the return button of my Menu on my master page called Menu2 by passing it the ID of the button.

*Method

public static Control FindControlRecursive(Control root, string id)
    {
        if (id == string.Empty)
            return null;

        if (root.ID == id)
            return root;

        foreach (Control c in root.Controls)
        {
            Control t = FindControlRecursive(c, id);
            if (t != null)
            {
                return t;
            }
        }
        return null;
    }

*Usage

Menu menu2 = (Menu)FindControlRecursive(Master.Page, "ReturnButton");

Using this method, I can now access the Menu of my master page from the content page.

I hope this helps!

Ok I understood the concept. Thank you. I need to learn more about this as I am very new to c#. I will study, solve this question and post the entire answer here. — A_AR, Dec 28 '12 at 20:19

HtmlGenericControl gives me an error "Object reference not set to an instance of an object"

1 Answers1