The field you are trying to update may be read only error while adding item to list programmatically

Question

I am adding a list item in from an application page which is redirected from web part page.

My code is :

protected void Page_Load(object sender, EventArgs e)
    {
        SPSite site = new SPSite("http://bipc11:1234/sites/webparts");
        SPWeb web = site.RootWeb;
        web.AllowUnsafeUpdates = true;
        SPUser currentUser = web.CurrentUser;
        SPList list = web.Lists["Cart"];
        SPListItem item = list.AddItem();
        item["Picture"] = Request.QueryString["title"];
        Response.Write("<script> alert('Pictrue Updated')</script>");
        item["User"] = currentUser;
        Response.Write("<script> alert('User Updated')</script>");
        item.Update();
        Response.Write("<script> alert('Item Updated')</script>");
        web.Update();
        web.AllowUnsafeUpdates = false;
        Response.Write("<script> alert('Value Added To Cart Successfully')</script>");
        Response.Redirect(Microsoft.SharePoint.Utilities.SPUrlUtility.CombineUrl(web.Url, "/SiteAssets/WebPartPage.aspx"));

    }

Here field Picture is of type look up in Cart list. User is the type of person or group. Java script to redirect to the application page is:

function myfunction(title)
     {
        window.location.assign("http://bipc11:1234/sites/webparts/_layouts/AddValueToCartList/AddValueToCartList.aspx?title="+title);
     }

When the page is redirected to the application page I am getting above mentioned error.

What am I missing? Please help.

Answer 1

I am unable to find the exact reason for the error: However pls go through the following points:

1) How to update a Readonly field.

First set Readonly to false and update the field and then revert it back as:

Field.ReadOnlyField = false;
Field.Update();

2) Updating the Lookupfield:

new SPFieldLookupValue(1, "ASP.Net");