Question

When I call DataBind() on a DropDownList in ASP.NET 4, it’s throwing an ArgumentNullException:

System.ArgumentNullException: Value cannot be null. Parameter name:
container at System.Web.UI.DataBinder.GetPropertyValue(Object
container, String propName) at
System.Web.UI.DataBinder.GetPropertyValue(Object container, String
propName, String format) at
System.Web.UI.WebControls.ListControl.PerformDataBinding(IEnumerable
dataSource) at
System.Web.UI.WebControls.ListControl.OnDataBinding(EventArgs e) at
System.Web.UI.WebControls.ListControl.PerformSelect() at
System.Web.UI.WebControls.BaseDataBoundControl.DataBind() at
MyNamespace.MyClass.MyFunction()

Here is my code:

myDropdown.Items.Clear();
myDropdown.ClearSelection();
myDropdown.Items.Add(new ListItem("-Select-", "-1"));
myDropdown.DataSource = myDataSource; //List<T> of my business objects
myDropdown.DataBind();

I found this forum post that seems to suggest taking away the DataValueField and DataTextField settings on the dropdown, which I’ve set in markup. When I do that and change the second-to-last line of my code to this so I still get meaningful text in the options:

myDropdown.DataSource = myDataSource.Select(elem => new ListItem(elem.Text, elem.Id));

…it still doesn’t work. It does work, though, when I take away the line that adds the “-Select-” option. And it only breaks when there is no element selected before it’s data bound.

What’s going on?

Answer 1

Further Google searching found this other post that suggests that null objects in the datasource can cause this problem. I changed my DataSource line to this:

myDropdown.DataSource = myDataSource.Where(elem => elem != null);

And it worked.

I have yet to determine why/how I have null values in that list. Hmm.