Question

I have recently found a strange behaviour inside of the ASP.NET DropDownList that I hope someone could explain.

Basically the problem I am running into is when databinding prior to postback and then setting the SelectedValue to a value that doesn’t exist within the list of dataitems the call simply has no effect. However on postback the same call will fail with a ArgumentOutOfRangeException()

‘cmbCountry’ has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

I’m using the following code.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        cmbCountry.DataSource = GetCountries();
        cmbCountry.DataBind();

        cmbCountry.SelectedValue = ""; //No effect
    }
    else
    {
        cmbCountry.SelectedValue = ""; //ArgumentOutOfRangeException is thrown
    }
}

protected List<Country> GetCountries()
{
    List<Country> result = new List<Country>();

    result.Add(new Country() { ID = Guid.NewGuid(), Description = "Test" });
    result.Add(new Country() { ID = Guid.NewGuid(), Description = "Test1" });
    result.Add(new Country() { ID = Guid.NewGuid(), Description = "Test2" });
    result.Add(new Country() { ID = Guid.NewGuid(), Description = "Test3" });

    return result;
}

public class Country
{
    public Country() { }
    public Guid ID { get; set; }
    public string Description { get; set; }
}

Could someone please clarify the cause of this behaviour for me and advise if there are any workarounds?

Answer 1

I’m not sure why it was designed this way, but the DropDownList only throws this exception on PostBack… here’s the setter code from ILSpy:

public virtual string SelectedValue
{
    get { ... }
    set
    {
        if (this.Items.Count != 0)
        {
            if (value == null || (base.DesignMode && value.Length == 0))
            {
                        this.ClearSelection();
                return;
            }
            ListItem listItem = this.Items.FindByValue(value);


/********** Checks IsPostBack here **********/
            bool flag = this.Page != null &&
                        this.Page.IsPostBack &&
                        this._stateLoaded;
            if (flag && listItem == null)
            {
                throw new ArgumentOutOfRangeException("value",
                    SR.GetString("ListControl_SelectionOutOfRange", new object[]
                        {
                            this.ID,
                            "SelectedValue"
                        }));
            }
            if (listItem != null)
            {
                this.ClearSelection();
                listItem.Selected = true;
            }
        }
        this.cachedSelectedValue = value;
    }
}

You can get around this by setting the SelectedValue to null instead of an empty string.