ASP dropdownlist missing value error

'DropDownList' has a SelectedValue which is invalid because it does not exist in the list of items.

This error is caused when the value is not in currently in the list items.

A solution:

In your ASPX page:

<asp:DropDownList ID="yourdropdownlist" runat="server" DataSourceID="SqlDataSource1"
   DataTextField="textfield" DataValueField="valuefield" AppendDataBoundItems="true"
   SelectedIndex='<%# GetSelectedIndex("yourdropdownlist",Eval("somevalue").ToString()) %>'
   SelectedValue='<%# Bind("somevalue") %>'>
</asp:DropDownList>

We pass the id of dropdownlist and the bound value to GetSelectedIndex in code behind which inserts the item in the list if item is not found in the list to prevent the error. you could then add validation during updating or inserting events to prevent the value if not allowed or use mapping to transform the illegal value.

In the code behind:

public int GetSelectedIndex(string id, string value)
{
    DropDownList list = (DropDownList)FindControlRecursive(Page.Master, Convert.ToString(id));
    int index = list.Items.IndexOf(list.Items.FindByValue(value));
    if (index == -1)
    {
        
    // the value was not found so we add it 
    
     list.Items.Insert(0, new ListItem(value, value));
        return 0;
    }
    else
    {
        return index;
    }
}

Can't remember where I found this snippet but it finds a control recursively and returns the first one found:

public Control FindControlRecursive(Control Root, string Id)
{
    if (Root.ID == Id)
        return Root;
    foreach (Control Ctl in Root.Controls)
    {
        Control FoundCtl = FindControlRecursive(Ctl, Id);
        if (FoundCtl != null)
            return FoundCtl;
    }
    return null;
}

Note: When Using with Formview, use only in edit mode. In insert mode you don't need the selectedindex.