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

I keep getting the above error in the title line and it makes no sense, because I am using a sample table with only 5 records and each record has a value as per the drop down menu.

This is my code used to declare the drop down list. I have joined two tables in my SQL data source to reflect what I want populated in the grid view and have hidden columns as necessary. I am using the same data source to populate the drop down list any help would be most appreciated

<asp:DropDownList ID="DropDownListCurrency" runat="server" 
                  CausesValidation="True" DataSourceID="GridView" 
                  DataTextField="Currency" DataValueField="Currency_ID" 
                  AppendDataBoundItems="True"> 

          <asp:ListItem Value="0" Text="&lt;Select&gt;" Enabled="True"></asp:ListItem>
</asp:DropDownList>

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    try 
    {
        GridViewRow row = GridView1.SelectedRow; 

        AccountNumber.Text = (string)row.Cells[0].Text;
        ....

        DropDownListCurrency.SelectedValue = (string)row.Cells[8].Text;
    }
    catch (Exception ex)
    {
        Console.WriteLine("{0} Exception caught.", ex);
    }        
}

Answer

Attempt to find the value in the drop down list before attempting to set the SelectedValue, like this:

if (DropDownListCurrency.Items.FindByValue(row.Cells[8].Text.ToString().Trim()) != null)
{
    DropDownListCurrency.SelectedValue = row.Cells[8].Text.ToString().Trim();
}

Note: The Trim() call will remove any leading or trailing spaces in your text box text, which could be a cause for a match not being found.

So your full code should be this:

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    try 
    {
        GridViewRow row = GridView1.SelectedRow; 

        AccountNumber.Text = (string)row.Cells[0].Text;
         ....

        if (DropDownListCurrency.Items.FindByValue(row.Cells[8].Text.ToString().Trim()) != null)
        {
            DropDownListCurrency.SelectedValue = row.Cells[8].Text.ToString().Trim();
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine("{0} Exception caught.", ex);
    }
}