I am new to XAML so apologies about this simple problem. I have a combobox on a page. When I try to set the comboxbox to a value read from a database I keep getting a null error. I am guessing its because I am using the selecteditem bit and as the combobox hasnt got anything selected it errors.. but that is only a gues

Design bit
----------
<ComboBox x:Name="cbox1" IsEnabled= "False" SelectionChanged="cbox1_SelectionChanged" Width="200" Margin="10,0,0,0" Height="60" Grid.Column="1" FontSize="20" >
<ComboBoxItem Tag="3" >0</ComboBoxItem>
<ComboBoxItem Tag="4">50</ComboBoxItem>
<ComboBoxItem Tag="5">100</ComboBoxItem>
<ComboBoxItem Tag="6">150</ComboBoxItem>
<ComboBoxItem Tag="7">200</ComboBoxItem>
<ComboBoxItem Tag="8">250</ComboBoxItem>
<ComboBoxItem Tag="9">300</ComboBoxItem>
<ComboBoxItem Tag="10">350</ComboBoxItem>

</ComboBox>

Code behind
-----------

SetResult = "600"; // This is where try to set the value in the box


code.....

public string SetResult
{
get
{
if (cbox1.SelectedIndex != -1)
{ return ((ComboBoxItem)cbox1.SelectedItem).ToString(); }
else
{ return ""; }
}
set
{
((ComboBoxItem)cbox1.SelectedItem).Content = value;
}
}


Can you anyone point out my obvious stupid mistake

thanks