Thanks Kevin,

that made me double check the control names and I had them incorrect. Once I set them up correctly it the designer OK.

I wish the error message was a bit more instructive though :(

>Frank,
>
>First, does it run? Sometimes the designer craps out but the app still runs. If it runs, see what the Output window has to say.
>
>Have you tried to bind the textbox to each source individually? One of them is failing, and it would be helpful to see which one.
>
>
>
>>In Visual Studio 2010, I have a XAML screen with a textbox defined like this:
>>
>>

                        < TextBox  Name="txtBalanceOutstanding"
>>                                  Grid.Row="9" Grid.Column="1" Grid.ColumnSpan="1" Height="23" 
>>                                  HorizontalAlignment="Stretch" Margin="0,0,100,0" 
>>                                  IsReadOnly="True" FontWeight="Bold" Focusable="False" VerticalContentAlignment="Center" >
>>
>>                        < /TextBox >
>>
>>

>>
>>When I change it to this (adding in the MultiBinding):
>>
>>

                        <TextBox  Name="txtBalanceOutstanding"
>>                                  Grid.Row="9" Grid.Column="1" Grid.ColumnSpan="1" Height="23" 
>>                                  HorizontalAlignment="Stretch" Margin="0,0,100,0" 
>>                                  IsReadOnly="True" FontWeight="Bold" Focusable="False" VerticalContentAlignment="Center">
>>                            <MultiBinding Converter="{StaticResource OutstandingBalanceConverter}">
>>                                <Binding ElementName="txtDepositTotal" Path="Text"></Binding>
>>                                <Binding ElementName="txtDep_Amount" Path="Text"></Binding>
>>                            </MultiBinding>
>>                        </TextBox>
>>
>>

>>
>>The screen designer shows an unhandled exception error has occurred:
>>
>>Unable to cast object of type 'MS.Internal.NamedObject' to type 'System.String'.
>> at System.Linq.Enumerable.< CastIterator >d__b1`1.MoveNext()
>> at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
>> at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
>> at SamaanSystems.IBC.WPF.OutstandingBalanceConverter.Convert(Object[] values, Type targetType, Object parameter, CultureInfo culture) in D:\Development\VS2010\IBC\IBC WPF\Converters\OutstandingBalanceConverter.cs:line 13
>> at System.Windows.Data.MultiBindingExpression.TransferValue()
>> at System.Windows.Data.MultiBindingExpression.Transfer()
>> at System.Windows.Data.MultiBindingExpression.UpdateTarget(Boolean includeInnerBindings)
>> at System.Windows.Data.MultiBindingExpression.AttachToContext(Boolean lastChance)
>> at System.Windows.Data.MultiBindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
>> at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
>> at MS.Internal.Data.DataBindEngine.Run(Object arg)
>> at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
>> at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
>>
>>Any idea what I'm doing wrong?