DataTrigger with a MultiBinding

Framework 3.5

My requirement is to tell the user when the one value in a data grid row is less than another value. I'm going to tell them by making the first cell's foreground red and adding a tooltip. So I will add a trigger that will use a multi value converter.

So step one is to write a multi value converter that will take two values which are passed by binding and an operator which is passed as the parameter and return true or false. It's a nice generic converter so I will be able to use it in many places.

Public Class CompareConverter

    Implements System.Windows.Data.IMultiValueConverter

    ' values(0) is decimal

    ' values(1) is decimal

    ' returns boolean

    Public Function Convert(values() As Object, targetType As System.Type, parameter As Object, culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IMultiValueConverter.Convert

        Dim LH As Decimal = 0

        Dim Op As String = "="

        Dim RH As Decimal = 0

        Dim Result As Boolean = False

        If values.Count > 0 Then Decimal.TryParse(values(0).ToString(), LH)

        Op = parameter.ToString()

        If values.Count > 1 Then Decimal.TryParse(values(1).ToString(), RH)

        Select Case Op

            Case "<" : Result = (LH < RH)

            Case ">" : Result = (LH > RH)

            Case "<>", "!=" : Result = (LH <> RH)

            Case "<=" : Result = (LH <= RH)

            Case ">=" : Result = (LH >= RH)

            Case Else : Result = (LH = RH)

        End Select

        Return Result

    End Function

    Public Function ConvertBack(value As Object, targetTypes() As System.Type, parameter As Object, culture As System.Globalization.CultureInfo) As Object() Implements System.Windows.Data.IMultiValueConverter.ConvertBack

        Throw New System.NotImplementedException

    End Function

End Class

Of course we will need to create a static resource so we can reference it.

<local:CompareConverter x:Key="CompareConverter"/>

Now we need to add a style to the column to define a trigger.

<DataGridTextColumn Header="Total Account Chgs" Binding="{Binding Path=Debit, StringFormat='{}{0:0.00}'}">

    <DataGridTextColumn.ElementStyle>

        <Style TargetType="TextBlock">

            <Setter Property="Foreground" Value="Black"></Setter>

            <Setter Property="TextBlock.ToolTip" Value=""></Setter>

            <Style.Triggers>

                <DataTrigger Value="True">

                    <DataTrigger.Binding>

                        <MultiBinding Converter="{StaticResource CompareConverter}" ConverterParameter="&lt;">

                            <Binding Path="Debit"></Binding>

                            <Binding Path="TotalPaid"></Binding>

                        </MultiBinding>

                    </DataTrigger.Binding>

                    <DataTrigger.Setters>

                        <Setter Property="Foreground" Value="Red"/>
                        <Setter Property="TextBlock.ToolTip" Value="The Total Account Charges are less than the Total Paid."/>
                    </DataTrigger.Setters>

                </DataTrigger>

            </Style.Triggers>

        </Style>

    </DataGridTextColumn.ElementStyle>

</DataGridTextColumn>