Today I got to play around with multibinding as I build a significant part of my application. My listboxitem template contains an image that needs to change based on up to 4 different text fields on the underlying data object. Multibinding works perfectly in this sort of situation. I was unable to do this through the blend GUI but I was able to do it in the XAML straight away. Here's how.
First you have to build a class than inherits IMultiValueConverter. Here's an example of one:
Option Strict On
Imports System.Windows.Data
Public Class ImageSourceConverter
Implements IMultiValueConverter
Public Function Convert(ByVal values() As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IMultiValueConverter.Convert
If Not values Is Nothing Then
If values.GetUpperBound(0) > 0 Then ' there should always be 2 values at least
Dim strPath As String = ""
Dim bBooleanCondition As Boolean = False
If values.GetUpperBound(0) >= 2 Then
strPath = CStr(values(2))
End If
If values.GetUpperBound(0) >= 3 Then
bBooleanCondition = CBool(values(3))
End If
Dim objImagePaths As New ImagePaths
' this class returns a path to the image in the format of "pack://application:,,,/myApp;component/Images/myApp.gif" based on the parameters fed in from here...this is an easy class to write
Dim strImagePath As String = objImagePaths.GetPathFromObjectInfo(Cint(values(0)), CInt(values(1)), strPath, bBooleanCondition)
objImagePaths = Nothing
Dim sourceConverter As New ImageSourceConverter
Return sourceConverter.ConvertFromString(strImagePath)
End If
End If
End Function
Public Function ConvertBack(ByVal value As Object, ByVal targetTypes() As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object() Implements System.Windows.Data.IMultiValueConverter.ConvertBack
'todo: implement
End Function
End Class
Note that unlike a single binding path where .net converts the imagesource from a string automatically, this doesn't work with multibinding paths. I think this is a bug in the framework. Anyhow the solution is easy, just do the conversion in your code as I've done here.
The second part is to create the converter resource in your XAML. First, make sure you have the namespace referenced in the root node of your XAML:
xmlns:util="clr-namespace:myApp;assembly=myApp"
Then create your reference under your Window.Resources (or in my case it was UserControl.Resources) section:
<util:ImageSourceConverter x:Key="FieldsToImageSourceConverter" />
Now in your datatemplate under your image remove any source attribute form the Image tag and beneath the Image tab put the following XAML:
<Image.Source>
<MultiBinding Converter="{StaticResource FieldsToImageSourceConverter" Mode="Default">
<Binding XPath="Field1" />
<Binding XPath="Field2" />
<Binding XPath="Field3" />
</MultiBinding>
</Image.Source>
Note I am using XPath because my datasource is an xml document...if you are using a non xml document I believe you would just use <Binding Path="Field1" /> etc. I first had Path and was getting an error because the values array contained objects of "MS.Internal Object" instead of what I was expecting.
Now, when this template is applied and the data is bound your ImageSourceConverter code is called and can choose the proper image file to return for the source of your image control.
The only gotcha I've seen is that there appears to be a bug in the Cider designer in VS 2008 if you have multibinding on a form (in my case a usercontrol). It won't render the control visibly anymore. I now have the following error when load my control in the visual designer:
Blend's designer still seems to work though so I'm not too concerned with this right now. Although MSFT should look at fixing this ASAP.
I hope this helps someone else out there looking to do multibinding...it is a very cool implementation that IMO is much more elegant than the "itemDataBound" methodologies I'd use in the past with ASP.net.