Why does a format string that works in a Binding not work in a MultiBinding?

Question

I was intrigued by this question: MultiBinding StringFormat of TimeSpan

If I have the following Binding defined where StartTime is of type TimeSpan:

<TextBlock Text={Binding Path=StartTime, StringFormat='{}From {0:hh\\:mm}'}" />

The above binding evaluates as expected. However, as the scenario in the original question shows, if I try to use the same format string in a MultiBinding, it fails with a FormatException:

<TextBlock>
    <TextBlock.Text>
        <MultiBinding StringFormat="{}From {0:hh\\:mm} to {1:hh\\:mm}">
            <Binding Path="StartTime" />
            <Binding Path="EndTime" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

The question is, does anyone know why? Is this a bug or expected behavior? It seems odd to me that to get the same output in a MultiBinding, I have to change the "\:" to a ':' in the format string (as I discovered in answering the original question).

Have you tried the solution presented in the accepted answer of the question that you linked to? http://stackoverflow.com/questions/6537595/multibinding-stringformat-of-timespan/6539134#6539134 — Brian Driscoll, Jun 30 '11 at 21:12
In your first binding, you've specified the `StringFormat` as part of the `Binding` `MarkupExtension` and demarcated it in single quotes. In the second example, you're passing it via an attribute. Try changing your first example to use an explicit `Binding` object and pass in the `StringFormat` in the same manner: ``. I'm guessing it won't work. Some kind of parsing/escaping issue with a typically unhelpful error message. — Kent Boogaart, Jun 30 '11 at 21:18
@Brian I answered the original question but I don't know why my answer is "correct" because I don't know why the multibinding can't use the binding's format string? — sellmeadog, Jun 30 '11 at 21:37
@Kent you're probably right. I'll try that as soon as I am back at my computer. — sellmeadog, Jun 30 '11 at 21:39

score 0 · Accepted Answer · answered Jul 01 '11 at 02:41

This appears to be a bug in WPF 4, if not it's at least a breaking change from WPF 3.5. Take the following code for example:

<Window x:Class="WpfSampleTestBed.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TextBlock Text="{Binding Path=StartTime, StringFormat='{}From {0:hh\\:mm}'}" />

        <TextBlock x:Name="textBlock2">
            <TextBlock.Text>
                <MultiBinding StringFormat="{}From {0:hh\\:mm} to {1:hh\\:mm}">
                    <Binding Path="StartTime" />
                    <Binding Path="EndTime" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>

        <TextBlock x:Name="textBlock3" Text="Three" />
        <TextBlock x:Name="textBlock4" Text="Four" />

        <TextBlock>
        <TextBlock.Text>
            <MultiBinding StringFormat="Three = {0}, Four = {1}">
                <Binding ElementName="textBlock3" Path="Text" />
                <Binding ElementName="textBlock4" Path="Text" />
            </MultiBinding>
        </TextBlock.Text>
        </TextBlock>
    </StackPanel>

</Window>

With the code behind like:

using System;
using System.Windows;

namespace WpfSampleTestBed {

    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            this.DataContext = new Test() {
                StartTime = TimeSpan.FromSeconds(90),
                EndTime = TimeSpan.FromSeconds(100),
            };
        }
    }

    public class Test {
        public TimeSpan StartTime { get; set; }
        public TimeSpan EndTime { get; set; }
    }

}

If you compile and run this code against .NET 3.5, the output (i.e. Window content) will look like this:

From 00:01:30
From 00:01:30 to 00:01:40
Three
Four
Three = Three, Four = Four

Taking the exact sample code/project and running it against .NET 4 you get:

From 00:01:30

Three
Four
Three = Three, Four = Four

I found one bug report that may be related, but the author never responded so Microsoft closed the issue as 'Not Reproducible'.

So it appears that depending on the how the child Bindings are used, the StringFormat may or may not work in .NET 4.

Why does a format string that works in a Binding not work in a MultiBinding?

1 Answers1