Error with textbox with stringformat and binding in WPF

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I have a simple view containing a Textbox and a Label.

<TextBox Text="{Binding MyInt, UpdateSourceTrigger=LostFocus, StringFormat='{}{##.##}'}"/>        
<Label Content="{Binding MyStr2}"/>

And in my ViewModel, I have

C#

private decimal myInt;

public decimal MyInt
{
    get { return myInt; }
    set
    {
        if (value == myInt) { return; }
        myInt = value;               
        OnPropertyChange();
        OnPropertyChange("MyStr2");
    }
}

public string MyStr2
{
    get
    {
        return myInt.ToString("N2", CultureInfo.CreateSpecificCulture("en-IN"));
    }
}

Simply speaking, the TextBox Text is bound to a decimal value, and the Label is supposed display the value in the textbox with proper formatting.

Since I have LostFocus as my UpdateSourceTriggerin TextBox, I press TAB in order for the validation and binding to work.

Without the StringFormat attribute in the TextBox, I cannot enter a decimal point in it. Hence, the StringFormat is necessary

So when I enter a decimal value, everything works properly. The Label properly displays the formatted number.

When I enter some garbage non-decimal value, the TextBox Border turns red indicating validation error. But, the very next time I enter some valid data, like 500, in the TextBox, and lose focus, the TextBox becomes blank, with the following error in Output Window:

System.Windows.Data Error: 6 : 'StringFormat' converter failed to convert value '500' (type 'Decimal'); fallback value will be used, if available. BindingExpression:Path=MyInt; DataItem='ViewModel' (HashCode=37975124); target element is 'TextBox' (Name='MyIntTextBox'); target property is 'Text' (type 'String') FormatException:'System.FormatException: Input string was not in a correct format.
   at System.Text.StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ParamsArray args)
   at System.String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)
   at System.String.Format(IFormatProvider provider, String format, Object[] args)
   at System.Windows.Data.BindingExpression.ConvertHelper(IValueConverter converter, Object value, Type targetType, Object parameter, CultureInfo culture)'

However, the Label displays 500.00 properly. I have put breakpoints in the ViewModel to see if the TextBox-bound property myInt is updated properly, and sure enough, it is. This only happens when I try to use StringFormat.

What I have tried:

I have tried to use many variations of the StringFormat, but in vain. In fact, some of the StringFormat values leads to erratic behavior of the cursor. But I am forced to use StringFormat, because, without it, I cannot enter a decimal point.

What exactly am I doing wrong?

Posted 19-Mar-17 6:18am

Sabyasachi Mukherjee

Updated 23-Mar-17 21:15pm

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Comments

Richard MacCutchan 19-Mar-17 13:33pm

I do not think the format '##.##' cannot accommodate the string '500' as it contains more than 2 significant digits. See Custom Numeric Format Strings[^].

Sabyasachi Mukherjee 19-Mar-17 14:55pm

But, when I enter 500 for the first time, it works. It only does not work when I enter it immediately after I enter some non-numeric value.

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Graeme_Grant · Accepted Answer · 2017-03-23T21:15:00

You don't need to format the string in code. You can do it in Xaml. Also, OnPropertyChange handling needs cleaning up. Below code will address both.

INotifyPropertyChanged wrapper

C#

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace WpfBindingEvents
{
    public abstract class ObservableObject : INotifyPropertyChanged
    {
        public void Notify([CallerMemberName] string caller = "")
        {
            PropertyChanged?.Invoke(this,
                new PropertyChangedEventArgs(caller));

        }
        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Data Model

C#

namespace WpfBindingEvents
{
    public class FloatFormatModel : ObservableObject
    {
        private float value;
        public float Value
        {
            get { return value; }
            set
            {
                this.value = value;
                Notify();
            }
        }
    }
}

ViewModel

C#

namespace WpfBindingEvents
{
    public class MainViewModel
    {
        public FloatFormatModel Model { get; } 
            = new FloatFormatModel { Value = 1234.5678F };
    }
}

Xaml

XML

<Window 
    x:Class="WpfBindingEvents.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfBindingEvents"
    mc:Ignorable="d" WindowStartupLocation="CenterScreen"
    Title="Numeric Formatting Example" Height="350" Width="525">

    <Window.DataContext>
        <local:MainViewModel/>
    </Window.DataContext>

    <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <TextBlock Text="Float:" 
                   Grid.Row="3" 
                   Margin="10 0"/>
        <TextBlock Text="{Binding Model.Value, StringFormat=N2}"
                   Grid.Column="1"
                   Width="80"/>
    </Grid>

</Window>

Take note of this binding:Text="{Binding Model.Value, StringFormat=N2}"