Problem with UpdateSourceTrigger=PropertyChanged and StringFormat in WPF

问题

I have a text box in my application which is data bound to a decimal field in my class and the binding mode is two way. I am using StringFormat={0:c} for currency formatting.

This works fine as long as I don't touch 'UpdateSourceTrigger'. If I set UpdateSourceTrigger=PropertyChanged , It stops formatting the text that I am entering.

here is my code example

Employee.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;

namespace Converter
{
    public class Employee : INotifyPropertyChanged
    {
        int _employeeNumber;
        string _firstName;
        string _lastName;
        string _department;
        string _title;
        decimal _salary;

        public Employee()
        {
            _employeeNumber = 0;
            _firstName =
                _lastName =
                _department =
                _title = null;
        }
        public int EmployeeNumber
        {
            get { return _employeeNumber; }
            set 
            { 
                _employeeNumber = value;
                OnPropertyChanged("EmployeeNumber");
            }
        }
        public string FirstName
        {
            get { return _firstName; }
            set 
            { 
                _firstName = value;
                OnPropertyChanged("FirstName");
            }
        }

        public string LastName
        {
            get { return _lastName; }
            set 
            { 
                _lastName = value;
                OnPropertyChanged("LastName");
            }
        }

        public string Department
        {
            get { return _department; }
            set 
            { 
                _department = value;
                OnPropertyChanged("Department");
            }
        }

        public string Title
        {
            get { return _title + " salary: " + _salary.ToString(); }
            set 
            { 
                _title = value;
                OnPropertyChanged("Title");
            }
        }

        public decimal Salary
        {
            get { return _salary; }
            set
            {
                _salary = value;                
                OnPropertyChanged("Salary");
                OnPropertyChanged("Title");
            }
        }

        public override string ToString()
        {
            return String.Format("{0} {1} ({2})", FirstName, LastName, EmployeeNumber);
        }


        protected void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChangedEventArgs args = new PropertyChangedEventArgs(propertyName);
                this.PropertyChanged(this, args);
            }
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion
    }
}

EmployeeList.cs:

using System.Collections.ObjectModel;

namespace Converter
{
    public class EmployeeList : ObservableCollection<Employee>
    {
    }
}

Window1.xaml:

<Window x:Class="Converter.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Converter"
    Title="Window1" Height="500" Width="500">
    <Window.Resources>
        <local:EmployeeList x:Key="myEmployeeList">
            <local:Employee EmployeeNumber="1" FirstName="John" LastName="Dow" Title="Accountant" Department="Payroll" Salary="25000.00" />
            <local:Employee EmployeeNumber="2" FirstName="Jane" LastName="Austin" Title="Account Executive" Department="Customer Management" Salary="25000.00" />
            <local:Employee EmployeeNumber="3" FirstName="Ralph" LastName="Emmerson" Title="QA Manager" Department="Product Development" Salary="25000.00" />
            <local:Employee EmployeeNumber="4" FirstName="Patrick" LastName="Fitzgerald" Title="QA Manager" Department="Product Development" Salary="25000.00" />
            <local:Employee EmployeeNumber="5" FirstName="Charles" LastName="Dickens" Title="QA Manager" Department="Product Development" Salary="25000.00" />            
        </local:EmployeeList>
        <local:StringToDecimalCurrencyConverter x:Key="StringToDecimalCurrencyConverter"></local:StringToDecimalCurrencyConverter>
    </Window.Resources>
    <Grid DataContext="{StaticResource myEmployeeList}">
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="240" />
            <RowDefinition Height="45" />
        </Grid.RowDefinitions>
        <ListBox Name="employeeListBox"  ItemsSource="{Binding Path=., Mode=TwoWay}" Grid.Row="0" />
        <Grid Grid.Row="1"  DataContext="{Binding ElementName=employeeListBox, Path=SelectedItem, Mode=TwoWay}">
            <Grid.RowDefinitions>
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
                <RowDefinition Height="40" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="80" />
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>
            <Label Grid.Row="0" Grid.Column="0">First Name</Label>
            <Label Grid.Row="1" Grid.Column="0">Last Name</Label>
            <Label Grid.Row="2" Grid.Column="0">Title</Label>
            <Label Grid.Row="3" Grid.Column="0">Department</Label>
            <Label Grid.Row="4" Grid.Column="0">Salary</Label>

            <TextBox Grid.Row="0" Grid.Column="1" Text="{Binding Mode=TwoWay, Path=FirstName}" />
            <TextBox Grid.Row="1" Grid.Column="1" Text="{Binding Mode=TwoWay, Path=LastName}" />
            <TextBox Grid.Row="2" Grid.Column="1" Text="{Binding Mode=TwoWay, Path=Title}" />
            <TextBox Grid.Row="3" Grid.Column="1" Text="{Binding Mode=TwoWay,  Path=Department}" />
            <TextBox Grid.Row="4" Grid.Column="1"  Text="{Binding Mode=TwoWay, StringFormat=\{0:c\}, UpdateSourceTrigger=PropertyChanged, Path=Salary}" />
            <TextBlock Grid.Row="5" Grid.Column="1"  Text="{Binding Mode=OneWay, Converter={StaticResource StringToDecimalCurrencyConverter}, Path=Salary}" />
        </Grid>        
    </Grid>
</Window>

If you remove 'UpdateSourceTrigger=PropertyChanged' from the above code it works fine.

I have also tried using Converter instead of the StringFormat, and still the problem is not solved.

回答1:

The problem is that if you use a converter and update on property changed then WPF will ignore property changes while it is the process of updating the source property. Since the PropertyChanged event is raised from the setter, WPF ignores it.

The reason it does this is that if you typed "1" in the text box, this would get converted to the decimal value 1.0 and then get converted back to the string "$1.00". This would change the text in the text box and reset the cursor, so if you tried to type "12" you'd end up with "2$1.00".

Note that your Employee object is getting updated, and the problem is just that the TextBox isn't getting a newly formatted value.

If you really do want that behavior, you can set IsAsync=True on the binding and WPF will no longer see it as a reentrant change and will allow it. This has also changed in .NET 4.0, so if you upgrade to the latest version of the framework then you should see the behavior you expect.

来源：https://stackoverflow.com/questions/3200464/problem-with-updatesourcetrigger-propertychanged-and-stringformat-in-wpf