输入时调用C＃getter两次

Question

Why does the c#-getter get called twice, if i write a letter into the TextBox ?

In my point of view, this is curious, because only one element ( Label ) bind to the property for getting values.

This is my xaml:

<Window x:Class="BindingDebug.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:BindingDebug"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <Label Grid.Row="0" Grid.Column="0" Content="Firstname" />
        <TextBox Grid.Row="0" Grid.Column="1"  x:Name="firstNameTextBox" Height="24" VerticalAlignment="Center" HorizontalAlignment="Stretch"
                    Text="{Binding FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

        <Label Grid.Row="1" Grid.Column="1" VerticalAlignment="Center" HorizontalAlignment="Left" Content="{Binding FirstName}" />
    </Grid>
</Window>

code behind:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.DataContext = new Model();
    }
}

the model

public class Model : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private string firstName;

    public string FirstName
    {
        get
        {
            System.Diagnostics.Debug.WriteLine("Get: " + firstName ?? "");
            return firstName;
        }

        set
        {
            firstName = value;
            System.Diagnostics.Debug.WriteLine("Set: " + value ?? "");
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("FirstName"));
            }
        }
    }
}

Or is it correct that the getter is called twice? The debug output goes to the Visual Studio output window.

Thank you!

Answer 1

Posted by Microsoft on 4/28/2010 at 10:10 AM This is not a bug. WPF (or any other code) can call your property-getter at any time for any reason; there's no rule that it will be called only once. WPF (and other callers) expects that your property follows the .Net guidelines; in particular that the property-getter is fast, and that it will return the same value from call to call unless you've raised a property-changed notification.

See here for links etc: WPF Binding View as Content