简单的字符串属性似乎不会在XAML GUI中绑定

Question

I have a Page where I want to display a string property in a Label . This is my code, but nothing will appear in the label.

This is my .xaml

<Page x:Class="MyProject.PageOne"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
          xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
          mc:Ignorable="d" 
          d:DesignHeight="300" d:DesignWidth="300"
        Title="PageOne"
          Name="pageOne>
    <Grid>
     <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    </Grid>
 <Label Grid.Column="0" Content="{Binding ElementName=pageOne, Path=aStr}" FontWeight="Normal" FontSize="43" HorizontalAlignment="Left" Margin="0,00,0,0" VerticalAlignment="Center" Foreground="White"/>
</Grid>

</Page>

And this is my .cs code

    public partial class PageOne: Page, IPageInterface
    {
       public String aStr{get;set;}
       public PageOne()
            {
                InitializeComponent();

            }
       public void Start()
         {
            aStr = "Test";

        }

    }

Answer 1

The only thing really wrong with the code you posted, in terms of the problem you describe, is that you have not implemented some way for property change notifications to occur. Because the aStr property is not set to the new value until after the Label content has been initially set, without a way to receive notification, the framework has no way to know it needs to update the Label content.

In WPF the two main ways this is typically done (indeed, AFAIK the only two fully supported ways) are to create DependencyProperty instances, or to implement INotifyPropertyChanged . Either will work fine.

Here is an example of how your code should look with INotifyPropertyChanged implemented:

public partial class PageOne : Page, IPageInterface, INotifyPropertyChanged
{
    private string _astr;
    public String aStr
    {
        get { return _astr; }
        set { _astr = value; OnPropertyChanged(); }
    }

    public Page1()
    {
        InitializeComponent();
    }

    public void Start()
    {
        aStr = "Test";
    }

    private void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

Implementing the interface involves a couple of simple steps:

1. Declare the event named PropertyChanged
2. Any time a property is changed, raise that event, passing the name of the property that's changing.

Note that to do this, you can't use auto-implemented properties. You need to implement each property yourself, with a backing field, and a call to a method that will raise the property.

.NET offers the convenient [CallerMemberName] attribute, which I show here. So in the setter method for your aStr property, after setting the backing field's value, you simply call the method without any parameters, and the runtime automatically fills in the correct property name for you.

Now, the code you posted has some other problems as well, in the XAML. First it won't compile because you left out a " character, and because you've got an extra </Grid> closing tag.

One other possible problem, though it's not possible to know for sure since we are missing the full context of how you display this Page object, is that the text's color is white. If you're putting the Label instance on a white background, then of course you won't be able to see the text, even if it were set correctly.

I note that commenter Franck has suggested that you should set the DataContext . The truth is, given the code you posted this is actually not necessary, and doing so wouldn't actually fix the problem you are having.

But if you do fix the underlying notification issue, then his suggestions are an alternative way that you can achieve the binding. By setting the DataContext to the object containing the property (here, your PageOne class), then when you are binding you can just specify the property name alone, without having to include the ElementName at all, and without having to use the Path= with the property name. You may find this technique more convenient, at least some of the time.

In the future, please take the time to provide a good, minimal , complete code example that reliably reproduces the problem. You are more likely to get an answer that way, and you will ensure that any answer you do get is as good as it can be.