数据绑定到用户控制错误数据.Binding不能转换为System.String

Question

我是WPF的新手，我在将文本绑定到我制作的用户控件时遇到了麻烦。它是一个简单的控件，它基本上就是一个带有文本和图像的按钮我想在几个视图中重用它

这是我的用户控制的。cs

public partial class MenuItemUserControl : UserControl
{
public string TextToDisplay { get; set; }
public MenuItemUserControl()
{
InitializeComponent();
DataContext = this;
}
}

这是我的用户控件的xaml

<UserControl x:Class="Class.Controls.MenuItemUserControl"
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" 
xmlns:local="clr-namespace:Class.Controls"
mc:Ignorable="d" 
d:DesignHeight="83.33" d:DesignWidth="512">
<Grid>
<Button Style="{StaticResource MenuItemStyle}" Height="Auto" Width="Auto">
<DockPanel LastChildFill="True" Width="512" Height="83.33" HorizontalAlignment="Center" VerticalAlignment="Center">
<Image Source="/Resources/MenuArrow.png" Stretch="None" HorizontalAlignment="Left" VerticalAlignment="Center" DockPanel.Dock="Left"/>
<TextBlock d:Text="sample" Text="{Binding TextToDisplay, Mode=OneWay}" HorizontalAlignment="Left" VerticalAlignment="Center" FontSize="{StaticResource MenuItems}"/>
</DockPanel>
</Button>
</Grid>
</UserControl>

这是我的视图xaml

<Page x:Class="Class.Views.MenuOperate"
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"
xmlns:uc="clr-namespace:Class.Controls"
xmlns:local="clr-namespace:Class.Views"
xmlns:properties="clr-namespace:Class.Properties"
xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls" xmlns:viewmodels="clr-namespace:Class.ViewModels" d:DataContext="{d:DesignInstance Type=viewmodels:MenuOperateViewModel}"
mc:Ignorable="d" 
d:DesignHeight="500" d:DesignWidth="1024"
Title="MenuOperate">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="83.33"/>
<RowDefinition Height="83.33"/>
<RowDefinition Height="83.33"/>
<RowDefinition Height="83.33"/>
<RowDefinition Height="83.33"/>
<RowDefinition Height="83.33"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="512"/>
<ColumnDefinition Width="512"/>
</Grid.ColumnDefinitions>

<uc:MenuItemUserControl TextToDisplay="{Binding StartStop, Mode=TwoWay}" Grid.Row="0" Grid.Column="0"/>
</Grid>
</Page>

这是我的视图模型。cs

namespace Class.ViewModels
{
public class MenuOperateViewModel : ObservableObject
{
private string? _StartStop;
public MenuOperateViewModel()
{
StartStop = Properties.Resources.MenuOperateStart;
}
public string? StartStop
{
get => _StartStop;
set => SetProperty(ref _StartStop, value);
}
}
}

这是我得到的错误在我的视图Xaml:

类型为"System.Windows.Data"的对象。绑定'不能转换为'System.String'类型。

Answer 1

有两件事可以防止表达式

TextToDisplay="{Binding StartStop}"

。

1. 绑定的目标属性，即TextToDisplay必须是一个依赖属性。

2. 你不能显式设置UserControl的DataContext。绑定将解析源属性路径相对于当前DataContext，即DataContext = this;在控件的构造函数中，它期望源属性StartStop在UserControl上，这显然是错误的。

详细信息请参见数据绑定概述

---

你的代码应该是这样的:

public partial class MenuItemUserControl : UserControl
{
public static readonly DependencyProperty TextToDisplayProperty =
DependencyProperty.Register(
nameof(TextToDisplay),
typeof(string),
typeof(MenuItemUserControl));
public string TextToDisplay
{
get { return (string)GetValue(TextToDisplayProperty); }
set { SetValue(TextToDisplayProperty, value); }
}
public MenuItemUserControl()
{
InitializeComponent();
}
}

UserControl的XAML中的绑定将使用RelativeSource绑定。

<TextBlock Text="{Binding TextToDisplay,
RelativeSource={RelativeSource AncestorType=UserControl}}" />