Slider控件ValueChanged事件报错：更新标签出现空引用异常

需求与问题场景 #

要实现：Slider控件值为1时，标签显示Player；值大于1时显示Players。编写的代码运行时触发Object reference not set to an instance of an object空引用异常，即使Slider初始值设为2，仍在else分支报错。

现有代码 #

XAML代码

<Viewbox Grid.Column="1" Margin="10">
  <Slider x:Name="sliderhunter" Minimum="1" Maximum="8" Width="200" TickFrequency="1" Value="2" 
          TickPlacement="BottomRight" ValueChanged="sliderhunter_ValueChanged" Margin="10" 
          IsMoveToPointEnabled="True" IsSnapToTickEnabled="True"/>
</Viewbox>
<TextBox Grid.Column="1" IsHitTestVisible="False"  Foreground="White" 
         Text="{Binding Value, ElementName=sliderhunter, UpdateSourceTrigger=PropertyChanged}" 
         TextAlignment="Left" Height="68" VerticalAlignment="Top" FontSize="50" 
         Background="Transparent" HorizontalContentAlignment="Left" BorderBrush="Transparent" 
         Margin="43,34,469,0" />
<Label Grid.Column="1" IsHitTestVisible="False" x:FieldModifier="Public"   Content="Hunters" 
       HorizontalContentAlignment="Left" Height="80" VerticalAlignment="Top" FontSize="50" 
       Background="Transparent" BorderBrush="Transparent" Margin="95,34,343,0" x:Name="txtHunter" />

后台VB代码

Private Sub sliderhunter_ValueChanged(sender As Object, e As RoutedPropertyChangedEventArgs(Of Double)) Handles sliderhunter.ValueChanged
    If e.NewValue > 1 Then
        txtHunter.Content = "Players"
    Else
        txtHunter.Content = "Player"
    End If
End Sub

错误信息

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=GameProject
  StackTrace:
   at GameProject.winSetGameSettings.sliderhunter_ValueChanged(Object sender, RoutedPropertyChangedEventArgs`1 e) in C:\Users\mgobl\source\repos\GameScoreBoard\GameScoreboard\winSetGameSettings.xaml.vb:line 2571

问题原因 #

空引用异常的核心原因是：Slider的初始值Value="2"会在窗口控件初始化完成前触发ValueChanged事件，此时txtHunter标签还没被实例化（未创建对象），代码里访问txtHunter.Content自然会报错。

新手友好的解决方案 #

方案一：先判断控件是否初始化完成（最简单快速） #

修改后台VB代码，在操作txtHunter前先检查它是否已创建：

Private Sub sliderhunter_ValueChanged(sender As Object, e As RoutedPropertyChangedEventArgs(Of Double)) Handles sliderhunter.ValueChanged
    ' 如果标签还没初始化，直接退出事件
    If txtHunter Is Nothing Then Return
    
    If e.NewValue > 1 Then
        txtHunter.Content = "Players"
    Else
        txtHunter.Content = "Player"
    End If
End Sub

方案二：延迟初始值设置（避免加载时触发事件） #

1. 先移除XAML中Slider的Value="2"属性，防止加载时提前触发事件：

<Slider x:Name="sliderhunter" Minimum="1" Maximum="8" Width="200" TickFrequency="1" 
        TickPlacement="BottomRight" ValueChanged="sliderhunter_ValueChanged" Margin="10" 
        IsMoveToPointEnabled="True" IsSnapToTickEnabled="True"/>

2. 在窗口的Loaded事件中设置初始值，此时所有控件都已初始化：

Private Sub winSetGameSettings_Loaded(sender As Object, e As RoutedEventArgs) Handles Me.Loaded
    ' 设置Slider初始值
    sliderhunter.Value = 2
    ' 同步标签内容
    txtHunter.Content = "Players"
End Sub

方案三：使用数据绑定（符合WPF设计思想，一劳永逸） #

不需要编写后台事件，通过值转换器直接绑定Slider的值到Label的Content：

1. 创建一个值转换器类（在项目中添加新的VB类）：

Public Class PlayerCountConverter
    Implements IValueConverter

    Public Function Convert(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.Convert
        ' 把Slider的Double值转换为对应的文本
        Dim count As Double = CDbl(value)
        Return If(count > 1, "Players", "Player")
    End Function

    Public Function ConvertBack(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.ConvertBack
        ' 反向转换不需要，返回Nothing即可
        Return Nothing
    End Function
End Class

2. 在XAML的窗口资源中声明转换器（注意替换命名空间为你的项目命名空间）：

<Window x:Class="GameProject.winSetGameSettings"
        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:GameProject" ' 这里是你的项目命名空间
        mc:Ignorable="d"
        Title="winSetGameSettings" Height="450" Width="800">
    <Window.Resources>
        <!-- 声明转换器 -->
        <local:PlayerCountConverter x:Key="PlayerCountConverter"/>
    </Window.Resources>
    <!-- 你的其他XAML内容 -->
</Window>

3. 修改Label的Content绑定，使用转换器：

<Label Grid.Column="1" IsHitTestVisible="False" x:FieldModifier="Public" 
       Content="{Binding Value, ElementName=sliderhunter, Converter={StaticResource PlayerCountConverter}}" 
       HorizontalContentAlignment="Left" Height="80" VerticalAlignment="Top" FontSize="50" 
       Background="Transparent" BorderBrush="Transparent" Margin="95,34,343,0" x:Name="txtHunter" />

这样Slider值变化时，Label内容会自动更新，完全不需要后台事件，也不会有初始化问题。

内容的提问来源于stack exchange，提问作者ExpertOnNothing