Silverlight Slider 控件“未将对象引用设置到对象的实例”错误解决方案

当在xaml文件中设置Value值之后,编译解决方案后，会报错“未将对象引用设置到对象的实例”

sColour.xaml文件代码

<UserControl x:Class="sample.sColour"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400">
<Grid x:Name="LayoutRoot" Background="White">
<Grid.RowDefinitions>
<RowDefinition Height="120"/>
<RowDefinition Height="120"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Rectangle Grid.Row="0" Grid.Column="1" x:Name="PreviewColour" Fill="#FFFF6600" Margin="15" ></Rectangle>
<StackPanel Grid.Row="1" Grid.Column="1">
<TextBlock FontSize="12">Color</TextBlock>
<TextBox x:Name="HexColour" Text="#FFFF6600" Height="30" Width="160" Margin="5" HorizontalAlignment="Left" FontSize="11"></TextBox>
</StackPanel>
<StackPanel Grid.Row="0" Grid.Column="0" Grid.RowSpan="2" VerticalAlignment="Center">
<TextBlock Text="Alpha" FontSize="11" Margin="10,15,0,0"/>
<Slider x:Name="SliderAlpha" Margin="20,0,10,0" Maximum="255" Value="255" ValueChanged="Slider_ValueChanged"/>
<TextBlock Text="Red" FontSize="11" Margin="10,15,0,0"/>
<Slider x:Name="SliderRed" Margin="20,0,10,0" Maximum="255" Value="255" ValueChanged="Slider_ValueChanged"/>
<TextBlock Text="Green" FontSize="11" Margin="10,15,0,0"/>
<Slider x:Name="SliderGreen" Margin="20,0,10,0" Maximum="255" Value="102" ValueChanged="Slider_ValueChanged"/>
<TextBlock Text="Blue" FontSize="11" Margin="10,15,0,0"/>
<Slider x:Name="SliderBlue" Margin="20,0,10,0" Maximum="255" Value="0" ValueChanged="Slider_ValueChanged"/>
</StackPanel>
</Grid>
</UserControl>

sColour.xaml.cs文件代码

using System.Windows.Documents;
 using System.Windows.Input;
 using System.Windows.Media;
 using System.Windows.Media.Animation;
 using System.Windows.Shapes;
namespace sample
{
public partial class sColour : UserControl
{
public sColour()
{
InitializeComponent();
}
private void Slider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
Color myColor = Color.FromArgb((byte)SliderAlpha.Value, (byte)SliderRed.Value, (byte)SliderGreen.Value, (byte)SliderBlue.Value);
　　　　　　 PreviewColour.Fill = new SolidColorBrush(myColor);
HexColour.Text = myColor.ToString();
}
}
}

仔细查看代码，貌似没有什么错误，但是总是会提示“未将对象引用设置到对象的实例”。后来经过查看资料，这可能是silverlight的一个bug，如果定义了ValueChanged，似乎不能给Slider的value属性在Xaml中赋初值。我们可以在sColour的构造函数中赋值：

public sColour()
{
　　　　 InitializeComponent();
this.SliderAlpha.Value = 255;
this.SliderRed.Value = 255;
this.SliderGreen.Value = 102;
this.SliderBlue.Value = 0;
}

运行界面，一个简易的取色器