求助：关于silverlight滚动条,在silverlight版块三天无人回复

在silverlight版块三天无人回复
grid内含ScrollViewer，ScrollViewer内含canvas，窗体大小800*600，但是CANVAS里绘出来的图超过600高度，造成部分内容被截断没显示出来，现在我把Canvas拉高到1500，然后把Canvas的绘制起点从高度/2也就是750开始绘制，暂时可以显示完全，问题：因为是从750开始绘制 所以页面出来的时候，上面大概有四五百的地方是空的，需要手动拉动滚动条去下面才能看到图

我用sv.ScrollToVerticalOffset（750）想实现页面在展现出来的时候 自动滚动到750的位置，目前失败，求思路或代码，谢

以下是代码：

<UserControl xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" x:Class="Dtsc.WorkFlow.FlowInfo.MainPage"
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"
xmlns:my="clr-namespace:Dtsc.WorkFlow.Silverlight.Controls;assembly=Dtsc.WorkFlow.Silverlight.Controls"
d:DesignHeight="600" d:DesignWidth="800">
<Grid x:Name="grid1">
<Grid.RowDefinitions>
<RowDefinition Height="20" />
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Image Height="20" HorizontalAlignment="Left" Margin="1,1,0,0" Name="ImgLegend" Stretch="Fill" VerticalAlignment="Top" Width="373" Source="Image/图例.png" Grid.RowSpan="2" />
<ScrollViewer VerticalScrollBarVisibility="Auto" Width="800" Height="600" Margin="0" HorizontalScrollBarVisibility="Auto" x:Name="sv" Grid.Row="1">
<Canvas Name="canva" Height="1500" />
</ScrollViewer>
</Grid>
</UserControl>




ublic MainPage(string rid, string vid, string showRule)
{
InitializeComponent();
RecordId = rid;
VersionId = vid;
ShowRule = showRule;

client.GetRecordModelCompleted += new EventHandler<WorkFlowWebService.GetRecordModelCompletedEventArgs>(client_GetRecordModelCompleted);
client.GetRecordModelAsync(RecordId);

ShowWhat(showRule);//包含在ScrollViewer中的Canvas上绘图
sv.ScrollToVerticalOffset(750);//滚动到750
}