如何在XAML中初始化一个空字符串

Question

有如下代码使用ObjectDataProvider绑定到一个接受参数的方法：

 <Window>
    <Window.Resources>
        <ObjectDataProvider x:Key="StudentServiceProvider"
                            ObjectType="{x:Type lib:StudentService}"
                            MethodName="QueryStudentsByName">
            <ObjectDataProvider.MethodParameters>
                <sys:String></sys:String>
            </ObjectDataProvider.MethodParameters>
        </ObjectDataProvider>
    </Window.Resources>
    <StackPanel>
        <TextBox Name="tbNameKeyword"
                      Text="{Binding BindsDirectlyToSource=True,
                      Source={StaticResource StudentServiceProvider}, Path=MethodParameters[0],
                      UpdateSourceTrigger=Explicit}" />
        <Button Content="Search Students" Click="DoSearchStudentsClick" />
        <ListBox ItemsSource="{Binding Source={StaticResource StudentServiceProvider}}"/>
    </StackPanel>
 </Window>

        private void DoQueryStudentClick(object sender, RoutedEventArgs e)
        {
            tbNameKeyword.GetBindingExpression(TextBox.TextProperty).UpdateSource();
        }

现在的问题是在定义ObjectDataProvider的参数时

<ObjectDataProvider.MethodParameters>
      <sys:String></sys:String>
</ObjectDataProvider.MethodParameters>

若不指定此string参数的值，运行时会抛出null reference的异常，我不希望为此string参数定义任何默认值，希望初始化为null或者string.Empty，请问如何在实现？

我试过如下：

            <ObjectDataProvider.MethodParameters>
                <sys:String xml:space="preserve"> </sys:String>
            </ObjectDataProvider.MethodParameters>

貌似不起作用。

 

 

Answer 1

你好，

你可以使用x:Static来引用String的静态成员Empty。

        <x:StaticExtension Member="sys:String.Empty"/>

---

Min Zhu [MSFT]
MSDN Community Support | Feedback to us
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Answer 2

你好，我尝试了如下代码：

<ObjectDataProvider.MethodParameters>
      <sys:String>
            <x:StaticExtension Member="sys:String.Empty"/>
      </sys:String>
</ObjectDataProvider.MethodParameters>

得到错误信息：The type 'String' does not support direct content. 请问如何解决？

Answer 3

<ObjectDataProvider.MethodParameters>
    <x:StaticExtension Member="sys:String.Empty"/>
 </ObjectDataProvider.MethodParameters>

这样

---

Bob Bao [MSFT]
MSDN Community Support | Feedback to us
Get or Request Code Sample from Microsoft
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