小编Oka*_*rin的帖子

我正在尝试编写一个 ValidationRule 来检查字符串是否为空：

public class NotNullValidationRule : ValidationRule
{
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {
    string str = value as string;

    return string.IsNullOrEmpty(str) ? new ValidationResult(false, Application.Current.FindResource("EmptyStringNotAllowed")) : ValidationResult.ValidResult;
  }
}

在我的窗口中，我像这样使用它：

<TextBox
    Name="TxtDescription"
    Width="Auto"
    controls:TextBoxHelper.Watermark="{DynamicResource Description}">
    <TextBox.Text>
        <Binding Path="MachineToEdit.Description">
            <Binding.ValidationRules>
                <validation:NotNullValidationRule ValidatesOnTargetUpdated="True"/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

如果我启动设计器，我会得到这个 NullReferenceException：

   at System.Windows.Data.BindingExpression.RunValidationRule(ValidationRule validationRule, Object value, CultureInfo culture)
   at System.Windows.Data.BindingExpression.ValidateOnTargetUpdated()
   at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
   at System.Windows.Data.BindingExpression.Activate(Object item)
   at System.Windows.Data.BindingExpression.AttachToContext(AttachAttempt attempt)
   at System.Windows.Data.BindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
   at MS.Internal.Data.DataBindEngine.Run(Object arg) …

我正在尝试向不同板条箱中的特征添加扩展方法。此特征具有指定的关联类型。

pub trait Test<W> {
    type Error;

    fn do_sth(&mut self) -> Result<W, Self::Error>;
}

为什么不能添加使用关联类型的方法Error？

impl dyn Test<u8> {
    fn use_do_sth(&mut self) -> Result<u8: Self::Error> {
        self.do_sth()
    }
}

操场