c# - 具有多个 XmlArrayItemAttribute 实例的 GetCustomAttribute

Question

我有一个 List<TransformationItem> . TransformationItem只是多个类的基类，例如ExtractTextTransform和 InsertTextTransform

为了使用内置的 XML 序列化和反序列化，我必须使用 XmlArrayItemAttribute 的多个实例，如 http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlarrayitemattribute%28v=vs.80%29.aspx 中所述

You can apply multiple instances of the XmlArrayItemAttribute or XmlElementAttribute to specify types of objects that can be inserted into the array.

这是我的代码:

[XmlArrayItem(Type = typeof(Transformations.EvaluateExpressionTransform))]
[XmlArrayItem(Type = typeof(Transformations.ExtractTextTransform))]
[XmlArrayItem(Type = typeof(Transformations.InsertTextTransform))]
[XmlArrayItem(Type = typeof(Transformations.MapTextTransform))]
[XmlArrayItem(Type = typeof(Transformations.ReplaceTextTransform))]
[XmlArrayItem(Type = typeof(Transformations.TextItem))]
[XmlArrayItem(ElementName = "Transformation")]
public List<Transformations.TransformationItem> transformations;

问题是，当我使用反射获取带有 GetCustomAttribute() 的 ElementName 属性时, 我得到了 AmbiguousMatchException .

我该如何解决这个问题，比如说，获取 ElementName ？

Answer 1

由于找到了多个属性，您需要使用 ICustomAttributeProvider.GetCustomAttributes()。否则，Attribute.GetCustomAttribute() 方法会抛出 AmbiguousMatchException，因为它不知道要选择哪个属性。

我喜欢把它包装成一个扩展方法，例如:

public static IEnumerable<TAttribute> GetAttributes<TAttribute>(this ICustomAttributeProvider provider, bool inherit = false)
    where TAttribute : Attribute
{
    return provider
        .GetCustomAttributes(typeof(TAttribute), inherit)
        .Cast<TAttribute>();
}

像这样调用:

var attribute = typeof(TransformationItem)
    .GetAttributes<XmlArrayItemAttribute>(true)
    .Where(attr => !string.IsNullOrEmpty(attr.ElementName))
    .FirstOrDefault();

if (attribute != null)
{
    string elementName = attribute.ElementName;
    // Do stuff...
}