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用Try-Catch块包装复杂的linq查询并捕获正确的异常

factory singleton exception-handling linq c#

Caspar Kleijne · 技术社区 · 14 年前

下面的代码是一个工厂类,它传递实现了GraphTypeAttribute的IGraph类型的对象。在GraphFactory的静态构造函数中,通过使用Linq收集工厂要交付的适当类来构建一个列表。通常没有Linq,我有一个buch循环,如果then的,可以很容易地用适当的try-catch块包装。 由于所有的查询都被塞进了一个查询中,所以我对如何在这里实现适当的异常处理有点困惑 .

所以我的问题是

我应该把它分成不同的查询还是根本不用linq?

查询的结果必须是工厂可以交付的所有类的列表。例如,用属性和实现的接口修饰。

    public sealed class GraphFactory 
    {
    static readonly GraphFactory _instance = new GraphFactory();
    static readonly IDictionary<string, Type> _items;
    static readonly Assembly _assembly = Assembly.GetExecutingAssembly();

    public static GraphFactory Instance { get { return _instance; } }
    GraphFactory() { }

    static GraphFactory() {
        try
        {
            _items = (from type in _assembly.GetTypes()
                      // filter from thatonly the classes with IGraph implemented
                      where type.GetInterface(typeof(IGraph).FullName) != null
                      // filter from thatonly the classes with GraphTypeAttribute imp.
                      from attribute in type.GetCustomAttributes(true)
                      where attribute is GraphTypeAttribute
                      select new { attribute, type })
                     // convert the result from anonymous to a dictionary
                      .ToDictionary(k => (k.attribute as GraphTypeAttribute).CustomType, 
                                          e => e.type);
        }
        /** EXH: non pokemon exception handling  * ........... * **/
    }

    public static IEnumerable<string> FriendlyNames  { get { return _items.Keys; } }

    public static IGraph CreateGraph(string friendlyName)
    {
        /** inspect argument, check it's a key 
            in the dictionary and throw exeptions if needed **/     

        IGraph result = null;
        try
        {
            result = _assembly.CreateInstance(_items[friendlyName].FullName) as IGraph;
        }
        /** non pokemon exception handling * ...........  * **/
        return result;
    }
}

接口(指定成员):

public interface IGraph { }

[AttributeUsage(AttributeTargets.Class, AllowMultiple=false,Inherited=true)]
public class GraphTypeAttribute : System.Attribute 
{ public GraphTypeAttribute(string friendlyName)  { } }

用属性修饰的类

[GraphTypeAttribute("piechart")]
public class PieChart : IGraph{ }

[GraphTypeAttribute("map")]
public class WorldMap : IGraph { }

[GraphTypeAttribute("horizontalbar")]
public class Bar : IGraph { }

[GraphTypeAttribute("verticalbar")]
public class VerticalBar : Bar { }

示例用法:

  foreach (string friendlyName in GraphFactory.FriendlyNames)
  {
   IGraph auth = GraphFactory.CreateGraph(friendlyName);
  }

感谢您对本课程的任何其他评论或建议。

1 回复 | 直到 14 年前

Steven 14 年前

我认为这是一个很好的例子 dynamic factory pattern

但是如果您真的想进行错误检查,您的LINQ查询将永远不会抛出异常。它是 ToDictionary

static GraphFactory()
{ 
    var items = (
        from type in _assembly.GetTypes()
        where type.GetInterface(typeof(IGraph).FullName) != null
        from attribute in type.GetCustomAttributes(true)
            .OfType<GraphTypeAttribute>
        select new { attribute, type }).ToArray();

    ValidateTypes(items);

    _item = items.ToDictionary(
        k => k.attribute.CustomType, e => e.type);
}

private static void ValidateTypes<T>(T[] items)
{
    var firstDoubleCustomType = (
        from item in items
        group item by item.attribute.CustomType into g
        where g.Count() > 1
        select g.Key).FirstOrDefault();

    if (firstDoubleCustomType != null)
    {
        throw new InvalidProgramException(
           "Doube: " + firstDoubleCustomType.ToString());
    }
}