2

问题

using System.Collections.Generic;
using System.Linq;

给定一个基类和 2 个派生类：

class Base{}
class Derived0 : Base{}
class Derived1 : Base{}

我有两个类型的字典：

IDictionary<String, Derived0> d0 = ...;
IDictionary<String, Derived1> d1 = ...;

我想找到两个字典的并集，它应该有 type IDictionary<String, Base>。（我已经知道两个字典中的键是唯一的，所以我不关心重复时的行为。）

尝试

如果它们是同一类型，我可以使用

var union = d0.Concat(d1);

但这会产生错误（使用编译dmcs）：

Test.cs(15,20): error CS0411: The type arguments for method `System.Linq.Queryable.Concat<TSource>(this System.Linq.IQueryable<TSource>, System.Collections.Generic.IEnumerable<TSource>)' cannot be inferred from the usage. Try specifying the type arguments explicitly

如果我明确给出Base作为类型参数：

IDictionary<string, Base> union = d0.Concat<Base>(d1);

它仍然不起作用：

Test.cs(15,42): error CS1928: Type `System.Collections.Generic.IDictionary<string,Derived0>' does not contain a member `Concat' and the best extension method overload `System.Linq.Enumerable.Concat<Base>(this System.Collections.Generic.IEnumerable<Base>, System.Collections.Generic.IEnumerable<Base>)' has some invalid arguments
/usr/lib/mono/gac/System.Core/4.0.0.0__b77a5c561934e089/System.Core.dll (Location of the symbol related to previous error)
Test.cs(15,42): error CS1929: Extension method instance type `System.Collections.Generic.IDictionary<string,Derived0>' cannot be converted to `System.Collections.Generic.IEnumerable<Base>'

原则上，方差在这里应该无关紧要，因为我正在创建一个新的字典对象，但我不知道如何在类型系统中表示它。

4

2 回答 2

4

IDictionary<TKey, TValue>并且KeyValuePair<TKey, TValue>不是变体......所以：

var sequence0 = d0.Select(kvp => new KeyValuePair<String, Base>(kvp.Key, kvp.Value));
var sequence1 = d1.Select(kvp => new KeyValuePair<String, Base>(kvp.Key, kvp.Value));
var dictionaryOfBase = sequence0.Concat(sequence1).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
于 2012-11-21T06:58:22.007 回答
0

您可以使用这些扩展方法：

    class Base { }
    class Derived0 : Base { }
    class Derived1 : Base { }

    class Program {
        static void Main(string[] args) {
            var d0 = new Dictionary<string, Derived0>();
            var d1 = new Dictionary<string, Derived1>();
            var b = d0.Merge<string, Derived0, Derived1, Base>(d1);
        }
    }

    public static class DictionaryExtensions {
        public static Dictionary<TKey, TBase> Merge<TKey, TValue1, TValue2, TBase>(this IDictionary<TKey, TValue1> thisDictionary, IDictionary<TKey, TValue2> thatDictionary)
            where TValue1 : TBase
            where TValue2 : TBase {
            var resultDictionary = new Dictionary<TKey, TBase>();
            resultDictionary.AddRange(thisDictionary);
            resultDictionary.AddRange(thatDictionary);

            return resultDictionary;
        }

        public static void AddRange<TKey, TBase, TValue>(this IDictionary<TKey, TBase> dictionary, IDictionary<TKey, TValue> dictionaryToAdd) where TValue : TBase {
            foreach (var kvp in dictionaryToAdd) {
                dictionary.Add(kvp.Key, kvp.Value);
            }
        }
    }
于 2012-11-21T07:07:22.897 回答