在c#中合并2个或更多字典(Dictionary<TKey, TValue>)的最佳方法是什么?
(像LINQ这样的3.0特性就可以了)。
我正在考虑一个方法签名,如下所示:
public static Dictionary<TKey,TValue>
Merge<TKey,TValue>(Dictionary<TKey,TValue>[] dictionaries);
or
public static Dictionary<TKey,TValue>
Merge<TKey,TValue>(IEnumerable<Dictionary<TKey,TValue>> dictionaries);
关于重复键的处理:在发生冲突的情况下,保存到字典中的值并不重要,只要它是一致的。
使用equalitycompararer进行合并,它将项目映射到不同的值/类型进行比较。这里我们将从KeyValuePair(枚举字典时的项类型)映射到Key。
public class MappedEqualityComparer<T,U> : EqualityComparer<T>
{
Func<T,U> _map;
public MappedEqualityComparer(Func<T,U> map)
{
_map = map;
}
public override bool Equals(T x, T y)
{
return EqualityComparer<U>.Default.Equals(_map(x), _map(y));
}
public override int GetHashCode(T obj)
{
return _map(obj).GetHashCode();
}
}
用法:
// if dictA and dictB are of type Dictionary<int,string>
var dict = dictA.Concat(dictB)
.Distinct(new MappedEqualityComparer<KeyValuePair<int,string>,int>(item => item.Key))
.ToDictionary(item => item.Key, item=> item.Value);
注意,如果你使用一个名为“Add”的扩展方法,你可以使用集合初始化器来组合尽可能多的字典,就像这样:
public static void Add<K, V>(this Dictionary<K, V> d, Dictionary<K, V> other) {
foreach (var kvp in other)
{
if (!d.ContainsKey(kvp.Key))
{
d.Add(kvp.Key, kvp.Value);
}
}
}
var s0 = new Dictionary<string, string> {
{ "A", "X"}
};
var s1 = new Dictionary<string, string> {
{ "A", "X" },
{ "B", "Y" }
};
// Combine as many dictionaries and key pairs as needed
var a = new Dictionary<string, string> {
s0, s1, s0, s1, s1, { "C", "Z" }
};
基于上面的答案,但添加一个Func-parameter,让调用者处理重复:
public static Dictionary<TKey, TValue> Merge<TKey, TValue>(this IEnumerable<Dictionary<TKey, TValue>> dicts,
Func<IGrouping<TKey, TValue>, TValue> resolveDuplicates)
{
if (resolveDuplicates == null)
resolveDuplicates = new Func<IGrouping<TKey, TValue>, TValue>(group => group.First());
return dicts.SelectMany<Dictionary<TKey, TValue>, KeyValuePair<TKey, TValue>>(dict => dict)
.ToLookup(pair => pair.Key, pair => pair.Value)
.ToDictionary(group => group.Key, group => resolveDuplicates(group));
}
@user166390的回答版本增加了一个IEqualityComparer参数,以允许不区分大小写的键比较。
public static T MergeLeft<T, K, V>(this T me, params Dictionary<K, V>[] others)
where T : Dictionary<K, V>, new()
{
return me.MergeLeft(me.Comparer, others);
}
public static T MergeLeft<T, K, V>(this T me, IEqualityComparer<K> comparer, params Dictionary<K, V>[] others)
where T : Dictionary<K, V>, new()
{
T newMap = Activator.CreateInstance(typeof(T), new object[] { comparer }) as T;
foreach (Dictionary<K, V> src in
(new List<Dictionary<K, V>> { me }).Concat(others))
{
// ^-- echk. Not quite there type-system.
foreach (KeyValuePair<K, V> p in src)
{
newMap[p.Key] = p.Value;
}
}
return newMap;
}