不幸的是,KeyValuePair<TKey, TValue>
是一个结构;并且结构在 .NET 中不会表现出差异。
您当然可以通过编写自己的协变Pair
接口和一些简单的助手来解决这个问题,以便在序列KeyValuePair
和自定义Pair
接口之间进行转换。这将使您可以:
var dict = new Dictionary<string, ICollection<string>>();
var view = dict.GetCovariantView(); // IEnumerable< IPair<string, ICollection<string> > >
// Notice that you can _widen_ both the key and the value types:
var dictView = view.CastPairs<object, IEnumerable<string>>(); // IEnumerable< IPair< object, IEnumerable<String> > >
// The `CastPairs` call is actually unnecessary provided you don't use `var` for the left-hand-side assignment.
// ...this is due to the implicit (and identity-preserving) variant interface conversion in C#, e.g.:
IEnumerable< IPair< Object, IEnumerable<String> > > dictView2 = view;
Console.WriteLine( Object.ReferenceEquals( view, dictView2 ) ); // --> True
这是一些示例代码,可让您实现此目的:
// `out TKey` is for demonstration purposes. In production-quality code you probably should be using invariant key types.
public interface IPair<out TKey, out TValue>
where TKey : notnull
{
TKey Key { get; }
TValue Value { get; }
}
public class Pair<TKey, TValue> : IPair<TKey, TValue>
where TKey : notnull
{
public TKey Key { get; }
public TValue Value { get; }
public Pair(TKey key, TValue value)
{
this.Key = key;
this.Value = value;
}
public Pair(KeyValuePair<TKey, TValue> pair)
: this(pair.Key, pair.Value)
{}
}
public static class PairSequenceExtensions
{
public static IEnumerable<IPair<TKey, TValue>> GetCovariantView<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source)
where TKey : notnull
{
if (source is null) throw new ArgumentNullException(nameof(source));
return source.Select(kvp => new Pair<TKey, TValue>(kvp));
}
public static IEnumerable<IPair<TKey, TValue>> CastPairs<TKey, TValue>(this IEnumerable<IPair<TKey, TValue>> source)
where TKey : notnull
{
if (source is null) throw new ArgumentNullException(nameof(source));
return source;
}
}