c# - 无法将 Dictionary> 转换为 IReadOnlyDictionary>

Question

谁能解释一下这个问题？

Dictionary<string, List<string>> x
  = new Dictionary<string, List<string>>();

IReadOnlyDictionary<string, IReadOnlyCollection<string>> y 
  = new Dictionary<string, IReadOnlyCollection<string>>();

y = x;  // CS0266: Cannot implicitly convert type...

Answer 1

您可以像这样接近您的目标:

Dictionary<string, List<string>> x = new Dictionary<string, List<string>>()
  {
    { "Foo", new List<string> {"A", "B", "C"} }
  };

IReadOnlyDictionary<string, ReadOnlyCollection<string>> y = 
    new ReadOnlyDictionary<string, ReadOnlyCollection<string>>(x.ToDictionary(k => k.Key, v => new ReadOnlyCollection<string>(v.Value)));
    
IReadOnlyCollection<string> foo = y["Foo"];

请注意 ReadOnlyCollection<T>包装您的原始列表。它不复制元素。 ReadOnlyDictionary<TKey, TValue> 相同.