为什么我不能使用HashSet <string> 实现一个IEnumerable <string> 接口属性？

Question

I was wondering why can't I use HashSet<string> to implement an IEnumerable<string> interface property ?

The code below gives me a compile error with the following error;

'Lookups' does not implement interface member 'ILookups.LastNames'. 'Lookups.LastNames' cannot implement 'ILookups.LastNames' because it does not have the matching return type of 'System.Collections.Generic.IEnumerable'.

public interface ILookups
{
    IEnumerable<string> FirstNames { get; set; }
    IEnumerable<string> LastNames { get; set; }
    IEnumerable<string> Companies { get; set; }
}

public class Lookups : ILookups
{
    public HashSet<string> FirstNames { get; set; }
    public HashSet<string> LastNames { get; set; }
    public HashSet<string> Companies { get; set; }
}

According to Resharper, this is the constructor signature for HashSet ; ...

// Type: System.Collections.Generic.HashSet`1
// Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Core.dll
...
  /// <summary>
  /// Represents a set of values.
  /// </summary>
  /// <typeparam name="T">The type of elements in the hash set.</typeparam>
  [DebuggerDisplay("Count = {Count}")]
  [DebuggerTypeProxy(typeof (HashSetDebugView<>))]
  [__DynamicallyInvokable]
  [Serializable]
  [HostProtection(SecurityAction.LinkDemand, MayLeakOnAbort = true)]
  public class HashSet<T> : ISerializable, 
      IDeserializationCallback, ISet<T>, 
      ICollection<T>, IEnumerable<T>, IEnumerable
  {

...and it looks like it definitely implements IEnumerable<T> huh! This is not critical, just annoying as the workaround is just lengthy and feels like a broken feature of the language and very er..Java-ish? (he he!). (I will post the work around here later as soon as it's done in case I've missed a trick). If anyone has an answer or a better way to do this, or why this is like this, that would be most appreciated?

txs,

Alan

UPDATE:1.1.15 1 day after most of the comments were written, so please take with a pinch of salt.

re: re: "You can't implement a property that's declared to return A with another one that returns B even when B inherits/implements A." I don't believe this is entirely correct, as the following code compiles perfectly ok; doh!

 void Main()
{
    var r = new PersonRepo();
    Console.WriteLine(r.GetPerson(2).Name);
}

public class PersonRepo : IPersonRepo
{
    public Person GetPerson(int id)
    {
        var m = new Manager()
        { 
            Department = "department" + id.ToString(), 
            Name = "Name " + id.ToString()
        };
        return m;
    }
}

public interface IPersonRepo
{
    Person GetPerson(int id);
}

public class Person 
{
    public string Name { get; set;}
}

public class Manager : Person
{
    public string Department { get; set; }
}

I've just seen my mistake, the code above will not compile if you change Person GetPerson(int id) to Manager GetPerson(int id) you will get a compile error, which actually does make sense! Ok, I think this is done and dusted! ;-D

Answer 1

To implement interface member signature must be exactly the same as declared in interface. You can't implement a property that's declared to return A with another one that returns B even when B inherits/implements A .

You can implement that member expliticly and route it to your property:

public class Lookups : ILookups
{
    public HashSet<string> FirstNames { get; set; }

    IEnumerable<string> ILookups.FirstNames { get { return this.FirstNames; } }
}

Why is that required? Consider following code:

var lookups = (ILookups)new Lookups();
// assigning List<string> to ILookups.FirstNames, which is IEnumerable<string>
lookups.FirstNames = new List<string>();

How would you like this to be solved? It's totally valid code, but with your Lookups implementation you've just assigned List<string> to HashSet<string> ! It doesn't matter for methods and/or getter-only properties, but maybe just for sake of consistency?