关于c＃：处理GetHashCode实现中的集合

Handling collections in GetHashCode implementation

我正在基于这个答案中的hashcode结构来实现gethashcode()。因为我的equals方法将考虑使用Enumerable.SequenceEqual()的集合，所以我需要在getHashCode()实现中包含这些集合。

作为起点，我使用jon skeet的嵌入式gethashcode()实现来测试hashcode结构实现的输出。使用下面的测试，可以按预期工作-

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private class MyObjectEmbeddedGetHashCode
{
public int x;
public string y;
public DateTimeOffset z;

public List<string> collection;

public override int GetHashCode()
{
unchecked
{
int hash = 17;

hash = hash * 31 + x.GetHashCode();
hash = hash * 31 + y.GetHashCode();
hash = hash * 31 + z.GetHashCode();

return hash;
}
}
}

private class MyObjectUsingHashCodeStruct
{
public int x;
public string y;
public DateTimeOffset z;

public List<string> collection;

public override int GetHashCode()
{
return HashCode.Start
.Hash(x)
.Hash(y)
.Hash(z);
}
}

[Test]
public void GetHashCode_CollectionExcluded()
{
DateTimeOffset now = DateTimeOffset.Now;

MyObjectEmbeddedGetHashCode a = new MyObjectEmbeddedGetHashCode()
{
x = 1,
y ="Fizz",
z = now,
collection = new List<string>()
{
"Foo",
"Bar",
"Baz"
}
};

MyObjectUsingHashCodeStruct b = new MyObjectUsingHashCodeStruct()
{
x = 1,
y ="Fizz",
z = now,
collection = new List<string>()
{
"Foo",
"Bar",
"Baz"
}
};

Console.WriteLine("MyObject::GetHashCode(): {0}", a.GetHashCode());
Console.WriteLine("MyObjectEx::GetHashCode(): {0}", b.GetHashCode());

Assert.AreEqual(a.GetHashCode(), b.GetHashCode());
}

下一步是在getHashCode()计算中考虑集合。这需要在myObjecteEmbeddedGetHashCode中对getHashCode()实现进行少量添加。

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public override int GetHashCode()
{
unchecked
{
int hash = 17;

hash = hash * 31 + x.GetHashCode();
hash = hash * 31 + y.GetHashCode();
hash = hash * 31 + z.GetHashCode();

int collectionHash = 17;

foreach (var item in collection)
{
collectionHash = collectionHash * 31 + item.GetHashCode();
}

hash = hash * 31 + collectionHash;

return hash;
}
}

然而，这在hashcode结构中有点困难。在本例中，当类型列表的集合传递到hash方法时，t是list，因此尝试将obj强制转换为ICollection或IEnumerable不起作用。我可以成功地强制转换为IEnumerable，但这会导致装箱，我发现我必须担心排除诸如实现IEnumerable的字符串之类的类型。

在这种情况下，是否有可靠地将obj强制转换为ICollection或IEnumerable的方法？

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public struct HashCode
{
private readonly int hashCode;

public HashCode(int hashCode)
{
this.hashCode = hashCode;
}

public static HashCode Start
{
get { return new HashCode(17); }
}

public static implicit operator int(HashCode hashCode)
{
return hashCode.GetHashCode();
}

public HashCode Hash<T>(T obj)
{
// I am able to detect if obj implements one of the lower level
// collection interfaces. However, I am not able to cast obj to
// one of them since T in this case is defined as List<string>,
// so using as to cast obj to ICollection<T> or IEnumberable<T>
// doesn't work.
var isGenericICollection = obj.GetType().GetInterfaces().Any(
x => x.IsGenericType &&
x.GetGenericTypeDefinition() == typeof(ICollection<>));

var c = EqualityComparer<T>.Default;

// This works but using IEnumerable causes boxing.
// var h = c.Equals(obj, default(T)) ? 0 : ( !(obj is string) && (obj is IEnumerable) ? GetCollectionHashCode(obj as IEnumerable) : obj.GetHashCode());

var h = c.Equals(obj, default(T)) ? 0 : obj.GetHashCode();
unchecked { h += this.hashCode * 31; }
return new HashCode(h);
}

public override int GetHashCode()
{
return this.hashCode;
}
}