c# - List.Contains 是否完全等同于 List.IndexOf？

Question

List.Contains() 的 MSDN 描述说

This method determines equality by using the default equality comparer, as defined by the object's implementation of the IEquatable.Equals method for T (the type of values in the list).

和List.IndexOf()的描述说

This method determines equality using the default equality comparer EqualityComparer.Default for T, the type of values in the list.

和EqualityComparer.Default描述说

The Default property checks whether type T implements the System.IEquatable interface and, if so, returns an EqualityComparer that uses that implementation. Otherwise, it returns an EqualityComparer that uses the overrides of Object.Equals and Object.GetHashCode provided by T.

这有点可疑 - Contains 的描述只提到了 IEquatable 并且可以将没有实现 IEquatable 的内容放入 列表。

所以我猜他们只是使用了相同的语义，也许 Contains() 重用了 IndexOf()。

那么它们在比较方面是否完全等效？

Answer 1

简答:

• 不，Contains()不重复使用 IndexOf()
• 是的，它们在比较方面是等价的

我反编译 (ReSharper) 并看到最终两者都使用 abstract EqualityComparer<T>.Default.Equals(T x, T y)方法。 Default EqualityComparer<T> 的实例根据类型 T 进行初始化(和缓存)。

列表.包含

EqualityComparer<T> @default = EqualityComparer<T>.Default;
// for loop
if (@default.Equals(this._items[index], item))
    return true;

List.IndexOf

return Array.IndexOf<T>(this._items, item, 0, this._size);

Array.IndexOf

public static int IndexOf<T>(T[] array, T value, int startIndex, int count)
{
    // Some assertions
    return EqualityComparer<T>.Default.IndexOf(array, value, startIndex, count);
}

EqualityComparer.IndexOf

internal virtual int IndexOf(T[] array, T value, int startIndex, int count)
{
    // for loop
    if (this.Equals(array[index], value))
        return index;
}

这就是 EqualityComparer.Default 的实例化

public static EqualityComparer<T> Default
{
  get
  {
    EqualityComparer<T> equalityComparer = EqualityComparer<T>.defaultComparer;
    if (equalityComparer == null)
    {
      equalityComparer = EqualityComparer<T>.CreateComparer();
      EqualityComparer<T>.defaultComparer = equalityComparer;
    }
    return equalityComparer;
  }
}

private static EqualityComparer<T> CreateComparer()
{
  RuntimeType genericParameter1 = (RuntimeType) typeof (T);

  if ((Type) genericParameter1 == typeof (byte))
    return (EqualityComparer<T>) new ByteEqualityComparer();
  // Some ifs go on
  else
    return (EqualityComparer<T>) new ObjectEqualityComparer<T>();
}