c# - 解释这些 Linq 代码

Question

public class Person {
    public string First { get; set; }
    public string Last { get; set; }
}

调用是:

var persons = new List<Person>();
persons.Add(new Person {First = "First 04", Last = "Last 04"});
persons.Add(new Person {First = "First 03", Last = "Last 03"});
persons.Add(new Person {First = "First 02", Last = "Last 02"});
persons.Add(new Person {First = "First 01", Last = "Last 01"});

var claim1 = new List<Person>(from c in persons select c);
var claim2 = new List<Person>(persons);

使用这个语句有什么区别或优势

var claim1 = new List<Person>(from c in persons select c); 

在这个声明上。

var claim2 = new List<Person>(persons);

Answer 1

第一种情况from c in persons select c返回 IEnumerable<Person> ，在第二个:List<Person>()但在这两种情况下public List(IEnumerable<T> collection)被调用，所以这两个语句之间没有真正的区别。

如果反汇编构造函数:

public List(IEnumerable<T> collection)
{
    if (collection == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
    }
    ICollection<T> collection2 = collection as ICollection<T>;
    if (collection2 == null)
    {
        this._size = 0;
        this._items = List<T>._emptyArray;
        using (IEnumerator<T> enumerator = collection.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {
                this.Add(enumerator.Current);
            }
        }
        return;
    }
    int count = collection2.Count;
    if (count == 0)
    {
        this._items = List<T>._emptyArray;
        return;
    }
    this._items = new T[count];
    collection2.CopyTo(this._items, 0);
    this._size = count;
}

我们可以看到，如果collection是ICollection<T> , ICollection<T>.CopyTo被调用，这比一个一个地复制更有效率。