C# Linq延迟查询的执行实例代码

C# Linq延迟查询

在定义linq查询表达式时，查询是不会执行，查询会在迭代数据项时运行。它使用yield return 语句返回谓词为true的元素。

var names = new List<string> { "Nino", "Alberto", "Juan", "Mike", "Phil" };
var namesWithJ = from n in names
                 where n.StartsWith("J")
                 orderby n
                 select n;
Console.WriteLine("First iteration");
foreach (string name in namesWithJ) 
{
    Console.WriteLine(name);
}
Console.WriteLine();

names.Add("John");
names.Add("Jim");
names.Add("Jack");
names.Add("Denny");

Console.WriteLine("Second iteration");
foreach (string name in namesWithJ)
{
    Console.WriteLine(name);
}

运行结果为

First iteration
Juan

Second iteration
Jack
Jim
John
Juan

从执行结果可以看出，当在定义namesWithJ时并不会执行，而是在执行每个foreach语句时进行，所以后面增加的“John”、“Jim”、“Jack”和“Denny”在第二次迭代时也会参与进来。

ToArray()、ToList()等方法可以改变这个操作，把namesWithJ的定义语句修改为

var namesWithJ = (from n in names
                  where n.StartsWith("J")
                  orderby n
                  select n).ToList();

运行结果为

First iteration
Juan

Second iteration
Juan

在日常工作中，我们常会使用 datas.Where(x=>x.XX == XXX).FirstOrDefault() 和 datas.FirstOrDefault(x=>x.XX == XXX)，其实这两种写法性能是等效的，如果真的要在性能上分个高低，请见下面

C# Linq.FirstOrDefault、Linq.Where、Linq.AsParallel、List.Exists、List.Find、Dictionar.TryGetValue、HashSet.Contains 性能的比较

今天我们来比较一下集合检索方法性能更优问题，测试代码

public class Entity
{
    public int Id { get; set; }
    public int No { get; set; }
    public string Col1 { get; set; }
    public string Col2 { get; set; }
    public string Col3 { get; set; }
    public string Col4 { get; set; }
    public string Col5 { get; set; }
    public string Col6 { get; set; }
    public string Col7 { get; set; }
    public string Col8 { get; set; }
    public string Col9 { get; set; }
    public string Col10 { get; set; }
}
static void TestFindVelocity(int totalDataCount, int executeCount)
{
    #region 构造数据
    List<Entity> datas = new List<Entity>();
    for (int i = 0; i < totalDataCount; i++)
    {
        var item = new Entity
        {
            No = i + 1,
            Col1 = Guid.NewGuid().ToString("N"),
            Col2 = Guid.NewGuid().ToString("N"),
            Col3 = Guid.NewGuid().ToString("N"),
            Col4 = Guid.NewGuid().ToString("N"),
            Col5 = Guid.NewGuid().ToString("N"),
            Col6 = Guid.NewGuid().ToString("N"),
            Col7 = Guid.NewGuid().ToString("N"),
            Col8 = Guid.NewGuid().ToString("N"),
            Col9 = Guid.NewGuid().ToString("N"),
            Col10 = Guid.NewGuid().ToString("N"),
        };
        datas.Add(item);
    }
    #endregion
    var dicDatas = datas.ToDictionary(x => x.No);
    var hashSetDatas = datas.ConvertAll<Tuple<int, int>>(x => new Tuple<int, int>(x.No, x.No + 1000)).ToHashSet();
    Stopwatch sw = new Stopwatch();
    Random random = new Random();
    Entity searchResult = null;
    bool searchResultBool = false;
    // 每次查询索引
    List<int> indexs = Enumerable.Range(1, executeCount).Select(x => random.Next(1, totalDataCount)).ToList();

    sw.Start();
    for (int i = 0; i < executeCount; i++)
    {
        searchResult = datas.FirstOrDefault(x => x.No == indexs[i]);
    }
    sw.Stop();
    Console.WriteLine($"list FirstOrDefault 耗时：{sw.ElapsedMilliseconds}");

    sw.Restart();
    for (int i = 0; i < executeCount; i++)
    {
        searchResult = datas.Where(x => x.No == indexs[i]).First();
    }
    sw.Stop();
    Console.WriteLine($"list Where+First 耗时：{sw.ElapsedMilliseconds}");

    sw.Restart();
    for (int i = 0; i < executeCount; i++)
    {
        searchResultBool = datas.Exists(x => x.No == indexs[i]);
    }
    sw.Stop();
    Console.WriteLine($"list Exist 耗时：{sw.ElapsedMilliseconds}");

    sw.Restart();
    for (int i = 0; i < executeCount; i++)
    {
        searchResult = datas.Find(x => x.No == indexs[i]);
    }
    sw.Stop();
    Console.WriteLine($"list Find 耗时：{sw.ElapsedMilliseconds}");

    sw.Restart();
    for (int i = 0; i < executeCount; i++)
    {
        dicDatas.TryGetValue(indexs[i], out searchResult);
    }
    sw.Stop();
    Console.WriteLine($"dictionary TryGetValue 耗时：{sw.ElapsedMilliseconds}");

    sw.Restart();
    for (int i = 0; i < executeCount; i++)
    {
        searchResultBool = hashSetDatas.Contains(new Tuple<int, int>(indexs[i], indexs[i] + 1000));
    }
    sw.Stop();
    Console.WriteLine($"Hashset contains 耗时：{sw.ElapsedMilliseconds}");
}

结果

应避免错误写法是 datas.Where(x=>x.XX == XXX).ToList()[0] 。

总结