C# Linq延迟查询的执行实例代码

编辑: admin 分类: c#语言 发布时间: 2021-11-25 来源:互联网

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
{
 【文章出处:http://www.1234xp.com/tbm.html转载请保留出处】   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

(集合数量，测试次数) Linq.FirstOrDefault Linq.Where+First List.Exists List.Find Dictionary.TryGetValue HashSet.Contains

(100, 5000000)

4544 3521 1992 1872 66 924

(1000, 5000000)

41751 29417 20631 19490 70 869

(10000, 5000000)

466918 397425 276409 281647 85 946

(50000, 5000)

6292 4602 4252 3559 0 2

(500000, 5000)

56988 55568 48423 48395 1 5

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

总结

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【文章转自：中东服务器】
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