c# - 使用 Linq 查询丢失 foreach

Question

我想知道是否可以(如果可能)用 LINQ 查询替换此 foreach:

在线围栏:https://ideone.com/PQEytf

using System;
using System.Collections.Generic;
using System.Linq;

public class Test
{
    public static void Main()
    {
        // dummy inputs for test sake
        var keys = new[] { "key1", "key2" };

        var services = new Dictionary<string, Service>
        {
            {"key1", new Service {Components = new Dictionary<string, string> {{"comp1", "value1"}}}},
            {"key2", new Service {Components = new Dictionary<string, string> {{"comp2", "value2"}}}}
        };


        var serviceComponents = GetServiceComponents(keys, services);
        // do something with it
    }

    public static IEnumerable<ServiceComponent> GetServiceComponents(string[] keys, Dictionary<string, Service> services)
    {
        var serviceComponents = new List<ServiceComponent>();

        // this is the foreach that I want to lose
        foreach (
            var key in
                keys.Select(
                    name =>
                        services.FirstOrDefault(
                            x => x.Key.Equals(name))))
        {
            serviceComponents.AddRange(key.Value.Components.Select(component => new ServiceComponent
            {
                Name = key.Key,
                Service = component.Key,
                Value = component.Value,
            }));
        }

        return serviceComponents.ToArray();

    }
}

public class Service
{
    public Dictionary<string, string> Components { get; set; }
}

public class ServiceComponent
{
    public string Name { get; set; }
    public string Service { get; set; }
    public string Value { get; set; }
}

Answer 1

是的，您正在寻找的是SelectMany。这允许您将序列中的每个项目转换为另一个序列，然后将所有这些序列展平为一个序列。 (通过将所有序列放入列表中，您可以完成相同的事情，而无需延迟执行。)

return keys.SelectMany(name => services.FirstOrDefault(x => x.Key.Equals(name))
    .Value.Components
    .Select(component => new ServiceComponent
    {
        Name = name.Key,
        Service = component.Key,
        Value = component.Value,
    }))
    .ToArray();

话虽如此，此查询所做的是获取您的每个键，使用线性搜索在服务中找到相应的项目，然后映射结果。无需使用 FirstOrDefault 进行线性搜索，您可以使用字典的 native 功能有效且高效地查找每个键的值:

return keys.SelectMany(key => services[key].Components
    .Select(component => new ServiceComponent
    {
        Name = key,
        Service = component.Key,
        Value = component.Value,
    }))
    .ToArray();