在接口的多个实现中打破 SOLID 原则

    public interface IWriter
    {
        void WriteToStorage(string data);
    }
    public class FileWriter : IWriter
    {
        public void WriteToStorage(string data)
        {
            //write to file
        }
    }
    public class DBWriter : IWriter
    {
        public void WriteToStorage(string data)
        {
            //write to DB
        }
    }

public interface IFactory
{
    IWriter GetType(string outputType);
}
public class Factory : IFactory
{
    public IWriter GetType(string outputType)
    {
        IWriter writer = null;
        if (outputType.Equels("db"))
        {
            writer = new FileWriter();
        }
        else if (outputType.Equels("db"))
        {
            writer = new DBWriter();
        }
    }
}

public interface ISaveDataFlow
{
    void SaveData(string data, string outputType);
}
public class SaveDataFlow : ISaveDataFlow
{
    private IFactory _writerFactory = null;
    public SaveDataFlow(IFactory writerFactory)
    {
        _writerFactory = writerFactory;
    }
    public void SaveData(string data, string outputType)
    {
        IWriter writer = _writerFactory.GetType(outputType);
        writer.WriteToStorage(data);
    }
}

public class SaveDataFlow : ISaveDataFlow
{
    private IWriter _dbWriter = null;
    private IWriter _fileWriter = null;
    public SaveDataFlow([Dependency("DB")]IWriter dbWriter,
                        [Dependency("FILE")]IWriter fileWriter)
    {
        _dbWriter = dbWriter;
        _fileWriter = fileWriter;
    }
    public void SaveData(string data, string outputType)
    {
        if (outputType.Equals("DB"))
        {
            _dbWriter.WriteToStorage(data);
        }
        else if (outputType.Equals("FILE"))
        {
            _fileWriter.WriteToStorage(data);
        }
    }
}

container.RegisterType<IWriter, DBWriter>("DB");
container.RegisterType<IWriter, FileWriter>("FILE");

我会简单地将其转换为策略模式：

namespace UnityMutliTest
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using Microsoft.Practices.Unity;
    class Program
    {
        static void Main(string[] args)
        {
            IUnityContainer container = new UnityContainer();
            container.RegisterType<IWriter, FileWriter>("file");
            container.RegisterType<IWriter, DbWriter>("db");
            container.RegisterType<IWriterSelector, WriterSelector>();
            var writerSelector = container.Resolve<IWriterSelector>();
            var writer = writerSelector.SelectWriter("FILE");
            writer.Write("Write me data");
            Console.WriteLine("Success");
            Console.ReadKey();
        }
    }
    interface IWriterSelector
    {
        IWriter SelectWriter(string output);
    }
    class WriterSelector : IWriterSelector
    {
        private readonly IEnumerable<IWriter> writers;
        public WriterSelector(IWriter[] writers)
        {
            this.writers = writers;
        }
        public IWriter SelectWriter(string output)
        {
            var writer = this.writers.FirstOrDefault(x => x.CanWrite(output));
            if (writer == null)
            {
                throw new NotImplementedException($"Couldn't find a writer for {output}");
            }
            return writer;
        }
    }
    interface IWriter
    {
        bool CanWrite(string output);
        void Write(string data);
    }
    class FileWriter : IWriter
    {
        public bool CanWrite(string output)
        {
            return output == "FILE";
        }
        public void Write(string data)
        {
        }
    }
    class DbWriter : IWriter
    {
        public bool CanWrite(string output)
        {
            return output == "DB";
        }
        public void Write(string data)
        {
        }
    }
}

您可以拥有任意数量的IWriter，只需注册它们：

container.RegisterType<IWriter, LogWriter>("log");

如果需要，您甚至可以在编写器上实现装饰器。

您使用(名称不好的(IWriterSelector作为如何选择作家的实现，这应该只关注获得作家！这里throw例外非常有用，如果没有适合您需求的实现，它将很快失败！！

如果您遇到Open Closed问题，请使用策略或模板模式来克服。

我一直使用这种模式，效果很好。

我创建了一个小的扩展方法来防止您必须命名实例：

static class UnityExtensions
{
    public static void RegisterMultipleType<TInterface, TConcrete>(this IUnityContainer container)
    {
        var typeToBind = typeof(TConcrete);
        container.RegisterType(typeof(TInterface), typeToBind, typeToBind.Name);
    }
}
container.RegisterMultipleType<IWriter, FileWriter>();

解决方案 1

在实例化之前选择并使用作用域

using(var scope = new Scope(unity))
{
    scope.register<IWriter, ConcreteWriter>();
    var flow = scope.Resolve<ISaveDataFlow>();
}

解决方案 2

在运行时注入您的策略。

ISaveDataFlow flow = ....
IWriter writer = GetWriterBasedOnSomeCondition();
flow.SaveData(data, writer);

我怀疑解决方案2更接近您要实现的目标。请记住，您无需传递字符串来描述要使用的strategy。

您可以传递要使用的实际strategy，在本例中为要使用的实际IWriter。

然后，您可以做的是在每个IWriter上都有元数据，以帮助用户选择要使用的IWriter。

例如

public interface IWriter
{
   void WriteData(data);
   string Name {get;}
}
void GetWriterBasedOnSomeCondition()
{
    Dictionary<string, IWriter> writers = ...ToDictionary(x => x.Name);
    var choice = Console.ReadLine();
    return writers[choice];
}

我倾向于使用这些方法之一。

1. 闯入不同的界面

public interface IWriter
{
    void WriteToStorage(string data);
}
public interface IFileWriter : IWriter
{
}
public interface IDBWriter: IWriter
{
}
public class FileWriter : IFileWriter 
{
    public void WriteToStorage(string data)
    {
        //write to file
    }
}
public class DBWriter : IDBWriter
{
    public void WriteToStorage(string data)
    {
        //write to DB
    }
}

优点：您可以根据接口注入正确的实现，这不会破坏 OCP。

缺点：你有空的接口。

---

2. 使用枚举将它们分开(策略模式(

public interface IWriter
{
    void WriteToStorage(string data);
    StorageType WritesTo { get; }
}
public enum StorageType 
{
    Db = 1,
    File = 2
}
public class Factory : IFactory
{
    public IEnumerable<IWriter> _writers;
    public Factory(IWriter[] writers)
    {
        _writers = writers;
    }
    public IWriter GetType(StorageType outputType)
    {
        IWriter writer = _writers.FirstOrDefault(x => x.WritesTo == outputType);
        return writer;
    }
}

优点：您可以同时注入它们，然后使用枚举使用您想要的那个。

缺点：我想它有点破坏 OCP 原则，就像你的第一个例子一样。

更多关于战略模式的信息，请看马克·西曼(Mark Seemann(的这个精彩回答。

---

3. 构建一个基于函数创建项目的工厂。

在您的注册中：

container.RegisterType<IWriter, DBWriter>("DB");
container.RegisterType<IWriter, FileWriter>("FILE");
container.RegisterType<IFactory, Factory>(
    new ContainerControlledLifetimeManager(),
    new InjectionConstructor(
        new Func<string, IWriter>(
            writesTo => container.Resolve<IWriter>(writesTo));

还有您的工厂

public class Factory : IFactory
{
    private readonly Func<string, IWriter> _createFunc;
    public Factory(Func<string, IWriter> createFunc)
    {
        _createFunc = createFunc;
    }
    public IWriter CreateScope(string writesTo)
    {
        return _createFunc(writesTo);
    }
}

优点：将整个依赖项移动到注册。

缺点： 服务定位器模式的包装器。可能有点难以阅读。

---

上面没有一个示例是完美的，因为它们中的每一个都有其优点和缺点。

类似的问题在这里：注入需要对象取决于构造函数注入中的条件

在 .NET Core 中(从问题中不清楚正在使用什么框架(，您可以使用内置 DI 使用很少的代码轻松实现策略模式。

在Startup.ConfigureServices：

services
    .AddScoped<IWriter, FileWriter>()
    .AddScoped<IWriter, DBWriter>()
    .AddScoped<ISaveDataFlow, SaveDataFlow>();

为策略算法添加一个要IWriter的方法：

public interface IWriter
{
    bool CanWrite(string outputType);
    void WriteToStorage(string data);
}
public class FileWriter : IWriter
{
    bool CanWrite(string outputType) => outputType == "FILE";
    public void WriteToStorage(string data) {}
}
public class DBWriter : IWriter
{
    bool CanWrite(string outputType) => outputType == "DB";
    public void WriteToStorage(string data) {}
}

然后将 SaveDataFlow 的构造函数更改为使用集合类型，并将SaveData更改为调用所有解析IWriter类型的算法方法。

public class SaveDataFlow : ISaveDataFlow
{
    private readonly IWriter _writers;
    public SaveDataFlow(IEnumerable<IWriter> writers)
    {
        _writers= writers;
    }
    public void SaveData(string data, string outputType)
    {
        _writers.Single(w => w.CanWrite(outputType)).WriteToStorage(data);
    }
}

这现在符合开放/封闭原则，因为具体选择仅在具体类本身内。