c# - 是否可以创建可以在 C# 中多次嵌套泛型类型的流畅接口(interface)

Question

我正在尝试创建一个描述嵌套结构的 Fluent API，因此我想使用泛型来键入生成的对象。我想做类似的事情:

var test = new Query<Entity>().Select(x => x.UniqueValue<string>())
                              .Select(x => x.UniqueValue<DateTime>())
                              .Select(x => x.UniqueValue<decimal>());

测试的类型:

Result<Entity, string, Result<Entity, DateTime, Result<Entity, decimal>>>

到目前为止，我的代码如下:

   class Program
{
    static void Main(string[] args)
    {
        // test : ResultWithResult<Entity,string, Result<Entity, DateTime, ChildLessResult<Entity, decimal>>>
        var test = new Query<Entity>().Select(x => x.UniqueValue<string>())
                                      .Select(x => x.UniqueValue<DateTime>())
                                      .Select(x => x.UniqueValue<decimal>());
    }
}

public class Entity
{
    public string Item1 { get; set; }
}

public class Aggregate<TEntity>
{
    public ChildLessResult<TEntity, TReturnValue> UniqueValue<TReturnValue>()
    {
        return new ChildLessResult<TEntity, TReturnValue>();
    }
}

public class Query<TEntity>
{
    public ChildLessResult<TEntity, TReturnValue> Select<TReturnValue>(Func<Aggregate<TEntity>, ChildLessResult<TEntity, TReturnValue>> predicate)
    {
        var aggregator = new Aggregate<TEntity>();
        return predicate(aggregator);
    }
}

public class ChildLessResult<TEntity, TReturnValue>
{
    public Result<TEntity, TReturnValue, TChild> Select<TChild>(Func<Aggregate<TEntity>, ChildLessResult<TEntity, TChild>> predicate)
    {
        return new Result<TEntity, TReturnValue, TChild>();
    }
}

public class Result<TEntity, TReturnValue, TChild>
{
    private ChildLessResult<TEntity, TChild> _child = new ChildLessResult<TEntity, TChild>();

    public ResultWithResult<TEntity, TReturnValue, Result<TEntity, TChild, ChildLessResult<TEntity, TNewChild>>> Select<TNewChild>(Func<Aggregate<TEntity>, ChildLessResult<TEntity, TNewChild>> predicate)
    {
        return new ResultWithResult<TEntity, TReturnValue,
            Result<TEntity, TChild, ChildLessResult<TEntity, TNewChild>>>();
    }
}

public class ResultWithResult<TEntity, TReturnValue, TChild>
{
}

如您所见，这是非常有限的，因为它只允许您深入到您定义的结构，并且您需要为每个更深的级别创建一个新结构。是否可以创建允许无限数量级别的类型(即，在没有相同相应数量类型的情况下，您可以根据需要选择任意数量的选择)。

Answer 1

是的，有点……可以或多或少地自动创建一些很长很深的嵌套类型。诀窍是你需要使用泛型类型推断，因为 constructors don't do inference ，你必须使用工厂方法。例如:

public class MyType<TData,TChild>
{
    public MyType(TData data, TChild child)
    {
    }
}

public class MyTypeFactory
{
   public static MyType<TData,TChild> Create<TData,TChild>(TData data, TChild child)
   {
      return new MyType<TData,TChild>(data, child);
   }
    
   public static MyType<TData,object> Create<TData>(TData data)
   {
      return new MyType<TData,object>(data, null);
   }
}

public static class Program
{
    static public void Main()
    {
        var grandchild  = MyTypeFactory.Create( 12 );
        var child       = MyTypeFactory.Create( 13D, grandchild );
        var parent      = MyTypeFactory.Create( 14M, child) ;
        var grandparent = MyTypeFactory.Create( 15F, parent );
        
        Console.WriteLine(  grandchild.GetType().FullName );
        Console.WriteLine(       child.GetType().FullName );
        Console.WriteLine(      parent.GetType().FullName );
        Console.WriteLine( grandparent.GetType().FullName );
    }
}

输出:

MyType'2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]

MyType'2[[System.Double, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]]

MyType'2[[System.Decimal, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Double, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]]

MyType'2[[System.Single, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Decimal, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Double, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[MyType'2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]], uy2zelya, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null]]

它甚至是类型安全的，这已经足够令人惊讶了。如果您添加必要的属性...

public class MyType<TData,TChild>
{
    protected readonly TChild _child;
    protected readonly TData _data;
    
    public MyType(TData data, TChild child)
    {
        _data = data;
        _child = child;
    }
    
    public TChild Child
    {
        get
        {
            return _child;
        }
    }
    
    public TData Data
    {
        get
        {
            return _data;
        }
    }
}

...这会起作用:

var grandchildValue = grandparent.Child.Child.Child.Data;
Console.WriteLine(grandchildValue); //12
Console.WriteLine(grandchildValue.GetType().Name); //int32

当然你可以使用构建器模式:

public static class ExtensionMethods
{
    static public MyType<TDataAdd,MyType<TDataIn,TResultIn>> AddColumn<TDataIn,TResultIn,TDataAdd>(this MyType<TDataIn,TResultIn> This, TDataAdd columnValue)
    {
        return MyTypeFactory.Create(columnValue, This);
    }
}

var grandparent = MyTypeFactory.Create( 12 )
    .AddColumn( 13D )
    .AddColumn( 14M )
    .AddColumn( 15F );

var grandchildValue = grandparent.Child.Child.Child.Data;
Console.WriteLine(grandchildValue);                       //12
Console.WriteLine(grandchildValue.GetType().FullName);    //System.Int32

See my code on DotNetFiddle

附言这是一个很好的例子，说明了为什么我们需要 var 关键字!否则你将永远打字。