當調用方法

Question

指定子類（或任何類）作爲返回類型時應該如何設置Apply方法，以便當我調用它時，可以指定我想要的返回類型BaseResult？

目的是讓調用代碼知道是什麼子類（實施BaseResult），該應用將返回（再次，考慮到從IRequest調用應用，而不是在它的實現）

的條件：

• 可能有許多實現IRequest
• 可能有很多實現的BaseResult

代碼：

void Main() 
{ 
    // For example purpose, let's pretend 'myreq' is retrieved from reflection. So, there's no way I would know it's type of MyRequest. And there will be many other implementations of IRequest. 
    var req = new MyRequest(); 

    var request = (IRequest)req; 

    // How should I setup Apply method so that when I call the method, I can specify what BaseResult return type that I want? 
    // In this example, I would like Apply method to return Result type, which inherits from BaseResult. 
    var res = req.Apply<Result>(); 
} 

// Define other methods and classes here 
public interface IRequest 
{ 
    string GetValue(); 
    T Apply<T>() where T : BaseResult; 
} 

public class MyRequest : IRequest 
{ 
    // How should I setup Apply method to allow returning any BaseResult I want? Each IRequest implementation of Apply method may return different BaseResult type. 
    public T Apply<T>() where T : BaseResult 
    { 
     // Doesn't work here 
     // Can't implicitly convert type BaseResult to T 
     return (BaseResult) new Result(); 
    } 

    public string GetValue() { return string.Empty; } 
} 

public class Result : BaseResult 
{ 
    public string Message { get; set;} 
} 

public class AnotherResult : BaseResult 
{ 
    public string Message { get; set; } 
} 

public class BaseResult 
{ 
} 

Answer 1

在你的應用方法，鑄就新的實例爲T.

public T Apply<T>() where T : BaseResult 
{ 
    var newInstance = new Result(); 
    return newInstance as T; 
} 

Answer 2

你能約束的Apply<T>類型有一個默認的構造函數？

public T Apply<T>() where T : BaseResult, new() 
{ 
    //you could put settable properties on BaseResult 
    //(or similar interface) and set them here, as well. 
    return new T(); 
} 

這也需要您指定接口上的約束：

public interface IRequest 
{ 
    string GetValue(); 
    T Apply<T>() where T : BaseResult, new(); 
} 

Answer 3

您可以更改Apply<T>返回類型BaseResult。

public interface IRequest 
{ 
    string GetValue(); 
    BaseResult Apply<T>() where T : BaseResult; 
} 

public class MyRequest : IRequest 
{ 
    public BaseResult Apply<T>() where T : BaseResult 
    { 
     return new Result(); 
    } 

    public string GetValue() { return string.Empty; } 
} 

編輯：想象一下，您需要使用此解決方案進行類型轉換，而不是在調用代碼中。所以它並不能真正解決問題。

編輯2：實際上，它看起來像返回泛型類型服務於此特定用途 - 以避免在調用代碼中投射。鑑於所需的類型在代碼Apply中已知，是否真的有必要？