在泛型方法中實例化泛型類型

Question

我有這樣的結構

void Main()
{
    var lol = ActionClass.GetTestTuple<Request, Response, RequestCallInfo>("lol", "user");
    lol.Dump();
}

public class ActionClass
{
    public static Tuple<TRes, TInfo> GetTestTuple<TReq, TRes, TInfo>(string resultMsg, string userName)
    where TReq : BaseRequest, new()
    where TRes : BaseResponse, new()
    where TInfo : CallInfo<TReq>, new()
    {
        var response = new TRes { Message = resultMsg };
        var eventsInfo = new TInfo();
        eventsInfo.Data.UserName = userName;

        return new Tuple<TRes, TInfo>(response, eventsInfo);
    }
}

public class Request : BaseRequest
{
}

public class Response : BaseResponse
{
}

public class RequestCallInfo : CallInfo<Request>
{
    public string Item { get; set; }
}

public class CallInfo<GenericType> : BaseCallInfo where GenericType : BaseRequest, new()
{
    public GenericType Data { get; set; }

    public CallInfo(GenericType x)
    {
        Data = x;
    }
}

public class BaseCallInfo
{
    public string CallItem { get; set; }
}

public class BaseRequest
{
    public string UserName { get; set; }
}

public class BaseResponse
{
    public string Message { get; set; }
}

當我執行它時，我得到UserQuery.CallInfo<UserQuery.Request> does not contain a constructor that takes 0 arguments,

所以我試過這種方式

public static Tuple<TRes, TInfo> GetTestTuple<TReq, TRes, TInfo>(string resultMsg, string userName)
    where TReq : BaseRequest, new()
    where TRes : BaseResponse, new()
    where TInfo : CallInfo<TReq>, new()
{
    var response = new TRes { Message = resultMsg };
    var eventsInfo = new TInfo(new TReq { UserName = userName });
    eventsInfo.Data.UserName = userName;

    return new Tuple<TRes, TInfo>(response, eventsInfo);
}

但我得到'TInfo': impossible to provide arguments when instantiating a type

如何將TInfo實例TReq給TInfo並僅在CallInfo<GenericType>保留參數構造函數？

Answer 1

new()約束只要求泛型類型具有公共參數構造函數; 無法在C＃中的泛型類型/約束上指定特定的構造簽名。 鑒於此，編譯器無法知道調用者可以使用哪些構造函數，因此您無法使用構造函數參數實例化泛型類型。

因此，如果您願意，可以在此處使用反射，但根據您提供的代碼，此解決方案似乎更簡單：

var eventsInfo = new TInfo() { Data  = new TReq { UserName = userName } };

Answer 2

您應該使用Activator.CreateInstance方法。

工作方案：

public class ActionClass
{
    public static Tuple<TRes, TInfo> GetTestTuple<TReq, TRes, TInfo>(string resultMsg, string userName)
        where TReq : BaseRequest, new()
        where TRes : BaseResponse, new()
        where TInfo : CallInfo<TReq>
    {
        var response = new TRes { Message = resultMsg };
        var eventsInfo = (TInfo)Activator.CreateInstance(typeof(TInfo), new []{ new TReq() });
        eventsInfo.Data.UserName = userName;

        return new Tuple<TRes, TInfo>(response, eventsInfo);
    }
}

public class BaseCallInfo
{
    public string CallItem { get; set; }
}

public class BaseRequest
{
    public string UserName { get; set; }
}

public class BaseResponse
{
    public string Message { get; set; }
}

public class Request : BaseRequest
{
}

public class Response : BaseResponse
{

}

public class RequestCallInfo : CallInfo<Request>
{
    public string Item { get; set; }

    public RequestCallInfo(Request x) : base(x)
    {

    }
}

public class CallInfo<GenericType> : BaseCallInfo where GenericType : BaseRequest, new()
{
    public GenericType Data { get; set; }

    public CallInfo(GenericType x)
    {
        Data = x;
    }
}

}