CODEWITHSUNDEEP
c#generics
Phate01
Phate01

Reputation: 1795

instantiate generic type in generic method

I have such a structure

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; }
}

and when I execute it, I get UserQuery.CallInfo<UserQuery.Request> does not contain a constructor that takes 0 arguments,

So i tried this way

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);
}

but i'm getting 'TInfo': impossible to provide arguments when instantiating a type

How can I pass an instance of TReq to TInfo and keep only the parameter constructor in CallInfo<GenericType>?

Upvotes: 3

Views: 386

Answers (2)

jarres
jarres

Reputation: 71

You should use Activator.CreateInstance method.

Working solution:

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;
    }
}

}

Upvotes: 0

CodingGorilla
CodingGorilla

Reputation: 19842

The new() constraint only requires that a generic type have a public parameter-less constructor; there is no way to specify specific construct signatures on a generic type / constraint in C#. Given that, the compiler cannot know what constructors would be available by callers, so you can't instantiate a generic type using constructor parameters.

So you could use reflection here if you would like, but this solution seems simpler based on the code you've provided:

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

Upvotes: 1

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