instantiate generic type in generic method

Question

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>?

OT: signature constraints are in the specs for c#7. i really hope they don't ditch it again. — Dbl
– Dbl, Commented Mar 23, 2016 at 17:30

CodingGorilla · Accepted Answer · 2016-03-23 16:53:06Z

1

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 } };

edited Mar 23, 2016 at 16:53

answered Mar 23, 2016 at 16:46

CodingGorilla

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15 Comments

Matthew Whited Over a year ago

It would be so nice if we could declare constructors on Interfaces.

Sriram Sakthivel Over a year ago

@MatthewWhited That can be easily worked-around with factories

D Stanley Over a year ago

@Phate01 then you violate the new() constraint that specifies that TInfo must have a parameterless constructor.

D Stanley Over a year ago

@icbytes That would compile , but since there's still no guarantee that the actual class has a constructor that matches the desired signature, it's susceptible to errors at runtime.

D Stanley Over a year ago

@icbytes CreateInstance does not do that automatically. You would have to look for a constructor that matches, and if one does not exist, then call the parameterless one. It's possible but messy.

|

invernomuto · Accepted Answer · 2016-03-27 16:20:12Z

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;
    }
}

}

As several people have commented on the other answer, this will fail at runtime for any class that does not have a matching constructor signature. While this will work given the specific code the OP provided, this is a very bad practice (IMO) as it presents potential bugs to another implementer of the CallInfo<TReq> in the future who isn't aware of this code.

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