Create instance of object with Generics

Question

I have the following:

public class Query : IAsyncRequest<Envelope<Reply>> { }

public class Reply { }

public class Flow<TQuery, TReply> where TQuery: IAsyncRequest<Envelope<TReply>>, IRequest<Envelope<TReply>> {

  public TQuery Query { get; set; }
  public TReply Reply { get; set; }

  public Flow(TQuery query, TReply reply) {
    Query = query;
    Reply = reply;
  }
}

When I try to create an instance of Flow as

Query query = service.GetQuery();
Reply reply = service.GetReply();
Flow flow = new Flow(query, reply);

I get the error:

Using the generic type 'Flow<TQuery, TReply>' requires 2 type arguments

Can't I create a Flow this way?

Possible duplicate of [Why can't the C# constructor infer type?](http://stackoverflow.com/questions/3570167/why-cant-the-c-sharp-constructor-infer-type) — NineBerry, Oct 09 '16 at 14:25

score 2 · Answer 1 · edited May 23 '17 at 10:27

No you can't, as said in @NineBerry's answer, the syntax new type must be the "real" type with mandatory generics arguments so you must write :

Flow<Query, Reply> flow = new Flow<Query, Reply> (query, reply);

That said, that constraint doesn't apply to a method, so you could write a static method to do the job :

static class Flow // not generic (same name isn't mandatory)
{
    public static Flow<TQuery, TReply> Create (TQuery query, TReply, reply) where /* constraint */
    {
        return new Flow<TQuery, TReply> (query, reply);
    }
}

// usage
Flow<Query, Reply> flow = Flow.Create (query, reply);
// or with var
var flow = Flow.Create (query, reply);

Ah, Factory pattern to the rescue :) ... nice alternative. – Bernhard Koenig Oct 09 '16 at 14:31 — Bernhard Koenig, Oct 09 '16 at 14:31

score 1 · Answer 2 · edited May 23 '17 at 11:55

1

You need to specify the types explicitly. Type inference is not supported here. See also Why can't the C# constructor infer type?

Query query = service.GetQuery();
Reply reply = service.GetReply();
var flow = new Flow<Query, Reply>(query, reply);

edited May 23 '17 at 11:55

Community

1
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answered Oct 09 '16 at 14:26

NineBerry

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you forgot generic arguments on the first Flow (flow's type) ;) – Sehnsucht Oct 09 '16 at 14:27
@Sehnsucht Thanks. Fixed – NineBerry Oct 09 '16 at 14:30

Bernhard Koenig · Answer 3 · 2016-10-09T14:29:47.240

0

No, you have to define the types you want to use:

Flow<Query, Reply> flow = new Flow<Query, Reply>(query, reply);

C# constructors do not support type inference.

edited Oct 09 '16 at 14:29

answered Oct 09 '16 at 14:27

Bernhard Koenig

1,342
13
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you forgot generic arguments on the first Flow (flow's type) ;) – Sehnsucht Oct 09 '16 at 14:29
Thanks :) ... I fixed that. – Bernhard Koenig Oct 09 '16 at 14:30

Create instance of object with Generics

3 Answers3