Cannot provide arguments when creating an instance of generic type

Question

I have an object that I want to have read only after it is created... namely because the properties in the constructor must be used in GetHashCode, and therefore can't change once created.

I this is one of many classes that are readonly:

public class AccountLabel
{ 
    private string result;

    public AccountLabel(string result)
    {
        // TODO: Complete member initialization
        this.result = result;
    }

    public string JSONRPCData { get { return this.result; } }
}

I have a generic result class like this

  public  class JsonResult<T>  where  T : JObject, new()
  {
    private T bhash;
    private string p;
    private JsonErrorResponse error;
    private int _id;
    private Newtonsoft.Json.Linq.JObject ret;

    public JsonResult(Newtonsoft.Json.Linq.JObject ret)
    { 
        this.ret = ret;

        var tempError = ret["error"];
        var tempid = ret["id"];
        JsonErrorResponse error = new JsonErrorResponse(tempError);
        this.error = error;
        this._id = 1;


        var tempresult = ret["result"];
        T someResult = new T(tempresult);  // <--- here is my problem
    }

My problem is that I want to pass an object into T's constructor but can't. When I type this the compiler tells me Cannot provide arguments when creating an instance of variable type

What is the best way to work around this situation?

Should I have an Interface that I can call that will update the property?
Will that previous Interface break encapsulation or allow changes to be made to my object?
How else should I approach this?

you could create a Factory class that knows how to create T (so JsonResult would call MyFactory<T>.create(tempresult) instead of using new — Paolo Falabella
– Paolo Falabella, Commented Dec 16, 2012 at 20:58
I have edited your title. Please see, "Should questions include “tags” in their titles?", where the consensus is "no, they should not". — John Saunders
– John Saunders, Commented Dec 16, 2012 at 20:58

Sergey Kalinichenko · Accepted Answer · 2018-05-03 21:53:48Z

34

You can remove the new type constraint, and use Activator.CreateInstance instead.

Instead of this

T someResult = new T(tempresult);

write this:

T someResult = (T)Activator.CreateInstance(
    typeof(T)
,   new object[] { tempresult }
);

This is likely to be somewhat slower because of going through reflection, and the static checks by the compiler would not be performed. However, considering your scenario, it does not look like either of these would present a significant problem.

edited May 3, 2018 at 21:53

answered Dec 16, 2012 at 21:01

Sergey Kalinichenko

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Comments

jam40jeff · Accepted Answer · 2016-01-13 16:46:50Z

8

You could pass in a factory delegate to the constructor of JSonResult<T>:

public class JsonResult<T> where T : JObject
{
    public JsonResult(Newtonsoft.Json.Linq.JObject ret, Func<object, T> factory)
    { 
        var tempresult = ret["result"];
        T someResult = factory(tempresult);
    }
}

object in Func<object, T> could be replaced with whatever the type of tempResult actually is.

edited Jan 13, 2016 at 16:46

answered Dec 23, 2012 at 15:01

jam40jeff

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