How to create instance of a generic type passed to the service method?

Question

I am confused a bit about creating instance of generic object. I belive, that it supposed to be casted(somehow). In my code I am calling a service generic method:

jockey = await _scrapDataService.ScrapSingleJockeyPlAsync<LoadedJockey>(id, jobType);

And in my service method I want to achieve this, but I am not sure how to do it:

public Task<T> ScrapGenericObject<T>(int id, string jobType)
        {
            var someObject = new T();
            return someObject;
        }

What do you plan to do with `id` and `jobType`? Do you need to pass those to `T` somehow? — John Wu, Jun 25 '19 at 03:26

TheGeneral · Accepted Answer · 2019-06-25T03:29:24.867

You need the new constraint

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor. To use the new constraint, the type cannot be abstract.

public Task<T> ScrapGenericObject<T>(int id, string jobType) where T : new()
{
    var someObject = new T();
    return someObject;
}

If you need to pass in any constructors you will have to use a different approach Activator.CreateInstance

Creates an instance of the specified type using the constructor that best matches the specified parameters.

 return (T)Activator.CreateInstance(typeof(T), new object[] { param1, param2 });

You are right, I will have to walk it around, if I want to use properties of the object. — bakunet, Jun 25 '19 at 03:37

score 1 · Answer 2 · answered Jun 25 '19 at 03:15

The compiler will only allow this if you have a generic type constraint that tells the compiler that your type has a public parameterless constructor.

You achieve this using the new() constraint:

public Task<T> ScrapGenericObject<T>(int id, string jobType) where T: new()

Then the compiler knows that whatever type you pass in can be constructed via new T().

How to create instance of a generic type passed to the service method?

2 Answers2