Create Generic object inside class [duplicate]

Question

I am making a base class from which other classes can be derived.

public class BaseClass<T> where T { public BaseClass() { TClassObject = new T("SomeText"); // Error here } public T TClassObject { get; set; } }

'T': cannot provide arguments when creating an instance of a variable type.

What I am missing here.

That works for no parameters constructor. My case is with a parameter, — fhnaseer
– fhnaseer, Commented Apr 9, 2013 at 13:10
Well, you don't read the post attentively enough. The post give you the exact procedure to resolve your problem. — Cyril Gandon
– Cyril Gandon, Commented Apr 9, 2013 at 13:12

Conrad Clark · Accepted Answer · 2013-04-09 12:59:37Z

5

From MSDN:

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor.

So it needs to be parameterless. You may want to look at Activator.CreateInstance

http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx

answered Apr 9, 2013 at 12:59

Conrad Clark

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6 Comments

fhnaseer Over a year ago

I have parameters to my generic object. Will Activator work for this?

Conrad Clark Over a year ago

Yes. msdn.microsoft.com/en-us/library/wcxyzt4d.aspx. Creates an instance of the specified type using the constructor that best matches the specified parameters. Check it out.

fhnaseer Over a year ago

What if I remove new()?

Conrad Clark Over a year ago

No problem, since you won't be calling new() anyway. Just be careful not to hurt your code readability and maintenance by using Activator.CreateInstance. You could also accept a Func<T> as a parameter for the BaseClass constructor, and write it like this: BaseClass<Orange> orangeBase = new BaseClass<Orange>(()=> new Orange("tasty");

Conrad Clark Over a year ago

(T) Activator.CreateInstance(typeof(T),"SomeText");

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Community · Accepted Answer · 2017-05-23 11:49:58Z

The where T : new() constraint states that T must have a parameterless constructor. Your code is calling into a constructor that takes a string parameter, and it isn't guaranteed that your T will have such a constructor.

It is not possible in C# to create a constraint on a specific constructor signature. If you need this functionality, you're better off using something like one of the answers in this thread.

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