Generics in C# - how can I create an instance of a variable type with an argument?

Question

I've got a generics class, where I want to instantiate an object with the generic type. I want to use an argument for the constructor of the type.

My code:

public class GenericClass<T> where T : Some_Base_Class, new()
{
    public static T SomeFunction(string s)
    {
        if (String.IsNullOrEmpty(s))
            return new T(some_param);
    }
}

I get an error on the

new T(some_param)

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

Any ideas how can I do this?

Are you going to do doing this frequently? What version of C#? — Ben Voigt, Jun 20 '11 at 11:26
[this](http://stackoverflow.com/questions/840261/c-generic-new-constructor-problem/840299#840299) is the best workaround i've been able to find.... — J. Ed, Jun 20 '11 at 11:22
Another way is to create a public property on `Some_Base_Class`, and then you can do the following: `new T() { SomeProperty = "value" }` — nZeus, Mar 13 '17 at 19:59

score 141 · Accepted Answer · edited Mar 26 '13 at 14:53

141

Take a look at Activator.CreateInstance. For instance:

var instance = Activator.CreateInstance(typeof(T), new object[] { null, null });

Obviously replacing the nulls with appropriate values expected by one of the constructors of the type.

If you receive a compiler error about cannot convert object to type T, then include as T:

var instance = Activator.CreateInstance(typeof(T), 
                  new object[] { null, null }) as T;

edited Mar 26 '13 at 14:53

p.campbell

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answered Jun 20 '11 at 11:20

Grant Thomas

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That's the correct way, right. – Ignacio Soler Garcia Jun 20 '11 at 11:21
This will work well, although it's slow and not a good choice if you have to construct the same object type many times. – Ben Voigt Jun 20 '11 at 11:51
9

Note the `as T` in `var instance = Activator.CreateInstance(typeof(T), new object[] { null, null }) as T;` – Valamas Oct 03 '11 at 23:19
4

And this works only if T is not a value type. For example if you need to use it with SystemNumerics.Complex which is a struct, then cast to (T) instead of using 'as T' – Lord of Scripts Feb 03 '15 at 16:00
1

I like neatness, so you can actually write it as this `Activator.CreateInstance(typeof(T), options) as T` which will work too – r3plica Oct 13 '20 at 09:33

Generics in C# - how can I create an instance of a variable type with an argument?

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