In C#, how to instantiate a passed generic type inside a method?

Question

How can I instantiate the type T inside my InstantiateType method below?

I\'m getting the error: \'T\' is a \'type parameter\' but is u

Answer 1

To extend on the answers above, adding where T:new() constraint to a generic method will require T to have a public, parameterless constructor.

If you want to avoid that - and in a factory pattern you sometimes force the others to go through your factory method and not directly through the constructor - then the alternative is to use reflection (Activator.CreateInstance...) and keep the default constructor private. But this comes with a performance penalty, of course.

Answer 2

You can also use reflection to fetch the object's constructor and instantiate that way:

var c = typeof(T).GetConstructor();
T t = (T)c.Invoke();

Answer 3

Using a factory class to build your object with compiled lamba expression: The fastest way I've found to instantiate generic type.

public static class FactoryContructor<T>
{
    private static readonly Func<T> New =
        Expression.Lambda<Func<T>>(Expression.New(typeof (T))).Compile();

    public static T Create()
    {
        return New();
    }
}

Here is the steps I followed to set up the benchmark.

Create my benchmark test method:

static void Benchmark(Action action, int iterationCount, string text)
{
    GC.Collect();
    var sw = new Stopwatch();
    action(); // Execute once before

    sw.Start();
    for (var i = 0; i <= iterationCount; i++)
    {
        action();
    }

    sw.Stop();
    System.Console.WriteLine(text + ", Elapsed: {0}ms", sw.ElapsedMilliseconds);
}

I've also tried using a factory method:

public static T FactoryMethod<T>() where T : new()
{
    return new T();
}

For the tests I've created the simplest class :

public class A { }

The script to test:

const int iterations = 1000000;
Benchmark(() => new A(), iterations, "new A()");
Benchmark(() => FactoryMethod<A>(), iterations, "FactoryMethod<A>()");
Benchmark(() => FactoryClass<A>.Create(), iterations, "FactoryClass<A>.Create()");
Benchmark(() => Activator.CreateInstance<A>(), iterations, "Activator.CreateInstance<A>()");
Benchmark(() => Activator.CreateInstance(typeof (A)), iterations, "Activator.CreateInstance(typeof (A))");

Results over 1 000 000 iterations:

new A(): 11ms

FactoryMethod A(): 275ms

FactoryClass A .Create(): 56ms

Activator.CreateInstance A (): 235ms

Activator.CreateInstance(typeof (A)): 157ms

Remarks: I've tested using both .NET Framework 4.5 and 4.6 (equivalent results).

Answer 4

Declare your method like this:

public string InstantiateType<T>(string firstName, string lastName) 
              where T : IPerson, new()

Notice the additional constraint at the end. Then create a new instance in the method body:

T obj = new T();    

Answer 5

Instead of creating a function to Instantiate the type

public T InstantiateType<T>(string firstName, string lastName) where T : IPerson, new()
    {
        T obj = new T();
        obj.FirstName = firstName;
        obj.LastName = lastName;
        return obj;
    }

you could have done it like this

T obj = new T { FirstName = firstName, LastName = lastname };

Answer 6

you want new T(), but you'll also need to add , new() to the where spec for the factory method