How to create an instance for a given Type?

Question

With generics you can

var object = default(T);

But when all you have is a Type instance I could only

constructor = type.GetConstructor(Type.EmptyTypes);
var parameters = new object[0];
var obj = constructor.Invoke(parameters);

or even

var obj = type.GetConstructor(Type.EmptyTypes).Invoke(new object[0]);

Isn't there a shorter way, like the generics version?

Solution 1

The closest available is Activator.CreateInstance:

object o = Activator.CreateInstance(type);

... but of course this relies on there being a public parameterless constructor. (Other overloads allow you to specify constructor arguments.)

I've used an explicitly typed variable here to make it clear that we really don't have a variable of the type itself... you can't write:

Type t = typeof(MemoryStream);
// Won't compile
MemoryStream ms = Activator.CreateInstance(t);

for example. The compile-time type of the return value of CreateInstance is always object.

Note that default(T) won't create an instance of a reference type - it gives the default value for the type, which is a null reference for reference types. Compare that with CreateInstance which would actually create a new object.

Solution 2

var myObject = Activator.CreateInstance(myType)

You have to cast if you want to use a typed parameter:

User user = (User)Activator.CreateInstance(typeof(User));

.. or with parameters

User user = (User)Activator.CreateInstance(typeof(User), new object[]{firstName, lastName});

You can also use generics:

public T Create<T>() where T : class, new()
{
    return new T();
}

var user = Create<User>();

Comments

[Jonas Van der Aa]

+1 for the second approach. I really like this one :)

[Brian Hinchey]

If the type is known at compile time, then you might as well use Activator.CreateInstance<T>().

[nawfal]

Why wouldn't you not mention Activator.CreateInstanc<T>() ?

[nawfal]

why restrict to classes? :)

[Jon Skeet]

@nawfal: Because the OP doesn't know T - only a Type (as an execution-time value).

[nawfal]

Jon yes, the last line of the question confused me.

[jgauffin]

@nawfal: Value objects are for unicorns.

[Hanfeng]

If I already know the Type User, why shouldn't I use the new method directly, the question is how to create object elegantly only with the System.Type variable