I need to figure out how to get the type of a generic parameter in order to
create an instance of this generic type using Activator.CreateInstance().
While there's Activator.CreateInstance<T>() that works with a generic
parameter, it doesn't accept parameters for constructors, so it looks like
CreateInstance() is what I need to use.

However, I can't figure out how to get the explicit type from the <T>
parameter. For example, here's my method signature:

public static TDataContext
GetWebRequestScopedDataContext<TDataContext>(strin g key, string
connectionString)
where TDataContext : DataContext, new()
{
// *** Doesn't work
Type t = typeof(TDataContext);

// *** doesn't work
t = TDataContext;

// *** This is what I need to do...
TDataContext context = Activator.CreateInstance(t,connectionString);
....
}

Is there any way to do this?

Looking at what CreateInstance<T> does with Reflector doesn't help as it
goes off into some external code that's not traceable.

Any ideas?

TIA,

+++ Rick ---

Hello Rick!

You have to construct the concrete Type before calling
Activator.CreateInstance.

e.g.

class X<T> {}

Type genericType = typeof(X<>);
Type concreteType = genericType.MakeGenericType(typeof(int));

object instance = Activator.CreateInstance(concreteType); // will be X<int>





OK?


GP

Hi Guenter,

Thanks for the help, but unfortunately this doesn't address the issue.

The problem is that I don't actually need create a type from the generic
parameter, but need to instantiate the type specified BY the parameter. The
generic parameter is an object and I need to instantiate that type. IOW,
it's not - for example - List<T> that I need to instantiate but T itself.

In essence what I need to do is:

Activator.CreateInstance<T>();

with the added requirement of passing constructor parameters.

+++ Rick ---

--

---
Rick Strahl
West Wind Technologies
www.west-wind.com/weblog


"Günter Prossliner" <g.prossliner/gmx/at> wrote

Type.GetElementType?

--
Sheng Jiang
Microsoft MVP in VC++
"Rick Strahl" <rickstrahl (AT) hotmail (DOT) com> wrote

Thanks Mattias,

I gave this another shot now and sure enough I did get it to work. Somewhere
I think the problem ended up being a missing cast - I apparently
misinterpreted the error as coming from the generic type conversion rather
than from method call signature being incorrect.

Thanks for a sanity check... <g>


+++ Rick ---

--

---
Rick Strahl
West Wind Technologies
www.west-wind.com/weblog


"Mattias Sjögren" <mattias.dont.want.spam (AT) mvps (DOT) org> wrote

Hello Rick!

You just have to use a typeof(T) in order to call the CTOR or to use
Activator:

class C<T> where T:Something{

void foo() {
Type t = typeof(T);
ConstructorInfo ctor = t.GetConstructor(
new Type[]{typeof(int), typeof(string)}
);

if(ctor == null)
throw new ArgumentException("T is invalid, because no CTOR(int,
string) is available!");

Something instance = (Something)ctor.Invoke(new object[]{1,
"hello"});

// do whatever you want
}

}


OK?



GP