Create List<> from runtime type

Question

I am looking to create a List, where the type of T is several unrelated classes (with the same constructor arguments) that I know through reflection.

    DataBase = new ArrayList();
    foreach (Type T in Types)
    {
        DataBase.Add(new List<T>);
    }

Unfortunately, Visual Studio says that 'The type or namespace T could not be found'. Is there some way I can implement this, or cast a List to type T? Thanks, Peter

Use the dynamic keyword, perhaps? Assuming the right version of .NET... — Dan J
– Dan J, Commented Jan 20, 2012 at 0:32
don't use ArrayList, worst case just use List<object> if you must — BrokenGlass
– BrokenGlass, Commented Jan 20, 2012 at 0:33
@BrokenGlass, I will probably do that, but it is not the ArrayList that is the issue, it is the creation of the List with runtime Type. — 3Pi
– 3Pi, Commented Jan 20, 2012 at 0:40

BrokenGlass · Accepted Answer · 2012-01-20 00:37:35Z

26

You can use reflection:

List<object> database = new List<object>();
foreach (Type t in Types)
{
   var listType = typeof(List<>).MakeGenericType(t);
   database.Add(Activator.CreateInstance(listType));
}

answered Jan 20, 2012 at 0:37

BrokenGlass

161k28 gold badges297 silver badges339 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Hans Passant Over a year ago

That works. But creating the List object is just a small part of problem. He'll also have to add elements to it. And worse, much worse, read them. That code won't know the element type either. An ORM is indicated here, given the word "database".

Amandalishus Over a year ago

That works to create the object, but it returns an object. I got that far, but I'm having trouble casting that object as a List<T>. I could of course use "as List<T>" if I knew T, but since that's not defined until runtime I'm not sure how to make it work.

doogle · Accepted Answer · 2012-01-20 00:38:40Z

0

You're confusing types and the System.Type class for Generic Type Parameters, here is the code to implement it the way you want:

var lt = typeof(List<>);
foreach (Type T in Types)
{
    var l = Activator.CreateInstance(lt.MakeGenericType(T));
    DataBase.Add(l);
}

answered Jan 20, 2012 at 0:38

doogle

3,4161 gold badge20 silver badges23 bronze badges

Comments

GGulati · Accepted Answer · 2012-01-20 00:40:11Z

0

You could... use List<object> (if you know that your type is a reference type, or boxing is acceptable); or create a List via reflection.

answered Jan 20, 2012 at 0:40

GGulati

1,0356 silver badges11 bronze badges

Collectives™ on Stack Overflow

Create List<> from runtime type

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related