Question

Maybe it is pretty simple, but I’m not in use with the type Type and it’s uses.

Say I want to create a List<T> either with T=Double or T=UInt32, depending on the result of some function, say public static Type CheckType(String input);

In code:

Type t = Program.CheckType(someInput); // it returns typeof(Double) or typeof(UInt32)
if (t == typeof(Double))
    List<Double> l = new List<Double>();
else
    List<UInt32> l = new List<UInt32>();

I know the above code won’t compile because I’m letting l to have two different meanings (a list of double and a list of unsigned int)… So it leads to my question:

• Is there a way to avoid the if in the above code?

Something similar to this:

Type t = Program.CheckType(someInput);
List<t> l = new List<t>(); // I know this won't compile either...

I mean, that would generically instantiate a generic List…

Thank you in advance!

EDIT:

This is NOT a duplicate of the marked question for only one reason: Double and UInt32 are not Anonymous types! The question, here, is how to determine the type of some input data (which will be Type T=typeof(Double) or Type T=typeof(UInt32), for example) and, thus, create a generic SampleClass<T> based on that input data type, T!

In other words: determine some Type T in the runtime and, then, instantiate a generic type with the determined type T. Sorry if I didn’t make that clear before…

PS: I used List<T> as an example for SampleClass<T>.

Answer 1

You can’t type the list as generic, since you don’t know the type parameter, but you can create a List instance at runtime.

Type t = Program.CheckType(someInput);
Type listType = typeof(List<>).MakeGenericType(t);
IList list = (IList) Activator.CreateInstance(listType);

If you try to add an object of the incorrect type, you will get an exception, so this approach has an advantage over using a collection like ArrayList, or List<object>