Question

Lets say I have an interface passed to my method:

public void AlphaToChar(iList _blah)
{

}

Out of IList I want to extract it’s members Type and use its type to create other Arrays or Lists in the method. See example below.

The “List = new List();” part doesn’t work because, as I assume it’s a type variable, not the actual type.
Any way around this ? How can I accomplish this and create a new collection of an extracted Type?

Type[] listTypes =  list.GetType().GetGenericArguments();
Type listType = null;
if (listTypes.Length>0)
{
   listType = listTypes[0];
}
List<listType> = new List<listType>();

Thank you.

Answer 1

You can do the List<> construction using the following:

// Find the generic argument type of your old list (theList)
Type genericType = theList.GetType().GetGenericArguments()[0];

// Create a new List with the same generic type as the old one
Type newListType = typeof(List<>).MakeGenericType(genericType);

// Create a new instance of the list with the generic type
var instance = Activator.CreateInstance(newListType);

But it’s only going to work if you are using generic lists. The example you gave was using a regular IList. You would have to change your method signature to use a generic IList<>:

public void AlphaToChar(IList<Something> _blah) { }

Or make it even more generic:

public void AlphaToChar<T>(IList<T> _blah) /* where T : ISomething, new() */ {}  

Without doing so, you should know what your IList is going to contain and you wouldn’t have to use reflection to figure out what its elements types are.