3

Following the example on this post, I found out how to create a list of generic types dynamically. Now, my issue is that I want to ADD items to the created list from an unknown source list - is there some way to achieve this?

EDIT I start with a source list containing business objects, but I absolutely need the right type of my output list, because of downstream bindings that require it.

My non-compiling code is as follows:

IList<object> sourceList; // some list containing custom objects
Type t = typeof(IList).MakeGenericType(sourceList[0].GetType());
IList res = (IList)Activator.CreateInstance(t);
foreach (var item in sourceList) {
    reportDS.Add(item); // obviously does not compile
}
Community
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neggenbe
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  • what happens if you cast item to the type of the list – Armand Jul 12 '16 at 12:51
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    What are `sourceList` and `reportDS`? Do you know what type _it_ is at compile time? If not why don't you just use a `List`? – D Stanley Jul 12 '16 at 12:52
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    You're using `typeof(IList)` when you actually need to say `typeof(List<>)` – Sean Jul 12 '16 at 12:54
  • `sourceList` is `IList`, but I need to cast to the correct type `typeof(sourceList[0])` because of downstream bindings on that list which require the list to have the correct type... I amen the post to avoid this obvious answer! – neggenbe Jul 12 '16 at 12:55
  • `IList` won't work like @Sean pointed out... even the example you reference shows that they are using `List` – The Bearded Llama Jul 12 '16 at 12:57
  • `AddItem` isn't a method on `IList`. Did you mean `Add` ? – Sean Jul 12 '16 at 13:00
  • Do the bindings require that the _list_ have a specific type or that the _objects_ within it be a specific type? You could call `Cast` or `OfType` downstream if you know the type _there_. Mixing reflection and generics is, as you can see, messy. – D Stanley Jul 12 '16 at 13:04
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    So why not just do `reportDS = sourceList.Cast()`? – DavidG Jul 12 '16 at 13:05

2 Answers2

14

There's a few reasons this code doesn't work.

Firstly, you need to create an instance of a concrete type. You're taking a non-generic interface (IList) and trying to create a generic type from it. You need typeof(List<>).

Secondly, you're calling AddItem which isn't a method on IList. You need to call Add, which is why the code doesn't compile.

This code will do what you want:

IList<object> sourceList; // some list containing custom objects
Type t = typeof(List<>).MakeGenericType(sourceList[0].GetType());
IList res = (IList)Activator.CreateInstance(t);

foreach(var item in sourceList)
{
    res.Add(item);
}

However, the assumption that sourceList[0] holds the correct type may come to bite you. If that list contains a sequence of objects that are not type compatible with the first item in the list then any attempt to add to res will fail. As was mentioned in the comment, you'd be better off creating a List<object> to hold the items.

Sean
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4

I would recommend moving your code into a generic class or function, moving the reflection to a higher level:

private static List<T> CloneListAs<T>(IList<object> source)
{
    // Here we can do anything we want with T
    // T == source[0].GetType()
    return source.Cast<T>().ToList();
}

To call it:

IList<object> sourceList; // some list containing custom objects
// sourceList = ...

MethodInfo method = typeof(this).GetMethod("CloneListAs");
MethodInfo genericMethod = method.MakeGenericMethod(sourceList[0].GetType());

var reportDS = genericMethod.Invoke(null, new[] {sourceList});
Dark Falcon
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