C# How to convert ArrayList of typed objects to a typed List?

Question

I have an ArrayList of objects that are of a certain type, and I need to convert this ArrayList in to a types list. Here is my code

Type objType = Type.GetType(myTypeName);

ArrayList myArrayList = new ArrayList();

object myObj0 = Activator.CreateInstance(type);
object myObj1 = Activator.CreateInstance(type);
object myObj2 = Activator.CreateInstance(type);

myArrayList.Add(myObj0);
myArrayList.Add(myObj1);
myArrayList.Add(myObj2);

Array typedArray = myArrayList.ToArray(objType);  // this is typed
object returnValue = typedArray.ToList();  // this is fake, but this is what I am looking for

There is no ToList() available for an Array, this is the behaviour I am looking for

object returnValue = typedArray.ToList();

So basically I have the type name as a string, I can create a Type from the name, and create a collection containing several objects typed, but how do I convert that to a List? I am hydrating a property and when I do a SetValue my property type needs to match.

Thank you very much.

dynamic, I am converting from JSON and I know the type as a string, and I can create the typed object array, but I need the output to be a List<myStrongType> — Jason Lavigne
– Jason Lavigne, Commented Aug 17, 2011 at 19:55
So... you're trying to covert an Array (typed) to a List<T>, is that right? And you have the name of "type T" stored in a string (and a Type object). — John McDonald
– John McDonald, Commented Aug 17, 2011 at 20:16

Jon Skeet · Accepted Answer · 2011-08-17 20:10:47Z

5

If you're using .NET 4, dynamic typing can help - it can perform type inference so you can call ToList, admittedly not as an extension method:

dynamic typedArray = myArrayList.ToArray(objType);    
object returnValue = Enumerable.ToList(typedArray);

Otherwise, you'll need to use reflection:

object typedArray = myArrayList.ToArray(objType);   
// It really helps that we don't need to work through overloads...
MethodInfo openMethod = typeof(Enumerable).GetMethod("ToList");
MethodInfo genericMethod = openMethod.MakeGenericMethod(objType);
object result = genericMethod.Invoke(null, new object[] { typedArray });

edited Aug 17, 2011 at 20:10

answered Aug 17, 2011 at 20:00

Jon Skeet

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3 Comments

digEmAll Over a year ago

If I'm not missing something, you can't call ToList<T> directly on ArrayList, because is not generic. You should call Cast<T> or OfType<T> before that...

Jon Skeet Over a year ago

@digEmAll: I'm not - I'm calling it on typedArray, which is a typed array resulting from the myArrayList.ToArray(objType) call.

Chris McGrath Over a year ago

dont argue with john skeet =)

lgtelles · Accepted Answer · 2011-08-17 19:54:26Z

2

Create a

List<YourType> list = new List<YourType>;

and then

list.AddRange(yourArray);

answered Aug 17, 2011 at 19:54

lgtelles

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3 Comments

dlev Over a year ago

This won't work, since Array doesn't implement IEnumerable<T>.

Jason Lavigne Over a year ago

I don't have <YourType> available at compile time, I am starting with a string that has the type name, and then I resolve the type as a Type

BoltClock Over a year ago

No need for AddRange() anyway, the List<T> constructor accepts an array of the same type.

Leon · Accepted Answer · 2011-08-17 19:50:24Z

1

Use an extenstion method: .ToList<myType>()

answered Aug 17, 2011 at 19:50

Leon

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2 Comments

dlev Over a year ago

ToList() is generic on the source sequence generic type, so that call will fail since ArrayList isn't generic at all.

Jason Lavigne Over a year ago

myType in my case is a Type variable, I don't have the actual type. Cheers.

Bernard · Accepted Answer · 2011-08-17 19:51:10Z

0

Use the generic List<> type instead.

answered Aug 17, 2011 at 19:51

Bernard

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6 Comments

BoltClock Over a year ago

Isn't that what he's asking? How to convert an array to a generic list?

Jason Lavigne Over a year ago

How do I use List<> when I only have the type as a variable? Thanks.

Bernard Over a year ago

@Jason Lavigne: Why is your type specified in a string?

Bernard Over a year ago

@BoltClock: He isn't using generics, so my suggestion is that he does.

Jason Lavigne Over a year ago

@Bernard, because I am converting JSON to a list of typed objects, I am able to populate the object just fine using reflection, but now I need a typed list.

|

AlG · Accepted Answer · 2011-08-17 20:10:57Z

0

Does it have to be List<T> or would an IEnumerable<T> do?

Taking a little from Linq you could do:

object returnValue = myArrayList.Cast<string>();

Which creates the following object (assuming T = string):

System.Linq.Enumerable+<CastIterator>d__b1`1[System.String]

answered Aug 17, 2011 at 20:10

AlG

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