I would like to write a function that takes a List<Object> and returns a the original list casted to a list of a specified object type List<ObjectType>, knowing that the objects within the original list are of type ObjectType. The trick is that ObjectType could be any type, which I find using Reflection. Sorry for the lack of code, but I have had no luck understanding how I might even go about doing this.
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1 Answer
If you know that each item in the list is of type ObjectType, you can do this:
List<object> sourceList = new List<object>() { 1, 2, 3 };
List<int> resultList = sourceList.Cast<int>().ToList();
If you actually want to convert each item in the list in a generic way, the simplest way would be to do something like this:
public static IEnumerable<T> ConvertTo<T>(this IEnumerable items)
{
return items.Cast<object>().Select(x => (T)Convert.ChangeType(x, typeof(T)));
}
This would be implemented as an extension method, so you can write:
List<object> sourceList = new List<object>() { 1, 2, 3 };
List<string> resultList = sourceList.ConvertTo<string>().ToList();
If the target type isn't known at compile-time, you would indeed need to use reflection. Something like this would work:
class ListUtil
{
public static List<T> ConvertToList<T>(this IEnumerable items)
{
// see method above
return items.ConvertTo<T>().ToList();
}
public static IList ConvertToList(this IEnumerable items, Type targetType)
{
var method = typeof(ListUtil).GetMethod(
"ConvertToList",
new[] { typeof(IEnumerable) });
var generic = method.MakeGenericMethod(targetType);
return (IList)generic.Invoke(null, new[] { items });
}
}
And now you can call it like:
List<object> sourceList = new List<object>() { 1, 2, 3 };
IList resultList = ListUtil.ConvertToList(sourceList, typeof(string));
resultList.GetType(); // List<string>
Of course, you loose any compile-time type safety with this method.
6 Comments
p.s.w.g
@bmused That only works when the all objects are of a type
T. It's basically a box / un-box. In my example using integers as input, .Cast<string> and .Cast<double> would fail.
BrokenGlass
If the type is only known at runtime above approach won't work
p.s.w.g
@BrokenGlass See my updated answer for a solution that work when the type is only known at run-time.
Brad Ross
I was looking for the last option you gave me. Awesome answer! I presume I could do a similar (but perhaps more complex) version of this to convert Arrays or Dictionaries from one type to another?
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