C# - Get the item type for a generic list

Question

What would be the best way of getting the type of items a generic list contains? It\'s easy enough to grab the first item in the collection and call .GetType(), but I can\'

Answer 1

Here's another way which works for non-generic collections, too:

static Type GetItemType(Type collectionType)
{
    return collectionType.GetMethod("get_Item").ReturnType;
}

That is, get the return type of foo[x], where foo is of the specified type.

Examples:

// Generic type; prints System.Int32
Console.WriteLine(GetItemType(typeof(List)));

// Non-generic type; prints System.String
Console.WriteLine(GetItemType(typeof(System.Collections.Specialized.StringCollection)));

---

The GetItemType method above has a couple issues, though:

• It throws a NullReferenceException if the type has no indexing operator.

• It throws an AmbiguousMatchException if the type has multiple overloads for the indexing operator (e.g. this[string] and this[int]).

Here is a more refined version:

public static Type GetItemType(this Type collectionType)
{
    var types =
        (from method in collectionType.GetMethods()
         where method.Name == "get_Item"
         select method.ReturnType
        ).Distinct().ToArray();
    if (types.Length == 0)
        return null;
    if (types.Length != 1)
        throw new Exception(string.Format("{0} has multiple item types", collectionType.FullName));
    return types[0];
}