C# - Get the item type for a generic list

Question

You could use the Type.GetGenericArguments method for this purpose.

List<Foo> myList = ...

Type myListElementType = myList.GetType().GetGenericArguments().Single();

For a more robust approach:

public static Type GetListType(object someList)
{
    if (someList == null)
        throw new ArgumentNullException("someList");

    var type = someList.GetType();

    if (!type.IsGenericType || type.GetGenericTypeDefinition() != typeof(List<>))
        throw new ArgumentException("Type must be List<>, but was " + type.FullName, "someList");

    return type.GetGenericArguments()[0];
}

But if your variable is typed List<T> then you can just use typeof(T). For example:

public static Type GetListType<T>(List<T> someList)
{
    return typeof(T);
}

Note that you don't really even need the someList parameter. This method is just an example for how you could use typeof if you are already in a generic method. You only need to use the reflection approach if you don't have access to the T token (the list is stored in a non-generic-typed variable, such as one typed IList, object, etc.).

list.GetType().GetGenericArguments()[0]

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<int>)));

// 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];
}

What about this, its all static (e.g. no instances required), and fast (no loops, no usage of linq), and it is simple :) these work for collections:

    [System.Diagnostics.DebuggerHidden]
    public static Type GetIndexedType(this ICollection poICollection)
    {
        PropertyInfo oPropertyInfo = poICollection == null ? null : poICollection.GetType().GetProperty("Item");
        return oPropertyInfo == null ? null : oPropertyInfo.PropertyType;
    }

    [System.Diagnostics.DebuggerHidden]
    public static Type GetEnumeratedType(this ICollection poICollection)
    {
        PropertyInfo oPropertyInfo = poICollection == null ? null : poICollection.GetType().GetMethod("GetEnumerator").ReturnType.GetProperty("Current");
        return oPropertyInfo == null ? null : oPropertyInfo.PropertyType;
    }

And a few simple unit tests:

        [Test]
        public void GetIndexedType()
        {
            Assert.AreEqual(null, ((ICollection)null).GetIndexedType());
            Assert.AreEqual(typeof(int), (new List<int>()).GetIndexedType());
            Assert.AreEqual(typeof(bool), (new SortedList<string, bool>()).GetIndexedType());
        }

        [Test]
        public void GetEnumeratedType()
        {
            Assert.AreEqual(null, ((ICollection)null).GetEnumeratedType());
            Assert.AreEqual(typeof(int), (new List<int>()).GetEnumeratedType());
            Assert.AreEqual(typeof(KeyValuePair<string, bool>), (new SortedList<string, bool>()).GetEnumeratedType());
        }

Notice the fact that there are two ways to look at this, one type may be returned by the indexer and an other type may be returned by the enumerator. The unit test do show both.

Have fun, Frans.

P.s. For enumerables:

    [System.Diagnostics.DebuggerHidden]
    public static Type GetEnumeratedType(this System.Collections.IEnumerable poIEnumerable)
    {
        PropertyInfo oPropertyInfo = poIEnumerable == null ? null : poIEnumerable.GetType().GetMethod("GetEnumerator").ReturnType.GetProperty("Current");
        return oPropertyInfo == null ? null : oPropertyInfo.PropertyType;
    }

And for enumerator:

    [System.Diagnostics.DebuggerHidden]
    public static Type GetEnumeratedType(this System.Collections.IEnumerator poIEnumerator)
    {
        PropertyInfo oPropertyInfo = poIEnumerator == null ? null : poIEnumerator.GetType().GetProperty("Current");
        return oPropertyInfo == null ? null : oPropertyInfo.PropertyType;
    }

    public Type GetType(IEnumerable<object> resultList)
    {
        return resultList.GetType().GetElementType();
    }

Old question new method with dynamic

void Foo(){
   Type type GetTypeT(data as dynamic);
}

private static Type GetTypeT<T>(IEnumerable<T> data)
{
    return typeof(T);
}