TypeSelector class

Type[] selectedTypes;

void OnGUI()
{
    // Use the selector manually. See the documentation for OdinSelector for more information.
    if (GUILayout.Button("Open My Selector"))
    {
        TypeSelector selector = new TypeSelector(customListOfTypes);
        TypeSelector selector = new TypeSelector(AssemblyTypeFlags.CustomTypes, supportsMultiSelect: true);
        selector.SetSelection(this.selectedTypes);
        selector.SelectionConfirmed += selection => this.selectedTypes = selection.ToArray();
        selector.ShowInPopup(); // Returns the Odin Editor Window instance, in case you want to mess around with that as well.
    }
}

private static Type currentSelectedType;
private static IEnumerable<Type> currentSource;
private static Func<Rect, OdinSelector<Type>> createTypeSelector = (rect) =>
{
    TypeSelector selector = new TypeSelector(currentSource, false);
    selector.SetSelection(currentSelectedType);
    selector.ShowInPopup(rect);
    return selector;
};

public static Type DrawTypeSelectorDropdown(GUIContent label, Type selectedType, IEnumerable<Type> source)
{
    currentSource = source;
    currentSelectedType = selectedType;

    var dropdownText = selectedType == null ? "None" : selectedType.GetNiceName();
    var selected = TypeSelector.DrawSelectorDropdown(label, dropdownText, createTypeSelector);
    if (selected != null && selected.Any())
    {
        selectedType = selected.FirstOrDefault();
    }
    return selectedType;
}