In-Built Classes in .NET

Abstract Classes

public abstract class Controller : ControllerBase, IActionFilter,  IAuthenticationFilter, IAuthorizationFilter, IDisposable, IExceptionFilter,  IResultFilter, IAsyncController, IController, IAsyncManagerContainer

public abstract class ApiController : IHttpController, IDisposable

public abstract class Array : ICloneable, IList, ICollection, IEnumerable, IStructuralComparable, IStructuralEquatable

public abstract class CollectionBase : IList, ICollection, IEnumerable

public abstract class DictionaryBase : IDictionary, ICollection, IEnumerable

Sealed Classes

public sealed class String : IComparable, ICloneable, IConvertible,
IEnumerable, IComparable<string>, IEnumerable<char>, IEquatable<string>

public sealed class StringBuilder : ISerializable

public sealed class Comparer : IComparer, ISerializable

Static Classes

public static class Math

public static class Enumerable

public static class Queryable

Collections (Concrete Classes)

public class ArrayList : IList, ICollection, IEnumerable, ICloneable

public class SortedList : IDictionary, ICollection, IEnumerable, ICloneable

public class Hashtable : IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, ICloneable

public class Stack : ICollection, IEnumerable, ICloneable

public class Queue : ICollection, IEnumerable, ICloneable