Below are the .Net inbuilt classes and each class is inheriting from different interfaces... At glance please look into the below sample classes..
public class Tuple<T1> : IStructuralEquatable, IStructuralComparable, IComparable, ITuple
public sealed class String : IComparable, ICloneable, IConvertible, IComparable<string>, IEnumerable<char>, IEnumerable, IEquatable<string>
public class SystemException : Exception
public sealed class StringBuilder : ISerializable
public abstract class StringComparer : IComparer, IEqualityComparer, IComparer<string>, IEqualityComparer<string>
public abstract class Stream : MarshalByRefObject, IDisposable
public class Hashtable : IDictionary, ICollection, IEnumerable, ISerializable, IDeserializationCallback, ICloneable
public class ArrayList : IList, ICollection, IEnumerable, ICloneable
public class SortedList : IDictionary, ICollection, IEnumerable, ICloneable
public class Stack : ICollection, IEnumerable, ICloneable