Le Thanh Hung's blog

Problem

public class Dictionary<K, V>
{
 public void Add(K key, V value)
 {
     ...
     if (key.CompareTo(x) < 0) {...} // Error, no CompareTo method
        ...
 }
}

Since the type argument specified for K  could be any type, the
only members that support CompareTo are  memberss that implement
IComparable Interface such as Equals, GetHashCode, ToString...
A Compile-time error occurs in the example above.

Solution

public class Dictionary<K, V> where K: ICompareable
{
  public void Add(K key, V value)
  {
      ...
      if (key.CompareTo(x) < 0) {...}
      ...
  }
}

The Declaration above ensure that any type argument supplied for K
is a type that implements IComparable. It is possible to specify
any number of interface and type parameters as constraints :

public class Dictionary<K, V>
where K: ICompareable<K>, IPersistable
where V: Entity
{
  public void Add(K key, V value)
  {
      ...
      if (key.CompareTo(x) < 0) {...}
      ...
  }
}