using System;

using System.Collections.Generic;

using System.Linq;

​

public static class Extensions {

    public static KeyValuePair<K, V> After<K, V>( this Dictionary<K, V> dictionary, K key ) {

        Int32 position = dictionary.Keys.ToList().IndexOf( key );

        if( position == -1 )

            throw new ArgumentException( "No match.", "key" );

        position++;

        if( position >= dictionary.Count )

            position = 0;

        K k = dictionary.Keys.ToList()[ position ];

        V v = dictionary.Values.ToList()[ position ];

        return new KeyValuePair<K, V>( k, v );

    }

    public static KeyValuePair<K, V> After<K, V>( this Dictionary<K, V> dictionary, V value ) {

        Int32 position = dictionary.Values.ToList().IndexOf( value );

        if( position == -1 )

            throw new ArgumentException( "No match.", "value" );

        position++;

        if( position >= dictionary.Count )

            position = 0;

        K k = dictionary.Keys.ToList()[ position ];

        V v = dictionary.Values.ToList()[ position ];

        return new KeyValuePair<K, V>( k, v );

    }

    public static KeyValuePair<K, V> Before<K, V>( this Dictionary<K, V> dictionary, K key ) {

        Int32 position = dictionary.Keys.ToList().IndexOf( key );

        if( position == -1 )

            throw new ArgumentException( "No match.", "key" );

        position--;

        if( position < 0 )

            position = dictionary.Count - 1;

        K k = dictionary.Keys.ToList()[ position ];

        V v = dictionary.Values.ToList()[ position ];

        return new KeyValuePair<K, V>( k, v );

    }

    public static KeyValuePair<K, V> Before<K, V>( this Dictionary<K, V> dictionary, V value ) {

        Int32 position = dictionary.Values.ToList().IndexOf( value );

        if( position == -1 )

            throw new ArgumentException( "No match.", "value" );

        position--;

        if( position < 0 )

            position = dictionary.Count - 1;

        K k = dictionary.Keys.ToList()[ position ];

        V v = dictionary.Values.ToList()[ position ];

        return new KeyValuePair<K, V>( k, v );

    }

}                   

public class Program

{

    public static void Main()

    {

        Dictionary<String, Int64>

            dictionary = new Dictionary<String, Int64>() {

                { "1000L", 1000L },

                { "2000L", 2000L },

                { "3000L", 3000L },

                { "4000L", 4000L },

                { "5000L", 5000L },

                { "6000L", 6000L },

            };

        Random

            random = new Random();

        while( true ) {

            Int64

                value = random.Next( 1, 7 ) * 1000L;

            Int32

                position = dictionary.Values.ToList().IndexOf( value );

            String

                key = dictionary.Keys.ToList()[ position ];

            KeyValuePair<String, Int64>

                after = dictionary.After( value ),

            before = dictionary.Before( value );

            Console.WriteLine( "Value: " + value );

            Console.WriteLine( "Key: " + key );

            Console.WriteLine();

            Console.WriteLine( "Next Value: " + after.Value );

            Console.WriteLine( "Next Key: " + after.Key );

            Console.WriteLine();

            Console.WriteLine( "Previous Value: " + before.Value );

            Console.WriteLine( "Previous Key: " + before.Key );

            Console.ReadLine();

            Console.Clear();

        }

    }

}