Public Class BTreeDictionary
    (Of TKey,TValue) 
   Implements IDictionaryEx(Of TKey,TValue) 

public class BTreeDictionary<TKey,TValue> : IDictionaryEx<TKey,TValue>  

BTreeDictionary<int, string> data = new BTreeDictionary<int, string>();
data.Add(1, "a");
data.Add(2, "b");
data.Add(3, "c");
data.Add(4, "d");
data.Add(5, "e");

Assert.AreEqual(1, data.First().Key);
Assert.AreEqual("a", data.First().Value);
data.Remove(1);
Assert.AreEqual(2, data.First().Key);
Assert.AreEqual("b", data.First().Value);

Assert.AreEqual(5, data.Last().Key);
Assert.AreEqual("e", data.Last().Value);
data.Remove(5);
Assert.AreEqual(4, data.Last().Key);
Assert.AreEqual("d", data.Last().Value);

data.Remove(4);
data.Remove(3);

KeyValuePair<int, string> kv;
Assert.IsTrue(data.TryGetLast(out kv));
Assert.IsTrue(data.TryGetFirst(out kv));
data.Remove(2);
Assert.IsFalse(data.TryGetLast(out kv));
Assert.IsFalse(data.TryGetFirst(out kv));

try
{
    data.First();
    Assert.Fail("Should raise InvalidOperationException");
}
catch (InvalidOperationException)
{
}
try
{
    data.Last();
    Assert.Fail("Should raise InvalidOperationException");
}
catch (InvalidOperationException)
{
}

Dim data As New BTreeDictionary(Of Integer, String)()
data.Add(1, "a")
data.Add(2, "b")
data.Add(3, "c")
data.Add(4, "d")
data.Add(5, "e")

Assert.AreEqual(1, data.First().Key)
Assert.AreEqual("a", data.First().Value)
data.Remove(1)
Assert.AreEqual(2, data.First().Key)
Assert.AreEqual("b", data.First().Value)

Assert.AreEqual(5, data.Last().Key)
Assert.AreEqual("e", data.Last().Value)
data.Remove(5)
Assert.AreEqual(4, data.Last().Key)
Assert.AreEqual("d", data.Last().Value)

data.Remove(4)
data.Remove(3)

Dim kv As KeyValuePair(Of Integer, String)
Assert.IsTrue(data.TryGetLast(kv))
Assert.IsTrue(data.TryGetFirst(kv))
data.Remove(2)
Assert.IsFalse(data.TryGetLast(kv))
Assert.IsFalse(data.TryGetFirst(kv))

Try
    data.First()
    Assert.Fail("Should raise InvalidOperationException")
Catch generatedExceptionName As InvalidOperationException
End Try
Try
    data.Last()
    Assert.Fail("Should raise InvalidOperationException")
Catch generatedExceptionName As InvalidOperationException
End Try