CA2000: Object is not disposed along all exception paths - asp.net

Question

I am having trouble trying to figure out why I'm getting this warning in following code.
CA2000 : Microsoft.Reliability : In method 'Encryption64.Decrypt(String, String)', object 'des' is not disposed along all exception paths. Call System.IDisposable.Dispose on object 'des' before all references to it are out of scope.
CA2000 : Microsoft.Reliability : In method 'Encryption64.Encrypt(String, String)', object 'des' is not disposed along all exception paths. Call System.IDisposable.Dispose on object 'des' before all references to it are out of scope.
Public Class Encryption64
Private key() As Byte = {}
Private IV() As Byte = {&H12, &H34, &H56, &H78, &H90, &HAB, &HCD, &HEF}
Public Function Decrypt(ByVal stringToDecrypt As String, ByVal sEncryptionKey As String) As String
Dim des As New DESCryptoServiceProvider()
Dim ms As New MemoryStream()
Dim ReturnValue As String = String.Empty
Try
Dim inputByteArray(stringToDecrypt.Length) As Byte
key = System.Text.Encoding.UTF8.GetBytes(Left(sEncryptionKey, 8))
inputByteArray = Convert.FromBase64String(stringToDecrypt)
Dim cs As New CryptoStream(ms, des.CreateDecryptor(key, IV),CryptoStreamMode.Write)
cs.Write(inputByteArray, 0, inputByteArray.Length)
cs.FlushFinalBlock()
Dim encoding As System.Text.Encoding = System.Text.Encoding.UTF8
ReturnValue = encoding.GetString(ms.ToArray())
Catch e As Exception
ReturnValue = e.Message
Finally
If des IsNot Nothing Then
des.Dispose()
End If
If ms IsNot Nothing Then
ms.Dispose()
End If
End Try
Return ReturnValue
End Function
Public Function Encrypt(ByVal stringToEncrypt As String, ByVal SEncryptionKey As String) As String
Dim des As New DESCryptoServiceProvider()
Dim ms As New MemoryStream()
Dim ReturnValue As String = String.Empty
Try
key = System.Text.Encoding.UTF8.GetBytes(Left(SEncryptionKey, 8))
Dim inputByteArray() As Byte = Encoding.UTF8.GetBytes(stringToEncrypt)
Dim cs As New CryptoStream(ms, des.CreateEncryptor(key, IV), CryptoStreamMode.Write)
cs.Write(inputByteArray, 0, inputByteArray.Length)
cs.FlushFinalBlock()
ReturnValue = Convert.ToBase64String(ms.ToArray())
Catch e As Exception
ReturnValue = e.Message
Finally
If des IsNot Nothing Then
des.Dispose()
End If
If ms IsNot Nothing Then
ms.Dispose()
End If
End Try
Return ReturnValue
End Function
End Class

Answer 1

Since you are declaring (and instantiating) your des objects outside of the Try ... Finally blocks, it is possible for your code to raise an exception in the line Dim ms As New MemoryStream() and your .Dispose() will not be called.
When you are working with objects that implement IDisposable, it is much preferable where possible to wrap them in a Using block instead of a Try...Finally block. For example:
Public Function Decrypt(ByVal stringToDecrypt As String, ByVal sEncryptionKey As String) As String
Dim ms As New MemoryStream()
Dim ReturnValue As String = String.Empty
Dim inputByteArray(stringToDecrypt.Length) As Byte
key = System.Text.Encoding.UTF8.GetBytes(Left(sEncryptionKey, 8))
inputByteArray = Convert.FromBase64String(stringToDecrypt)
Using ms as New MemoryStream
Using des As New DESCryptoServiceProvider
Dim cs As New CryptoStream(ms, des.CreateDecryptor(key, IV),CryptoStreamMode.Write)
cs.Write(inputByteArray, 0, inputByteArray.Length)
cs.FlushFinalBlock()
End Using ' des
Dim encoding As System.Text.Encoding = System.Text.Encoding.UTF8
ReturnValue = encoding.GetString(ms.ToArray())
End Using ' ms
Catch e As Exception
ReturnValue = e.Message
End Try
Return ReturnValue
End Function

Answer 2

Just a guess, but maybe it's not smart enough to realize that this code line will always be true:
If des IsNot Nothing Then
In other words, it might assume that because there is a conditional statement, the Dispose() call might not be executed.
To check, you can try commenting out the "if" and see if the warning goes away.