I’ve run into issues with using a GET to read large attachments using HttpWebRequest.   Because of this, I use System.Net.WebClient instead.  Big attachments will have an error thrown back with HttpWebRequest, however it’s possible to GET the smaller ones.   So, since System.Net.WebClient seems to work just as good, I use it for getting attachments.  I use HttpWebRequest for everything else. 

       Public Function SaveAttachmentToFile(ByVal sUri As String, ByVal sFileName As String) As Boolean
            Dim bRet As Boolean = False
            Dim wcClient As New System.Net.WebClient
            Dim myUri As System.Uri
            Dim sErrorText As String = ""

            Try
                myUri = New System.Uri(sUri)
            Catch exa As Exception
               
                sErrorText = exa.ToString
                MsgBox(sErrorText)
                Return False
            End Try

            'Try
            '--------------------------- Set Credentials --------------------------------------
            Dim myCred As NetworkCredential
            Dim MyCredentialCache As CredentialCache

            Select Case m_Connection.AuthenticationType
                Case AuthenticationTypes.Basic
                    If m_Connection.Domain.Trim.Length = 0 Then
                        myCred = New NetworkCredential(m_Connection.UserName, m_Connection.Password)
                    Else
                        myCred = New NetworkCredential(m_Connection.UserName, m_Connection.Password, m_Connection.Domain)
                    End If
                    MyCredentialCache = New CredentialCache
                    MyCredentialCache.Add(myUri, "Basic", myCred)
                    wcClient.Credentials = MyCredentialCache
                Case AuthenticationTypes.NTLM
                    If m_Connection.Domain.Trim.Length = 0 Then
                        myCred = New NetworkCredential(m_Connection.UserName, m_Connection.Password)
                    Else
                        myCred = New NetworkCredential(m_Connection.UserName, m_Connection.Password, m_Connection.Domain)
                    End If
                    MyCredentialCache = New CredentialCache
                    MyCredentialCache.Add(myUri, "NTLM", myCred)
                    wcClient.Credentials = MyCredentialCache
                Case AuthenticationTypes.Windows
                    wcClient.Credentials = CredentialCache.DefaultCredentials
                Case AuthenticationTypes.FBA
                    Dim strPassedCookies As String
                    If m_Connection.strReusableCookies.Trim.Length = 0 Then
                        strPassedCookies = m_Connection.FBACookies() ' Get new cookies
                        If strPassedCookies = "Error" Then
                            ' had an error getting the cookie!
                            strPassedCookies = ""  ' Reset
                            m_Connection.strReusableCookies = "" ' Reset
                            Return ""
                        Else
                            ' No errors getting the cookie
                            m_Connection.strReusableCookies = strPassedCookies ' Use New Cookies
                        End If
                    Else
                        strPassedCookies = m_Connection.strReusableCookies ' Use Existing cookies
                    End If
                    wcClient.Headers.Add("Cookie", strPassedCookies)
                Case AuthenticationTypes.Certificate
                    MsgBox(m_Connection.AuthenticationType.ToString & " Not Implemented as yet")
                Case AuthenticationTypes.None
                    'MsgBox(m_Connection.AuthenticationType.ToString & " Unknown authentication type")
            End Select

            '-------------------- Set Additional Headers if needed (From Connect screen)------------------
            If m_Connection.ConnectAdditionalHeaders.AnAdditionalHeaderIsSet = True Then
                SetAdditionalConnectionHeaders(wcClient)
            End If


            ' Now Download the file!!!
            Try
                wcClient.DownloadFile(sUri, sFileName)
                bRet = True
            Catch ex As Exception
                sErrorText = ex.ToString
  MsgBox(sErrorText)
                bRet = False
            End Try
            Return bRet
        End Function

Read Attachments - Webdav: System.Net.HttpWebRequest  vs  System.Net.WebClient