Convert HTML to MHTML using ASP.NET

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Introduction

Ever wanted to make a report out of an html document and have it sent to the client for offline use in Word or Excel? An RFC - compliant Multipart MIME Message (mhtml web archive) is one single file containing all related material such as linked documents and images serialized to their Base64 inline encoding representations. There is no native support for creating mhtml archives in .NET but thanks to the Windows CDO library this is easy accomplished.

The code

The projects contains 3 classes; mht, mhtImage and mhtImageCollection. The mht class contains the conversion functions like convertWebControlToMHTString which takes a webControl and a collection of images and returns a string representation of the created mht archive. Use this function when the converting webControl is dependent on user specific Session and Application variables for rendering or when you use dynamically created images.

Public Function convertWebControlToMHTString(ByVal control As WebControl, _
  ByVal MHTimages As mhtImageCollection) As String
  'Render WebControl to html
  Dim html As String = getHtml(control)

  'If WebControl has images, make the html Word compatible
  If Not MHTimages Is Nothing Then
    fixImageLocation(html, MHTimages)
  End If

  Dim msg As New CDO.MessageClass
  Dim stm As ADODB.Stream = Nothing
  Dim MS As System.IO.MemoryStream = Nothing

  Dim iBp As CDO.IBodyPart

  'Make a multipart mhtml document
  Dim mainBody As CDO.IBodyPart
  mainBody = msg
  mainBody.ContentMediaType = "multipart/related"

  'Make the html part of the document
  iBp = mainBody.AddBodyPart()
  iBp.ContentMediaType = "text/html"
  iBp.ContentTransferEncoding = "quoted-printable"
  stm = iBp.GetDecodedContentStream
  stm.WriteText(html)
  stm.Flush()

  'Make the image parts of the document
  If Not MHTimages Is Nothing Then
    Dim oMhtImage As mhtImage
    For Each oMhtImage In MHTimages
      iBp = mainBody.AddBodyPart()
      With iBp
        .ContentMediaType = "image/" + _
 oMhtImage.ImageFormat.ToString().ToLower()
        .ContentTransferEncoding = "base64"

        'ContentLocation must be the same as in the 
        'html part to make them linked
        .Fields.Append("urn:schemas:mailheader:content-location", _
    DataTypeEnum.adBSTR, , , oMhtImage.ContentLocation)
        .Fields.Update()
        .Fields.Refresh()
      End With

      Try
        MS = New System.IO.MemoryStream
        oMhtImage.Image.Save(MS, oMhtImage.ImageFormat)
        Dim bytearray As Byte() = MS.ToArray()
        stm = iBp.GetDecodedContentStream
        stm.Write(bytearray)
        stm.Flush()
      Finally
        MS.Close()
        stm.Close()
      End Try
    Next
  End If

  stm = mainBody.GetStream()
  Return stm.ReadText(stm.Size)
End Function

The convertWebPageToMHTString function converts an html document from a specific URL to a mht archive, all images included. Use this function for public html documents not dependent on user specific Session and Application variables.

Public Function convertWebPageToMHTString(ByVal url As String) As String
    Dim msg As New CDO.MessageClass
    Dim stm As ADODB.Stream = Nothing

    Try
        msg.MimeFormatted = True
        msg.CreateMHTMLBody(url, CDO.CdoMHTMLFlags.cdoSuppressNone, "", "")
        stm = msg.GetStream()
        Return stm.ReadText(stm.Size)
    Finally
        stm.Close()
    End Try
End Function

The fixImageLocation appends the string "http://" at the beginning of each ContentLocation if not already there, for Word compliance

Private Sub fixImageLocation( _
      ByRef html As String, ByRef MHTimages As mhtImageCollection)
    Dim curContentLocation As String
    Dim curIndex As Integer
    Dim oMhtImage As mhtImage
    For Each oMhtImage In MHTimages
        curContentLocation = oMhtImage.ContentLocation
        If curContentLocation.IndexOf(":") = -1 Then
            curIndex = html.IndexOf(curContentLocation)
            While curIndex <> -1
                html = html.Insert(curIndex, "http://")
                curIndex = html.IndexOf(curContentLocation, curIndex + _
   curContentLocation.Length)
            End While
            oMhtImage.ContentLocation = "http://" + curContentLocation
        End If
    Next
End Sub

The mhtImage class contains image information. Property Image contains the actual image. Property ContentLocation contains the path to the image, must be exactly the same as the source for the image in the html part. Property ImageFormat contains the image format (jpg, gif, bmp...)

The mhtImageCollection class contains a collection of mhtImages.

Using the code

Example on how to make a mht archive from a Panel webControl containing one image.

Dim oMhtCol As New mhtImageCollection
oMhtCol.add(New mhtImage(System.Drawing.Image.FromFile( _
  Server.MapPath("/mhtml/images/myComputer.jpg")), _
  "images/myComputer.jpg", System.Drawing.Imaging.ImageFormat.Jpeg))
sendMHTFile(ConvertWebControlToMHTString(Panel1, oMhtCol), "myFirstMht.mht")