vb.net image save to memory stream results in an ArgumentNullException

Question

In VB.NET I have the following function that allows me to calculate a hash for an image I have not yet saved to a file :

Public Function pictureHash(ByVal image As System.Drawing.Image) As String
  Try
    If image Is Nothing Then Return Nothing
    Dim ha As HashAlgorithm = HashAlgorithm.Create()
    Dim ms As New MemoryStream()
    image.Save(ms, image.RawFormat)
    ms.Position = 0
    Dim imageHash As Byte() = ha.ComputeHash(ms)
    ms.Close()
    Return BitConverter.ToString(imageHash)
  Catch ex As Exception
    Return Nothing
  End Try
End Function

The problem is I get an ArgumentNullException on the instruction image.Save(ms, image.rawFormat).

Here is the detail of the exception :

System.ArgumentNullException occurred
  Message="Value cannot be null. Parameter name: encoder"
  ParamName="encoder"
  Source="System.Drawing"
  StackTrace:
       at System.Drawing.Image.Save(Stream stream, ImageCodecInfo encoder, EncoderParameters encoderParams)
       at System.Drawing.Image.Save(Stream stream, ImageFormat format)
       at MyProgram.pictureHash(Image image)

The thing is, when I look at the stack trace, the last call to Image.Save with 3 parameters (the one that crashes) isn't even made by me, but by the previous Image.Save call.

Any idea what should I do ?

Many thanks in advance for your help,

Regards,

Joël

Answer 1

According to the community content for the Image.Save method, certain image formats do not have an associated encoder and fail with the error you are reporting.

Could you not use a standard format (e.g. bmp or png) when saving the file to the MemoryStream? To do this replace the call to Save with something similar to:

image.Save(ms, ImageFormat.Png)