I'm getting a casting error using the code below. The error on the web page (when you click the error icon) is:

Sys.WebForms.PageRequestManagerServerErrorExceptio n: Conversion from string "application.msword" to type "Long" is not valid.

It's to do with declaring the mimeout integer pointer to the "<MarshalAs(UnmanagedType.LPWStr)> ByRef ppwzMimeOut As String" value in the function call.

If I change mimeout declaration to: "Dim mimeout as string" and return that and comment out the other mimeout referenced code it returns "application/msword" correctly but obviously the rest of the function won't work because there is no pointer to the original referenced mimeout address.

Has anyone got any ideas of what I'm doing wrong?



Imports System.Security.Permissions

Public Declare Function FindMimeFromData Lib "urlmon.dll" (ByVal pBC As IntPtr, <MarshalAs(UnmanagedType.LPWStr)> ByVal pwzUrl As String, <MarshalAs(UnmanagedType.LPArray, ArraySubType:=UnmanagedType.I1, SizeParamIndex:=3)> ByVal pBuffer As Byte(), ByVal cbSize As Integer, <MarshalAs(UnmanagedType.LPWStr)> ByVal pwzMimeProposed As String, ByVal dwMimeFlags As Integer, <MarshalAs(UnmanagedType.LPWStr)> ByRef ppwzMimeOut As String, ByVal dwReserved As Integer) As Integer

Public Shared Function getMimeFromFile(ByVal file As String) As String

        Dim mimeout As IntPtr

        If Not System.IO.File.Exists(file) Then
            Throw New FileNotFoundException(file + " not found")
        End If

        Dim MaxContent As Integer = CInt(New FileInfo(file).Length)

        If MaxContent > 4096 Then
            MaxContent = 4096
        End If

        Dim fs As New FileStream(file, FileMode.Open)

        Dim buf(MaxContent) As Byte
        fs.Read(buf, 0, MaxContent)

        Dim result As Integer = FindMimeFromData(IntPtr.Zero, file, buf, MaxContent, Nothing, 0, mimeout, 0)

        If result <> 0 Then
            'Throw Marshal.GetHRForExceptionresult)
        End If

        Dim mime As String = Marshal.PtrToStringUni(mimeout)


        Return mime

    End Function

Can nobody help me with this?

I've posted the same question in six forums and no-one seems to know what I've done wrong in my coding. I'm sure it's a simple case of using the wrong variable type either in the function or pointer declaration but for the life of me I don't know where!

Please advise.


I've fixed this by using "<MarshalAs(UnmanagedType.U4)> ByRef ppwzMimeOut As Integer" in the function decalre.. but I don't really understand why it works, probably because I don't really understand the way MarshalAs works either. But at least it works!

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