substring doesn't match string

Question

I've got the following function that when it opens a file to establish the file type doesn't return the correct value. What am I doing wrong?

The first 8 characters are in this format %PDF-1.4

Function GetFileType(xFile As String) As Boolean
    Dim fileBytes As Byte()

    fileBytes = System.IO.File.ReadAllBytes(xFile)

    Dim s As String
    s = System.Text.Encoding.UTF8.GetString(fileBytes)

    Select Case s
        Case s.Substring(1, 3) = "PDF"
            GetFileType = False
        Case Else
            GetFileType = True
    End Select

End Function

It doesn't match the case statement, it's always moves to the else part which has me stumped.

VB.NET doesn't actually support this kind of Case statement syntax. Option Strict On allows the compiler to tell you about it. Consider a simple If-statement instead. — Hans Passant, Sep 05 '19 at 22:08
For file signature detection, you'd be better off doing a byte-wise comparison. Besides being more reliable (converting to UTF-8 could result in invalid characters about which you don't care), you don't have to read the whole file to determine that those 8 bytes match the signature. Open a `FileStream`, read up to 8 bytes, and compare (against the characters of the signature's *ASCII* characters as byte values). — madreflection, Sep 05 '19 at 22:15
madreflection, I'm open to improvements, can you point me to some code to get started? — user2496011, Sep 05 '19 at 22:25
@user2496011: [This answer in C#](https://stackoverflow.com/a/16014518) is pretty close to what I was suggesting. It also compares just the constant part of the header, `"%PDF-"`, so you don't get false negatives on other versions (assuming you don't want to be limited to 1.4 files). — madreflection, Sep 05 '19 at 23:07

score 2 · Accepted Answer · answered Sep 05 '19 at 22:16

2

Why are you trying to use a case statement?

In this case (see what I did there?), a simple If will work:

Function GetFileType(xFile As String) As Boolean
    Dim fileBytes As Byte()

    fileBytes = System.IO.File.ReadAllBytes(xFile)

    Dim s As String
    s = System.Text.Encoding.UTF8.GetString(fileBytes)

    if s.Substring(1, 3) = "PDF" then
            GetFileType = False
    Else
            GetFileType = True
    End If

End Function

answered Sep 05 '19 at 22:16

daShier

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Or just GetFileType = s.SubString(1, 3) <> "PDF". – Hans Passant Sep 05 '19 at 22:23
@HansPassant, excellent catch! But I still would suggest the simple `If Then Else` since it provides a more generalized method that works for more situations than this special case. – daShier Sep 05 '19 at 22:27

substring doesn't match string

1 Answers1