Question

I have a binary file with the following contents:

The following code is used to read this contents in old VB6 program:

Private Type tpClient
    Firstname As String
    LastName As String
    Birth As String
    Adres As String
    Geslacht As String
    IDNummer As Long
    SSNummer As String
    DatabaseID As Long
    Telefoon1 As String
    Telefoon2 As String
End Type

Open strFilePath For Random Access Read As #intFileNumber
Get #intFileNumber, 1, ClientData ' ClientData is of type tpClient

Now I’m trying to read this with my new C# program:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct PatientStruct
{
    [MarshalAs(UnmanagedType.BStr)]
    public string FirstName;

    [MarshalAs(UnmanagedType.BStr)]
    public string LastName;

    [MarshalAs(UnmanagedType.BStr)]
    public string BirthDate;

    [MarshalAs(UnmanagedType.BStr)]
    public string Address;

    [MarshalAs(UnmanagedType.BStr)]
    public string Gender;

    [MarshalAs(UnmanagedType.BStr)]
    public string IdNumber;

    [MarshalAs(UnmanagedType.BStr)]
    public string SsNumber;

    [MarshalAs(UnmanagedType.BStr)]
    public string DatabaseId;

    [MarshalAs(UnmanagedType.BStr)]
    public string Telephone1;

    [MarshalAs(UnmanagedType.BStr)]
    public string Telephone2;
}

byte[] buffer = new byte[stream.Length];
stream.Read(buffer, 0, (int)stream.Length);
GCHandle handle = GCHandle.Alloc(buffer, GCHandleType.Pinned);
T stuff = (PatientStruct)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T));
handle.Free();

However, I get an AccessViolationException on the call to Marshal.PtrToStructure.

Any suggestions?

Answer 1

First of all, your struct should not be a struct at all, but a class. Structs are intended for small, immutable types that represent a single value.

Making a data type marshal exactly as you want is really tricky, and as you are not doing interop you really don’t need marshalling at all. It’s easier to just use a BinaryReader to read the data from the file.

The simple data types can be read straight off, and the strings can be read like this:

string value = reader.ReadChars(reader.ReadShort());

Specify an appropriate single byte encoding when opening the reader, for example windows-1252.