Hi, I'm just getting started with Interop, and I would appreciate any help
you might give me on this question (and others upcoming, I'm sure).

I have been assigned a project which requires me to interact with a regular
non-COM DLL.  I don't have any definitions for the library functions, but I
have detailed VB examples of how to work with the DLL.

The Visual Basic examples show the following:
   Type BVLogonRecord
       Directory               As String * 256
       UserID                  As String * 12
       Password                As String * 8
       LogonDate               As String * 8
   End Type

   Declare Function BVLogonWithDir Lib "bvapi.dll" (LogonRecord As
BVLogonRecord) As Integer

VB function:
   Dim LogonRecord As BVLogonRecord

   LogonRecord.Directory = "C:\DBDIR"
   LogonRecord.UserID = "USER"
   LogonRecord.Password = "PASSWORD"
   LogonRecord.LogonDate = "20041117"

   result = BVLogonWithDir(LogonRecord)

So, I tried this in C#:
[StructLayout(LayoutKind.Sequential, Pack=1, CharSet=CharSet.Ansi)]
public struct BVLogonRecord
{
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst=256)]
 public String Directory;
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst=12)]
 public String UserID;
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst=8)]
 public String Password;
 [MarshalAs(UnmanagedType.ByValTStr, SizeConst=8)]
 public String LogonDate;
}

[DllImport(@"C:\BV\bvapi.dll", CharSet=CharSet.Ansi)]
public static extern int BVLogonWithDir
([MarshalAs(UnmanagedType.Struct)]BVLogonRecord LogonRecord);

public void Logon()
{
 String dir = @"C:\DBDIR".PadRight(256,' ');
 String usr = "USER".PadRight(12,' ');
 String pwd = "PASSWORD".PadRight(8,' ');
 String dt = "20041117".PadRight(8,' ');
 BVLogonRecord lr = new BVLogonRecord(dir,usr,pwd,dt);

 int result = BVTest.BVLogonWithDir(lr);
}

So I have been able to come up with three results:
1)  if I append all the strings together and store them in Directory, then
the function works. (regardless of what's stored in the other variables)
2)  when I use UnmanagedType.ByValTStr  I get "Object reference not set to
an instance of an object." on the call to the BVTest.BVLogonWithDir(lr).  As
soon as I comment out the [MarshalAs...] on the struct properties, i don't
get the error any more, but the function still doesn't work.
3)  everything else I do gives me bad parameter errors, missing paramters,
etc.

Sorry for the long post, but I'm hoping that if someone can help me figure
out where I'm going wrong.  Plus any website references to help me convert
VB examples to working C# functions/call woud be invaluable.

Thanks very, very, very much in advance for any help.

Sincerely,
Trevor Braun