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Passing String To DLL Function - String Not Returned Properly

I'm trying to call a DLL function, passing it a string, expecting it to modify the string and return it.

Function Prototype:

Declare Function GetLastError Lib "DBAPI2.dll" (ByRef msg As StringBuilder) As Integer

String Declaration:

 <MarshalAs(UnmanagedType.LPStr)> Public tmsg As New StringBuilder("Default" & Chr(0), 4096)
According to documentation, the function should expect to be passed a string of size 4096.

Current Usage:

Result = GetLastError(tmsg)

Calling GetLastError should return an error message placed in tmsg. Whenever I call GetLastError though, I'm either returned an empty string, or 1 weird character.

Am I doing something wrong in my implementation here? Any insight?
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diablo089
Asked:
diablo089
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1 Solution
 
nffvrxqgrcfqvvcCommented:
Can you post the documentation and examlpe in the original language it was written?
You can marshal it <Out()> Byval


Declare Function GetLastError Lib "DBAPI2.dll" (Byval msg As StringBuilder) As Integer

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diablo089Author Commented:
The API and libraries are Embarcadero (Borland) Dephi Win32.

Here's the original function prototype in Delphi:

function GetLastError(Msg: pChar): integer;
  stdcall; external 'DBAPI2.dll';

I was told by one of the guys that works with the API is that the function expects a string of size 4096.

Attached is some info I got from one of the developers that occasionally works with the API he wrote in C# that apparently works.



Prototype:

namespace TestDBAPI
{
                class DBAPI
                {
 [DllImport(dllname, CharSet = CharSet.Ansi)]
public static extern int GetLastError(StringBuilder msg);
                }
}

Usage:

StringBuilder msg = new StringBuilder(4096);
DBAPI.GetLastError(msg);

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nffvrxqgrcfqvvcCommented:
Okay.. You can try something like this
<DllImport("DBAPI2.dll", CharSet:=CharSet.Ansi)> _
    Private Shared Function GetLastError(ByVal msg As System.Text.StringBuilder) As Integer
    End Function
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim msg As New System.Text.StringBuilder(4096)
        GetLastError(msg)
    End Sub

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diablo089Author Commented:
Okay, I tried what you suggested as is, and I can't get anything put into "msg". If I switch to ByRef msg, I either get one character or a string of characters, but it's either a random character or a mess of random characters. Nothing discernible as a word or phrase.
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nffvrxqgrcfqvvcCommented:
Do you know if it always returns "something" regardless? For example would it say "no error" if you never called into ANY other exported routine that would write the error from that routine into the buffer.
Lets just says the DLL exported a function called Initalize() if initialize failed it would write the error string into the buffer but if you never called any other method it could just return NULL unless there is *always* a default string that is suppose to be returned if only calling GetLastError...
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diablo089Author Commented:
Hmm... not sure if it always returns something. I'll see if can find out.
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