Returning a string from unmanaged dll to .net
I write most of my code in unmanaged languages such as Delphi and C/C++. Sometimes I need to interface my code to .net code in which case I create a dll.
A recurring thing is that I need to return string to .net.
There are many ways to do this of course but in all cases we need to manage memory: who will allocate the memory for the string and who is responsible for freeing it?
Windows API Method
Windows API often requires the caller to pass in an allocated buffer and it’s maximum size and returns the actual size. An example is the GetComputerName API:
BOOL WINAPI GetComputerName( _Out_ LPTSTR lpBuffer, _Inout_ LPDWORD lpnSize );
A typical call to this API looks like this in .net (VB.NET example):
Imports System.Runtime.InteropServices Imports System.Text Module Module1_ Public Function GetComputerName( _ ByVal Name As StringBuilder, _ ByRef CharCount As Integer) _ As Boolean End Function Sub Main() Const MAX_COMPUTERNAME_LENGTH As Integer = 15 Dim Buffer As New StringBuilder(MAX_COMPUTERNAME_LENGTH) Dim bRes As Boolean Dim iLen As Integer iLen = MAX_COMPUTERNAME_LENGTH bRes = GetComputerName(Buffer, iLen) Buffer.Length = iLen If bRes Then System.Console.WriteLine("Computername: " & Chr(34) & Buffer.ToString() & Chr(34)) End If End Sub End Module
As you can see we need to allocate the string before the API call and set the correct length after the API call. For that reason I used the StringBuilder class because the regular .net string class does not have a way to change the string length.
BSTR Method
For this reason I prefer to use the BSTR type because it has much easier memory allocation and deallocation.
An example using ATL’s CString class in C:
BOOL __stdcall ReturnString(BSTR* TheString) { CString s = _T("Hello World!"); try { *TheString = s.AllocSysString(); return TRUE; } catch (CAtlException Exception) { return FALSE; } }
Imports System.Runtime.InteropServices Module Module1_ Public Function ReturnString( _ ByRef TheString As String) _ As Boolean End Function Sub Main() Dim bRes As Boolean Dim TheString As String = Nothing bRes = ReturnString(TheString) If bRes Then System.Console.WriteLine("The String " & TheString) End If End Sub End Module
Below an example in Delphi for the ReturnString implementation (VB code stays the same). Note that in all examples I am not passing the string as the function return value because of compiler implementation differences as described here.
function ReturnString(TheString: WideString): Boolean; stdcall; begin try TheString := 'Hello World'; finally Result := True; end; end;
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