Returning a string from unmanaged dll to .net

13 Mar

I write most of my code in unmanaged languages such as Delphi and C/C++. Sometimes customers ask me to interface my code to their .net code in which case I create a dll for them.

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
);

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
    <DllImport("kernel32.dll", CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Unicode, SetLastError:=True)> _
    Public Function GetComputerName( _
        <[In]()> <[Out]()> <MarshalAs(UnmanagedType.LPWStr)> ByVal Name As StringBuilder, _
        <[In]()> 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

Imports System.Runtime.InteropServices

Imports System.Text

Module Module1

<DllImport("kernel32.dll", CallingConvention:=CallingConvention.StdCall, CharSet:=CharSet.Unicode, SetLastError:=True)> _

Public Function GetComputerName( _

<[In]()> <[Out]()> <MarshalAs(UnmanagedType.LPWStr)> ByVal Name As StringBuilder, _

<[In]()> 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;
	}

}

BOOL __stdcall ReturnString(BSTR* TheString)

{

CString s = _T("Hello World!");

try

{

*TheString = s.AllocSysString();

return TRUE;

}

catch (CAtlException Exception)

{

return FALSE;

}

And an example implementation in VB.NET:

Imports System.Runtime.InteropServices

Module Module1
    <DllImport("mydll.dll", CallingConvention:=CallingConvention.StdCall)> _
    Public Function ReturnString( _
       <[Out]()> <MarshalAs(UnmanagedType.BStr)> 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

Imports System.Runtime.InteropServices

Module Module1

<DllImport("mydll.dll", CallingConvention:=CallingConvention.StdCall)> _

Public Function ReturnString( _

<[Out]()> <MarshalAs(UnmanagedType.BStr)> 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):

function ReturnString(var TheString: WideString): Boolean; stdcall;
begin
  try
    TheString := 'Hello World';
  finally
    Result := True;
  end;
end;

function ReturnString(var TheString: WideString): Boolean; stdcall;

begin

try

TheString := 'Hello World';

finally

Result := True;

end;

Note: In all examples I am not passing the string as the function return value because of compiler implementation differences regarding the calling convention for the return value. A good description can be found here on stackoverflow.

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Filed under: .NET, C#, C++, Delphi

3 Responses for "Returning a string from unmanaged dll to .net"

David Heffernan April 23rd, 2013 at 16:34 1
In the final Delphi example you declare the function like this:
function ReturnString(TheString: WideString): Boolean;
Isn’t that a mistake. Surely it needs to be an out parameter.
Remko Weijnen April 23rd, 2013 at 18:58 2
Need to check but I thought that Delphi passes a WideString by reference (since it’s basically a pointer)…
David Heffernan April 23rd, 2013 at 21:52 3
Well, I’ve not tested but I’m 100% sure your version won’t work because you need to pass by ref.