Visual basic WPF 加载 ATL C++ dll 程序退出时返回字符串类型

Question

I'm using VB.net WPF with dll made with ATL for this project but every time I run dll function, program exit with no error please let me know why is not working and how to fix it.

c++ function

extern "C" __declspec(dllexport) BSTR __cdecl sprint(LPSTR str1, LPSTR str2) {
    TCHAR test[100];
    sprintf_s(test, 100, "%s %s", str1, str2);

    BSTR test2 = L"helloworld";
    return test2;
}

VB.net code

declaration

    <DllImport("database.dll", CallingConvention:=CallingConvention.Cdecl)>
    Private Shared Function sprint(ByVal str1 As String, ByVal str2 As String) As String
    End Function

usage

    Private Sub On_Submit_Btn_Clk(sender As Object, e As RoutedEventArgs)
        testStr = sprint("hello", "world")
        MessageBox.Show(testStr)
    End Sub

issue:

program exit when function "sprint" called.

no issue:

program not exit when function "sprint" return integer.

only BSTR, LPTSTR, LPSTR type returns make proram exit.

Answer 1

In COM programming, a BSTR is a special LPWSTR which is allocated by SysAllocString , freed by SysFreeString . The characters are preceded by the string length, at offset -1. That means you can pass a BSTR to all functions that expect an LPCWSTR

L"helloworld" is a C++ literal, which happens to almost match the LPWSTR type, except for the const part. And VC++ will gloss over that last small problem. The easiest solution in MSVC is to use the _bstr_t wrapper type for BSTR , which does SysAllocString for you. Else you'll need to do it manually.