Question

我正在Visual Studio 2013中用VB.NET编写一个小应用程序，到目前为止我写的是以下代码：

Public Class MainMenu

    Private Declare Function Version_Get Lib "mypath/mydll.dll" () As String   

    ' Before anyone asks, yes, the DLL is present in the mypath folder

    Private Sub MainMenu_Load(sender As Object, e As EventArgs) Handles Me.Load

        Dim Temp As String
        Dim Dummy As Integer

        On Error GoTo Handler
        Temp = Version_Get
        Dummy = 1 ' This line is never reached in debug mode

Handler:
        If Not IsNothing(Err.GetException()) Then
            MsgBox("Error " & Str(Err.Number) & " generated by the application " & Err.Source & ControlChars.CrLf & Err.Description, vbCritical, "Error")
            End
        End If

    End Sub

End Class

DLL应该返回BSTR（当然这是extern "C"等等，但为了理解我正在编写纯粹而简单的函数）：

档案.h

#ifdef EVALFUNC_EXPORTS
#define EVALFUNC_API __declspec(dllexport)
#else
#define EVALFUNC_API __declspec(dllimport)
#endif

extern "C"
{
    EVALFUNC_API BSTR __stdcall Version_Get();
}

档案.cpp

BSTR __stdcall Version_Get()
{
    CRegKey Key;
    CString sValue;
    BSTR Str;

    LONG nA = Key.Open(HKEY_LOCAL_MACHINE, _T("SOFTWARE\\my Software"), KEY_READ);

    // Before anyone asks, yes, the application is present in the system

    ULONG nValueLength = 0;
    LONG nB = Key.QueryStringValue(_T("Version"), NULL, &nValueLength);

    if (nValueLength > 0) LONG nC = Key.QueryStringValue(_T("Version"), sValue.GetBufferSetLength(nValueLength - 1), &nValueLength);    

    Str = sValue.AllocSysString();
    return Str;
}

问题在于，即使我编写了错误处理程序，代码也不会遇到错误，Version_Get会导致代码崩溃而不会出现任何错误（无论如何都会加载MainMenu表单。）

我在另一个VB环境（Excel）上使用相同的声明尝试了DLL。

Private Declare Function Version_Get Lib "mypath/mydll.dll" () As String

在这种情况下，字符串变量用正确的文本填充。

我做错了什么？

Answer 1

我必须通过编组BSTR字符串来创建一个导入Dll的类：

Imports System.Runtime.InteropServices

Public Class ImportDll

    <DllImport("myDll.dll", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)> _
    Public Shared Function Version_Get() As <MarshalAs(UnmanagedType.BStr)> String
    End Function
End Class

可以按如下方式调用代码：

Version = ImportDll.Version_Get

DLL不在visual studio 2013 VB代码中导出字符串

1 个答案: