vb.net调用非托管函数aa.dll，aa.dll包括2个入参数，2个出参数。请教怎么调用?解决送100分

[qiri07]

在VC++中调用
定义 int GetContent(IN char* lpszURL, IN char* lpszSubject, OUT char* lpszPushContent, OUT int &nTotalPacket);
调用 int rc = GetContent((char *)(LPCTSTR)m_strURL, (char *)(LPCTSTR)m_strSubject, szContent, nPacketCount);

在VB.net中如何调用？？？
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本人试验

<DllImport("aa.DLL", EntryPoint:="GetContent", SetLastError:=True, CharSet:=CharSet.Unicode, ExactSpelling:=True, CallingConvention:=CallingConvention.StdCall)> Public Shared Function GetContents(ByVal src As String, ByVal dst As String, ByVal contents As String, ByVal intpage As Integer) As Integer
' Leave function empty - DllImport attribute forces calls
End Function

调用时 RetVal = GetContents(sendurl.Text, sendtext.Text, contents, intpage)

提示：调用行 ：未将对象引用设置到对象的实例。
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如果这样
<DllImport("aa.DLL", EntryPoint:="GetContent", SetLastError:=True, CharSet:=CharSet.Unicode, ExactSpelling:=True, CallingConvention:=CallingConvention.StdCall)> Public Shared Function GetContents(ByVal src As String, ByVal dst As String) As Integer
' Leave function empty - DllImport attribute forces calls
End Function

调用时 RetVal = GetContents(sendurl.Text, sendtext.Text)
倒是可以调用，但无法 得到函数返回值...

[qiri07]

本人无比倾佩 Sunmast(速马.上海) 兄弟！！！
对你的敬仰如 滔滔江水连绵不绝，犹如 黄河泛滥一发不可收拾
无条件奉送100分。

[速马]

<DllImport("aa.DLL", EntryPoint:="GetContent", SetLastError:=True, CharSet:=CharSet.Unicode, ExactSpelling:=True, CallingConvention:=CallingConvention.StdCall)> Public Shared Function GetContents(ByVal src As String, ByVal dst As String, ByVal contents As StringBuilder, ByRef intpage As Integer) As Integer
' Leave function empty - DllImport attribute forces calls
End Function
调用前，第三个参数，ByVal contents As StringBuilder，要先初始化，以开辟字符串缓冲区

[速马]

OUT int &nTotalPacket应该对应ByRef intpage As Integer
还有这里为什么用CharSet:=CharSet.Unicode？既然原形是char*。