using System.Runtime.InteropServices;
[DllImport("sde.dll")]//还有其他的属性可参考MSDN
public static extern Int32 SE_connection_create(string server, string instance, string database, string username, string pwd, ref SE_ERROR error, out IntPtr pSdeConn);

 '--------------↓声明SE_ERROR结构体↓-----------------

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode)> _
Public Structure SE_ERROR
Public sde_error As Int32
Public ext_error As Int32
'<MarshalAs(UnmanagedType.ByValArray, SizeConst:=512)> _
            Public err_msg1 As Char()
'<MarshalAs(UnmanagedType.ByValArray, SizeConst:=4096)> _
            Public err_msg2 As Char()
End Structure
'--------------↑声明SE_ERROR结构体↑-----------------

'-------------------调用SE_connection_create函数----------------------
        '------------------连接SDE-------------------------
        '返回结果
        'SE_SUCCESS     0
        'SE_FAILURE     -1
        'SE_NO_ACCESS   -15
        'SE_SDE_NOT_STARTED     -5
        'SE_IOMGR_NOT_AVAILABLE     -101
        'SE_INVALID_DATABASE        -162
        'SE_INVALID_SERVER      -100
        'SE_INVALID_POINTER     -65
        'SE_INVALID_USER        -9
        'SE_LOGIN_NOT_ALLOWED       -8
        'SE_DBMS_DOES_NOT_SUPPORT       -1008
        'SE_NET_FAILURE     -10
        'SE_NET_TIMEOUT     -11
        'SE_SERVICE_NOT_FOUND       -102
        'SE_TASKS_EXCEEDED      -7

<DllImport("sde.dll", SetLastError:=True,ThrowOnUnmappableChar:=True)> _
Public Shared Function SE_connection_create(ByVal server As String, ByVal instance As String, _
ByVal database As String, ByVal username As String, ByVal password As String, ByRef error1 As SE_ERROR, _
ByRef connection As IntPtr) As Int32
End Function
'----------------------------调用SE_connection_free函数-------------------------
        '----------------------------释放SDE连接----------------------
        <DllImport("sde.dll", SetLastError:=True, ThrowOnUnmappableChar:=True)> _
Public Shared Sub SE_connection_free(ByVal Byrefconnection As IntPtr)
End Sub