首先打开vsto,创建窗体，添加一个按钮，双击按钮录入代码：

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim cad As Object
        cad = CreateObject("autocad.Application")
        cad.Visible = True
        cad.documents.Add

        cad.application.documents.Open("D:\ZD\你的文件.dwg")
    End Sub
End Class

由下图可见，已打开默认cad版本：