Solidworks API and Winforms C# swApp (Standalone)

这是solidworks告诉您引用swApp所做的事情，但我一直在引用NullReferenceException。

有人知道为什么吗?或者我如何着手修理?

我以后可以引用它吗，比如swApp = ????

private void button5_Click(object sender, EventArgs e)
{
    //Save Drawing
    ModelDoc2 swDoc = null;
    int longstatus = 0;
    swDoc = ((ModelDoc2)(swApp.ActiveDoc));
    longstatus = swDoc.SaveAs3(
            @"C:'Engineering'Engineering'SW Automation'Linear Actuator Technology'MLD Series'Prints'Configured Prints'" + 
            textBox1.Text + ".SLDDRW", 0, 2);
}
public SldWorks swApp;

SldWorks swApp = null;
swApp = (SldWorks)Activator.CreateInstance(
            Type.GetTypeFromProgID("SldWorks.Application"));