Object reference not set to an instance of an object.

Question

Hi All,

I am using C# for ArcObjects desktop. Kindly clarify me on the below to declarations

1.   IApplicatin app = IMxDocument.doc as IApplication;
      ----
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throws an exception (NUllReferenceException) when i use 
app.OpenDocument(filename)------ where filename is a string in the openFileDialog1

2.    IApplication app = (IMxDocument.doc as IDocument).Parent
---
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This does not throw an exception. My doubt is why does the first option throw an error while second option does not even though both IApplication & IDocument.Parent both refer to Application.

Thanks a lot in advance...!

Gautham_SChejarla · Answer

IMxDocument is not the same as IApplication, they are in two different co-classes.  The document is derived from the application so you can't cast a document as application but you can cast the parent of the document as application because the parent of a document is the application.  Hopefully that explanation helps.

Thanks a lot....

SteveFang · Answer

IMxDocument is not the same as IApplication, they are in two different co-classes.  The document is derived from the application so you can't cast a document as application but you can cast the parent of the document as application because the parent of a document is the application.  Hopefully that explanation helps.