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am doing a dll in vc++ for blocking ports(like tcp sniffer).while blocking udp port in local machine its works fine.while am taking remote computer from my my system its raises on exception 

      "system.NullReferenceException'occurred in system.windows.forms.dll"

i incorporated the dll in c# application.please give your suggestion

maxy
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2 Answers2

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With very limited information, The most probable cause that I can think of is:

Your C# application is trying to create an instance of the C++ dll's class and most probably that fails resulting in a NULL pointer. The later call of a method through that instance is resulting in this exception.

To resolve this, you should check first whether instance was created successfully or not.

Aamir
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Too bad you did not collect and share this information. Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

john
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