NullReferenceException after method return

Question

I have encountered a NullReferenceException in an odd location. You can see where it is in the picture:

The exception was thrown outside of try catch and after return. And both Move and MoveResult data types are structures and are not reference type.

Here are the details of the exception:

System.NullReferenceException was unhandled
      Message=Object reference not set to an instance of an object.
      Source=MatinChess.Net
      StackTrace:
           at MatinChess.Net.MatinChess.MovePiece(Move move) in C:\Users\Matin\Documents\GitHub\MatinChessDLL\dotnet\MatinChess.Net\MatinChess.cs:line 37
           at MatinChess.Net.Demo.Program.Main(String[] args) in C:\Users\Matin\Documents\GitHub\MatinChessDLL\dotnet\MatinChess.Net.Demo\Program.cs:line 31
           at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
           at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
           at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
           at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
           at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
           at System.Threading.ThreadHelper.ThreadStart()
      InnerException:

I am using .NET Framework 2.0 on Visual Studio 2015 community edition.

Edit

When I comment the ExternMethods.MovePiece line, it does not throw the exception.

Here is the definition of this method:

[DllImport(dll, CallingConvention = CallingConvention.Cdecl)]
public extern static void MovePiece(Move movement, ref MoveResult result);

@SelmanGenç The stack trace is given in the question. And yes I tried clearing and rebuilding the solution. — Matin Lotfaliee, Dec 03 '16 at 11:18
@MatinLotfaliee okey I didn't see it at the first glance, try manually deleting bin and obj folder and debug the code again, use F11 to go through every step and you should find where it blows. — Selman Genç, Dec 03 '16 at 11:20
@Enigmativity it is not a duplicate. I know what does this exception mean. — Matin Lotfaliee, Dec 03 '16 at 11:20
Btw, what's in line 37? since we can't see the line numbers in editor. — Selman Genç, Dec 03 '16 at 11:21
@SelmanGenç when I use F11, it runs the return and then goes to the } mark, after another F11 it throws the exception. Line 37 is the } mark — Matin Lotfaliee, Dec 03 '16 at 11:22
@AfzaalAhmadZeeshan As I said, Move and MoveResult are structs. They are not reference type and can not be null. On the other hand, The whole method is in try catch. — Matin Lotfaliee, Dec 03 '16 at 11:25
Ah right, can you share how you are calling this? Also check if there is an InnerException detail possible. — Afzaal Ahmad Zeeshan, Dec 03 '16 at 11:27
@AfzaalAhmadZeeshan I call this method from my Main method. I don't know how to share how am I calling this. The InnerException is set to null. — Matin Lotfaliee, Dec 03 '16 at 11:31
Right, just share the `new Class().MovePiece(move);` code from Main function. Can you consider providing a fiddle (htp://dotnetfiddle.net)? — Afzaal Ahmad Zeeshan, Dec 03 '16 at 11:34
@AfzaalAhmadZeeshan here you are: https://dotnetfiddle.net/S3eAaS — Matin Lotfaliee, Dec 03 '16 at 11:42
My guess is that the exception is the result of a corrupted call stack, either because of an error in the native function you are calling, either because you declared it incorrectly. Are you sure about the Cdecl calling convention? Are your structs declared correctly? — Kris Vandermotten, Dec 03 '16 at 18:08
@KrisVandermotten why doesn't it throw an exception on the MovePiece method call? I believe if it was the error of declared struct, it would throw BadImageFormatException. — Matin Lotfaliee, Dec 03 '16 at 18:20
Please show the declaration of that external method in the other dll, ie. the C code that declares it or whatever programming language it is. Since you've narrowed it down to being related to this external method call, we need to know what that other method/function looks like. — Lasse V. Karlsen, Dec 03 '16 at 18:23
Since the exception is thrown at method end *after* you've called an external method, my guess is that the method signatures doesn't match so the stack is corrupted. — Lasse V. Karlsen, Dec 03 '16 at 18:24
@LasseV.Karlsen you are right. I rechecked my MoveResult structure and changed it. now the exception is gone. Write the answer with references if possible and gain your reputation. — Matin Lotfaliee, Dec 03 '16 at 19:30

score 1 · Accepted Answer · answered Dec 03 '16 at 19:34

You've edited in a vital clue: The method being called, the one that somehow provokes this exception, is defined using [DllImport(...)].

Since the exception is thrown at the end of the method that calls this external function, most likely the signature specified in C# does not match the signature of the function compiled into this dll.

This may lead to the stack becoming corrupted when you call it. Symptoms of such corruption may include:

Odd exceptions when the method call returns
Odd exceptions after the method call returned
Data corruption, like stack variables overwritten, etc.

Since you've now verified that after changing the MoveResult structure, the exception has gone away, it is safe to say that this is what happened.

NullReferenceException after method return

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