C# Task - LazyAsyncResult - NullReferenceException

Question

There is something wrong with the following method:

   public async Task<ExtendedProductDTO> GetProductByUniqueIDAsync(string id)  
            {  
                var product = await _productRepo.FindFirstOrDefaultAsync(i => i.UniqueID == id).ConfigureAwait(false);  
                return new ExtendedProductDTO(product != null, product);  
            **}**  

product is populated correctly and using a debugger I can reach the final } without any problems, but after that I got a weird NullReferenceException.
The exception happens after:

Notes:

• ExtendedProductDTO just sets 2 fields (a boolean "Found" property and the actual product)
• product is correctly retrieved by FindFirstOrDefaultAsync
• In order to understand what's happening I am invoking the method as follows, but the execution halts after the first call. So product4 = ... and the catch clause are never reached
  
• I also have a sync method (very similar, just sync) and it works without any problems
• There are also references to .NET Framework calls, see images below:
  
  

I really don't understand what's happening here...

Answer 1

I found the root cause of this issue.

It was not in the:

• MongoDB driver (async)
• My repository (async)
• Application layer (async)
• ASP.NET controller (async)

The async controller method was invoked synchronously by the caller and that caused the exception: invoking it in the proper way (=asynchronously) avoids the exception.

Thank you for your help!

Answer 2

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=<Cannot evaluate the exception source>
StackTrace:
<Cannot evaluate the exception stack trace>

Your app has entered a break state, but there is no code to show because all threads were executing external code (typically system or framework code).

Answer 3

You are using async/await so when something goes wrong inside the auto-generated code to support this the debugger throws you into the framework code as you're seeing. Unfortunately debugging is harder with async/await right now.

Nevertheless the issue is that you're referencing an instance member on a null object. Given the code you posted the only case I see where that happens is on the _productRepo instance. Therefore I assume it is null and causes the exception. You set a breakpoint on that line but keep in mind it is running async so it may not fire until later.

You also create an instance of ExtendedProductDTO. That too accepts a reference object, Product, and can blow up. If you look at the callstack for the NullReferenceException it should identify exactly where the exception is being thrown. Since we cannot see what your ExtendedProductDTO is doing you might start with it. Maybe the product you're passing it has nulls where it doesn't expect them and it blows up.

Answer 4

var product = await _productRepo.FindFirstOrDefaultAsync(i => i.UniqueID == id).ConfigureAwait(false);

is always populated correctly:

and ExtendedProductDTO is simply:

public class ExtendedProductDTO  
    {  
        public bool Found { get; }  
  
        public ExtendedProduct Product { get; }  
  
        public ExtendedProductDTO(bool found, ExtendedProduct product)  
        {  
            Found = found;  
            Product = product;  
        }  
    }  

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=<Cannot evaluate the exception source>
StackTrace:
<Cannot evaluate the exception stack trace>

Your app has entered a break state, but there is no code to show because all threads were executing external code (typically system or framework code).

Answer 5

This does not work:

  public Task<T> FindFirstOrDefaultAsync(Expression<Func<T, bool>> predicate)
            {
                return _collection.Find(predicate).FirstOrDefaultAsync();
            }

This works:

 public T FindFirstOrDefault(Expression<Func<T, bool>> predicate)
        {
            return _collection.Find(predicate).FirstOrDefaultAsync().Result;
        }

That method is invoked by:

public async Task<ExtendedProductDTO> GetProductByUniqueIDAsync(string id)
        {
            var product = await _productRepo.**FindFirstOrDefaultAsync**(i => i.UniqueID == id).ConfigureAwait(true);
            return new ExtendedProductDTO(product != null, product);
        }

And GetProductByUniqueIDAsync is invoked as:

 try
            {
                var product3 = await _productService.GetProductByUniqueIDAsync("9988ad0a66f28fe8611962a06a593865c1c8103f87258e3920986fc2a4ba2d0e");
                var product4 = await _productService.GetProductByUniqueIDAsync("6641020d80e10877a13a973592d934165de39c743f7a2fa3f78a8671ac2e9c5b");
            }
            catch (Exception ex)
            {
                Logger.Fatal(ex.Message, ex);
            }

As said, I can reach the } of GetProductByUniqueIDAsync and all data is there (product and then ExtendedProductDTO).

When the exception is triggered (=always), the execution of the caller method is interrupted: no lines after " var product3 = ..." is reached.