NSubstitute, try catch is not working on async method configured to throw an exception

Question

I'm using NSubstitute for mocking and faking. I'm working with EF6 and would like to setup the SaveChangesAsync-Method of the database context to throw an exception:

context.SaveChangesAsync().Throws(new DbUpdateException("", SqlExceptionHelper.CreateSqlException(2627)));

SaveChangesAsync is called within a method of my data repository like this:

try
{
     var fromDatabase = await context.Entries.OfType<Document>().FirstOrDefaultAsync(d => d.Id == doc.Id);

      if (fromDatabase == null)
      {
           fromDatabase = new Document();
           context.Entries.Add(fromDatabase);
       }

       PatchEntity(fromDatabase, doc);

       await context.SaveChangesAsync();
   }
   catch (DbUpdateException ex)
   {
       var innerException = ex.InnerException as SqlException;

       if (innerException != null && innerException.Number == 2627)
       {
           errors.Add(new DbValidationError(nameof(doc.Name), "A entry with the same Name already exists under the selected parent."));
        }
    }

And this is the line within my unit test:

var result = await repository.TryAddOrUpdateDocument(doc);

Unfortunately my test keeps failing with the reason, that my test method(!) is throwing the exception, I'm trying to catch. Adding a general exception catch block isn't working either, the exception is not being catched at all. The exception is bubbling up.

My test is declared as "public async Task...", but turning it into simply void and calling .Result on the async method of my repository doesn't help either. What is going on?

score 2 · Accepted Answer · answered Mar 17 '17 at 00:45

I think the problem is that the exception is thrown from the original call instead of from inside the returned Task as described here.

Try something like:

Func<int> throwDbEx = () => { 
    throw new DbUpdateException("", SqlExceptionHelper.CreateSqlException(2627));
}; 
context.SaveChangesAsync().Returns(Task.Run(throwDbEx));

score 1 · Answer 2 · answered Mar 16 '17 at 16:09

I cannot be 100% sure, but you are probably facing the problem that exceptions thrown by async with void return type cannot be caught naturally. Read the section "Avoid Async Void" here: https://msdn.microsoft.com/en-us/magazine/jj991977.aspx Even if it won't answer your problem, it is worth reading anyway...

NSubstitute, try catch is not working on async method configured to throw an exception

2 Answers2