C# Mongo FirstOrDefaultAsync hangs

Question

using the 2.0 driver the following code will sometimes hang and never return.

public async Task<T> GetFirst(FilterDefinition<T> query)
{
    return await GetCollection.Find(query).FirstOrDefaultAsync();
}

if I debut and put a break point on the return line, everything returns normally. In the shell the query being run is something like this:

db.Customers.find({"Name" : /test$/i})

You probably need to add a ConfigureAwait to the end of the call so that you don't deadlock your main thread. return await GetCollection.Find(query).FirstOrDefaultAsync().ConfigureAwait(false); — Craig Wilson, Apr 15 '15 at 19:35
However, in this case, there really isn't any reason you need to use the async and await keywords... just return GetCollection.Find(query).FirstOrDefaultAsync(); — Craig Wilson, Apr 15 '15 at 19:36
removing the async and await worked... not sure why, I assumed because it was FirstOrDefaultAsync it needed an await flag — Chadit, Apr 15 '15 at 23:44
How exactly are you calling `GetFirst()`? Are you using something like `GetFirst(query).Result`? — svick, Apr 15 '15 at 23:55
I call the GetFirst with the await flag, so something like this await repo.GetFirst(filterQuery); — Chadit, Apr 16 '15 at 11:34

Craig Wilson · Accepted Answer · 2015-04-16T11:52:13.727

8

There are 2 solutions:

Add a ConfigureAwait(false) at the end:

return await GetCollection.Find(query).FirstOrDefaultAsync().ConfigureAwait(false);

Just return the Task<T>, since the result of FirstOrDefaultAsync() is the same type as the result you want to return.

public Task<T> GetFirst(FilterDefinition<T> query)
{
    return GetCollection.Find(query).FirstOrDefaultAsync();
}

edited Apr 16 '15 at 11:52

answered Apr 16 '15 at 00:52

Craig Wilson

1 Answers1