public static Task<TSource> FirstAsync<TSource>(
	this IMongoQueryable<TSource> source,
	Expression<Func<TSource, bool>> predicate,
	CancellationToken cancellationToken = null
)

<ExtensionAttribute>
Public Shared Function FirstAsync(Of TSource) ( 
	source As IMongoQueryable(Of TSource),
	predicate As Expression(Of Func(Of TSource, Boolean)),
	Optional cancellationToken As CancellationToken = Nothing
) As Task(Of TSource)

[<ExtensionAttribute>]
static member FirstAsync : 
        source : IMongoQueryable<'TSource> * 
        predicate : Expression<Func<'TSource, bool>> * 
        ?cancellationToken : CancellationToken 
(* Defaults:
        let _cancellationToken = defaultArg cancellationToken null
*)
-> Task<'TSource>