public static Task<Nullable<decimal>> AverageAsync<TSource>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, Nullable<decimal>>> selector
)

<ExtensionAttribute>
Public Shared Function AverageAsync(Of TSource) ( 
	source As IQueryable(Of TSource),
	selector As Expression(Of Func(Of TSource, Nullable(Of Decimal)))
) As Task(Of Nullable(Of Decimal))