public static Task<Nullable<double>> SumAsync<TSource>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, Nullable<double>>> selector,
	CancellationToken cancellationToken
)

<ExtensionAttribute>
Public Shared Function SumAsync(Of TSource) ( 
	source As IQueryable(Of TSource),
	selector As Expression(Of Func(Of TSource, Nullable(Of Double))),
	cancellationToken As CancellationToken
) As Task(Of Nullable(Of Double))