public static Task<object> CustomAggregateAsync<T>(
    this IQueryable<T> query,
    string customAggregateName,
    Expression<Func<T, object>> argument,
    CancellationToken cancellationToken = default(CancellationToken)
)

<ExtensionAttribute>
Public Shared Function CustomAggregateAsync(Of T)(
    query As IQueryable(Of T),
    customAggregateName As String,
    argument As Expression(Of Func(Of T, Object)),
    cancellationToken As CancellationToken = Nothing
) As Task(Of Object)