public static Task<object> CustomAggregateAsync<T>(
    this IQueryable<T> query,
    string customAggregateName,
    params Expression<Func<T, object>>[] arguments
)

<ExtensionAttribute>
Public Shared Function CustomAggregateAsync(Of T)(
    query As IQueryable(Of T),
    customAggregateName As String,
    ParamArray arguments As Expression(Of Func(Of T, Object))()
) As Task(Of Object)