Microsoft.Extensions.DependencyModel.LinqHelpers
public static IEnumerable<TResult> SelectMany<TSource, TResult, TParam1>(IEnumerable<TSource> src, Func<TSource, TParam1, IEnumerable<TResult>> func, TParam1 param1)
public static IEnumerable<TResult> SelectMany<TSource, TResult, TParam1, TParam2>(IEnumerable<TSource> src, Func<TSource, TParam1, TParam2, IEnumerable<TResult>> func, TParam1 param1, TParam2 param2)