Renamed existing Flatten method to FlattenAsync and added new Flatten method. Also fixed ClientHelper using incorrect guild batch count.

src/Discord.Net.Commands/Readers/UserTypeReader.cs

@@ -13,7 +13,7 @@ namespace Discord.Commands
         public override async Task<TypeReaderResult> Read(ICommandContext context, string input)
         {
             var results = new Dictionary<ulong, TypeReaderValue>();
-            IReadOnlyCollection<IUser> channelUsers = (await context.Channel.GetUsersAsync(CacheMode.CacheOnly).Flatten().ConfigureAwait(false)).ToArray(); //TODO: must be a better way?
+            IAsyncEnumerable<IUser> channelUsers = context.Channel.GetUsersAsync(CacheMode.CacheOnly).Flatten(); // it's better
             IReadOnlyCollection<IGuildUser> guildUsers = ImmutableArray.Create<IGuildUser>();
             ulong id;
 
@@ -46,7 +46,7 @@ namespace Discord.Commands
                 ushort discriminator;
                 if (ushort.TryParse(input.Substring(index + 1), out discriminator))
                 {
-                    var channelUser = channelUsers.FirstOrDefault(x => x.DiscriminatorValue == discriminator &&
+                    var channelUser = await channelUsers.FirstOrDefault(x => x.DiscriminatorValue == discriminator &&
                         string.Equals(username, x.Username, StringComparison.OrdinalIgnoreCase));
                     AddResult(results, channelUser as T, channelUser?.Username == username ? 0.85f : 0.75f);
 
@@ -58,8 +58,9 @@ namespace Discord.Commands
 
             //By Username (0.5-0.6)
             {
-                foreach (var channelUser in channelUsers.Where(x => string.Equals(input, x.Username, StringComparison.OrdinalIgnoreCase)))
-                    AddResult(results, channelUser as T, channelUser.Username == input ? 0.65f : 0.55f);
+                await channelUsers
+                    .Where(x => string.Equals(input, x.Username, StringComparison.OrdinalIgnoreCase))
+                    .ForEachAsync(channelUser => AddResult(results, channelUser as T, channelUser.Username == input ? 0.65f : 0.55f));
                 
                 foreach (var guildUser in guildUsers.Where(x => string.Equals(input, x.Username, StringComparison.OrdinalIgnoreCase)))
                     AddResult(results, guildUser as T, guildUser.Username == input ? 0.60f : 0.50f);
@@ -67,8 +68,9 @@ namespace Discord.Commands
 
             //By Nickname (0.5-0.6)
             {
-                foreach (var channelUser in channelUsers.Where(x => string.Equals(input, (x as IGuildUser)?.Nickname, StringComparison.OrdinalIgnoreCase)))
-                    AddResult(results, channelUser as T, (channelUser as IGuildUser).Nickname == input ? 0.65f : 0.55f);
+                await channelUsers
+                    .Where(x => string.Equals(input, (x as IGuildUser)?.Nickname, StringComparison.OrdinalIgnoreCase))
+                    .ForEachAsync(channelUser => AddResult(results, channelUser as T, (channelUser as IGuildUser).Nickname == input ? 0.65f : 0.55f));
 
                 foreach (var guildUser in guildUsers.Where(x => string.Equals(input, (x as IGuildUser).Nickname, StringComparison.OrdinalIgnoreCase)))
                     AddResult(results, guildUser as T, (guildUser as IGuildUser).Nickname == input ? 0.60f : 0.50f);

src/Discord.Net.Core/Extensions/AsyncEnumerableExtensions.cs

@@ -1,14 +1,64 @@
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Discord
 {
     public static class AsyncEnumerableExtensions
     {
-        public static async Task<IEnumerable<T>> Flatten<T>(this IAsyncEnumerable<IReadOnlyCollection<T>> source)
+        /// <summary>
+        /// Flattens the specified pages into one <see cref="IEnumerable{T}"/> asynchronously
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <param name="source"></param>
+        /// <returns></returns>
+        public static async Task<IEnumerable<T>> FlattenAsync<T>(this IAsyncEnumerable<IEnumerable<T>> source)
         {
-            return (await source.ToArray().ConfigureAwait(false)).SelectMany(x => x);
+            return await source.Flatten().ToArray().ConfigureAwait(false);
+        }
+
+        public static IAsyncEnumerable<T> Flatten<T>(this IAsyncEnumerable<IEnumerable<T>> source)
+        {
+            return new PagedCollectionEnumerator<T>(source);
+        }
+        
+        internal class PagedCollectionEnumerator<T> : IAsyncEnumerator<T>, IAsyncEnumerable<T>
+        {
+            readonly IAsyncEnumerator<IEnumerable<T>> _source;
+            IEnumerator<T> _enumerator;
+
+            public IAsyncEnumerator<T> GetEnumerator() => this;
+
+            internal PagedCollectionEnumerator(IAsyncEnumerable<IEnumerable<T>> source)
+            {
+                _source = source.GetEnumerator();
+            }
+
+            public T Current => _enumerator.Current;
+
+            public void Dispose()
+            {
+                _enumerator?.Dispose();
+                _source.Dispose();
+            }
+
+            public async Task<bool> MoveNext(CancellationToken cancellationToken)
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+                
+                if(!_enumerator?.MoveNext() ?? true)
+                {
+                    if (!await _source.MoveNext(cancellationToken).ConfigureAwait(false))
+                        return false;
+
+                    _enumerator?.Dispose();
+                    _enumerator = _source.Current.GetEnumerator();
+                    return _enumerator.MoveNext();
+                }
+
+                return true;
+            }
         }
     }
 }

src/Discord.Net.Rest/ClientHelper.cs

@@ -79,7 +79,7 @@ namespace Discord.Rest
             ulong? fromGuildId, int? limit, RequestOptions options)
         {
             return new PagedAsyncEnumerable<RestUserGuild>(
-                DiscordConfig.MaxUsersPerBatch,
+                DiscordConfig.MaxGuildsPerBatch,
                 async (info, ct) =>
                 {
                     var args = new GetGuildSummariesParams
@@ -106,7 +106,7 @@ namespace Discord.Rest
         }
         public static async Task<IReadOnlyCollection<RestGuild>> GetGuildsAsync(BaseDiscordClient client, RequestOptions options)
         {
-            var summaryModels = await GetGuildSummariesAsync(client, null, null, options).Flatten();
+            var summaryModels = await GetGuildSummariesAsync(client, null, null, options).FlattenAsync().ConfigureAwait(false);
             var guilds = ImmutableArray.CreateBuilder<RestGuild>();
             foreach (var summaryModel in summaryModels)
             {

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -356,7 +356,7 @@ namespace Discord.Rest
         async Task<IReadOnlyCollection<IGuildUser>> IGuild.GetUsersAsync(CacheMode mode, RequestOptions options)
         {
             if (mode == CacheMode.AllowDownload)
-                return (await GetUsersAsync(options).Flatten().ConfigureAwait(false)).ToImmutableArray();
+                return (await GetUsersAsync(options).FlattenAsync().ConfigureAwait(false)).ToImmutableArray();
             else
                 return ImmutableArray.Create<IGuildUser>();
         }