Merge pull request #395 from LassieME/issue/359

Conflicts:
	src/Discord.Net.WebSocket/DiscordSocketClient.cs

src/Discord.Net.Core/Utils/Cacheable.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace Discord
+{
+    /// <summary>
+    /// Contains an entity that may be cached.
+    /// </summary>
+    /// <typeparam name="TEntity">The type of entity that is cached</typeparam>
+    /// <typeparam name="TId">The type of this entity's ID</typeparam>
+    public struct Cacheable<TEntity, TId>
+        where TEntity : IEntity<TId>
+        where TId : IEquatable<TId>
+    {
+        /// <summary>
+        /// Is this entity cached?
+        /// </summary>
+        public bool HasValue { get; }
+        /// <summary>
+        /// The ID of this entity.
+        /// </summary>
+        public TId Id { get; }
+        /// <summary>
+        /// The entity, if it could be pulled from cache.
+        /// </summary>
+        /// <remarks>
+        /// This value is not guaranteed to be set; in cases where the entity cannot be pulled from cache, it is null.
+        /// </remarks>
+        public TEntity Value { get; }
+        private Func<Task<TEntity>> DownloadFunc { get; }
+
+        internal Cacheable(TEntity value, TId id, bool hasValue , Func<Task<TEntity>> downloadFunc)
+        {
+            Value = value;
+            Id = id;
+            HasValue = hasValue;
+            DownloadFunc = downloadFunc;
+        }
+
+        /// <summary>
+        /// Downloads this entity to cache.
+        /// </summary>
+        /// <returns>An awaitable Task containing the downloaded entity.</returns>
+        /// <exception cref="Discord.Net.HttpException">Thrown when used from a user account.</exception>
+        /// <exception cref="NullReferenceException">Thrown when the message is deleted.</exception>
+        public async Task<TEntity> DownloadAsync()
+        {
+            return await DownloadFunc();
+        }
+
+        /// <summary>
+        /// Returns the cached entity if it exists; otherwise downloads it.
+        /// </summary>
+        /// <returns>An awaitable Task containing a cached or downloaded entity.</returns>
+        /// <exception cref="Discord.Net.HttpException">Thrown when used from a user account.</exception>
+        /// <exception cref="NullReferenceException">Thrown when the message is deleted and is not in cache.</exception>
+        public async Task<TEntity> GetOrDownloadAsync() => HasValue ? Value : await DownloadAsync();
+    }
+}