feature: add emote formatting/parsing, +tests

src/Discord.Net/Models/Emotes/AttachedGuildEmote.cs

@@ -30,7 +30,7 @@ namespace Discord
         public IGuild Guild { get; set; }
 
         // IMentionable
-        public string Mention => throw new System.NotImplementedException();
+        public string Mention => EmoteUtilities.FormatGuildEmote(Id, Name);
 
         public Task DeleteAsync()
             => Discord.Rest.DeleteGuildEmojiAsync(Guild.Id, Id);

src/Discord.Net/Models/Emotes/Emote.cs

@@ -13,6 +13,6 @@ namespace Discord
         public ulong Id { get; set; }
         public string Name { get; set; }
 
-        public string Mention => throw new System.NotImplementedException(); // TODO: EmojiUtils
+        public string Mention => EmoteUtilities.FormatGuildEmote(Id, Name);
     }
 }

src/Discord.Net/Utilities/EmoteUtilities.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Discord
+{
+    public static class EmoteUtilities
+    {
+        public static string FormatGuildEmote(ulong id, string name)
+            => $"<:{name}:{id}>";
+
+        public static (ulong, string) ParseGuildEmote(string formatted)
+        {
+            if (formatted.IndexOf('<') != 0 || formatted.IndexOf(':') != 1 || formatted.IndexOf('>') != formatted.Length-1)
+                throw new ArgumentException("passed string does not match a guild emote format", nameof(formatted)); // TODO: grammar
+
+            int closingIndex = formatted.IndexOf(':', 2);
+            if (closingIndex < 0)
+                throw new ArgumentException("passed string does not match a guild emote format", nameof(formatted));
+
+            string name = formatted.Substring(2, closingIndex-2);
+            string idStr = formatted.Substring(closingIndex + 1);
+            idStr = idStr.Substring(0, idStr.Length - 1); // ignore closing >
+            ulong id = ulong.Parse(idStr); // TODO: TryParse here?
+
+            return (id, name);
+        }
+    }
+}

test/Discord.Tests.Unit/Utilities/EmoteTests.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xunit;
+
+namespace Discord.Tests.Unit
+{
+    public class EmoteTests
+    {
+        [Fact]
+        public void Parse()
+        {
+            string input = "<:gopher:243902586946715658>";
+            var (resultId, resultName) = EmoteUtilities.ParseGuildEmote(input);
+            Assert.Equal(243902586946715658UL, resultId);
+            Assert.Equal("gopher", resultName);
+
+            Assert.Throws<ArgumentException>(() => EmoteUtilities.ParseGuildEmote("foo"));
+            Assert.Throws<ArgumentException>(() => EmoteUtilities.ParseGuildEmote("<foo"));
+            Assert.Throws<ArgumentException>(() => EmoteUtilities.ParseGuildEmote("<:foo"));
+            Assert.Throws<ArgumentException>(() => EmoteUtilities.ParseGuildEmote("<:foo>"));
+        }
+
+        [Fact]
+        public void Format()
+        {
+            string result = EmoteUtilities.FormatGuildEmote(243902586946715658, "gopher");
+            Assert.Equal("<:gopher:243902586946715658>", result);
+        }
+    }
+}