diff --git a/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs b/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
index 5edd72c042..3207d2c44f 100644
--- a/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
+++ b/BizHawk.Client.Common/cheats/GbGgGameGenieDecoder.cs
@@ -6,11 +6,9 @@ namespace BizHawk.Client.Common.cheats
 	/// <summary>
 	/// Decodes Gameboy and Game Gear Game Genie codes
 	/// </summary>
-	public class GbGgGameGenieDecoder
+	public static class GbGgGameGenieDecoder
 	{
-		private readonly string _code;
-
-		private readonly Dictionary<char, int> _gbGgGameGenieTable = new Dictionary<char, int>
+		private static readonly Dictionary<char, int> _gbGgGameGenieTable = new Dictionary<char, int>
 		{
 			['0'] = 0,
 			['1'] = 1,
@@ -30,65 +28,67 @@ namespace BizHawk.Client.Common.cheats
 			['F'] = 15
 		};
 
-		public GbGgGameGenieDecoder(string code)
+		public static IDecodeResult Decode(string _code)
 		{
-			_code = code;
-			Decode();
-		}
+			if (_code == null)
+			{
+				throw new ArgumentNullException(nameof(_code));
+			}
 
-		public int Address { get; private set; }
-		public int Value { get; private set; }
-		public int? Compare { get; private set; }
+			if (_code.LastIndexOf("-") != 7 || _code.IndexOf("-") != 3)
+			{
+				return new InvalidCheatCode("All Master System Game Genie Codes need to have a dash after the third character and seventh character.");
+			}
 
-		public void Decode()
-		{
 			// No cypher on value
 			// Char # |   0   |   1   |   2   |   3   |   4   |   5   |   6   |   7   |   8   |
 			// Bit  # |3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|3|2|1|0|
 			// maps to|      Value    |A|B|C|D|E|F|G|H|I|J|K|L|XOR 0xF|a|b|c|c|NotUsed|e|f|g|h|
 			// proper |      Value    |XOR 0xF|A|B|C|D|E|F|G|H|I|J|K|L|g|h|a|b|Nothing|c|d|e|f|
+			var result = new DecodeResult { Size = WatchSize.Byte };
+
 			int x;
 
 			// Getting Value
 			if (_code.Length > 0)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[0], out x);
-				Value = x << 4;
+				result.Value = x << 4;
 			}
 
 			if (_code.Length > 1)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[1], out x);
-				Value |= x;
+				result.Value |= x;
 			}
 
 			// Address
 			if (_code.Length > 2)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[2], out x);
-				Value = x << 8;
+				result.Value = x << 8;
 			}
 			else
 			{
-				Value = -1;
+				result.Value = -1;
 			}
 
 			if (_code.Length > 3)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[3], out x);
-				Address |= x << 4;
+				result.Address |= x << 4;
 			}
 
 			if (_code.Length > 4)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[4], out x);
-				Address |= x;
+				result.Address |= x;
 			}
 
 			if (_code.Length > 5)
 			{
 				_gbGgGameGenieTable.TryGetValue(_code[5], out x);
-				Address |= (x ^ 0xF) << 12;
+				result.Address |= (x ^ 0xF) << 12;
 			}
 
 			// compare need to be full
@@ -101,8 +101,10 @@ namespace BizHawk.Client.Common.cheats
 				_gbGgGameGenieTable.TryGetValue(_code[8], out x);
 				comp |= (x & 0xC) >> 2;
 				comp |= (x & 0x3) << 6;
-				Compare = comp ^ 0xBA;
+				result.Compare = comp ^ 0xBA;
 			}
+
+			return result;
 		}
 	}
 }
diff --git a/BizHawk.Client.Common/cheats/NesGameGenieDecoder.cs b/BizHawk.Client.Common/cheats/NesGameGenieDecoder.cs
index 106540b08a..04cddfa884 100644
--- a/BizHawk.Client.Common/cheats/NesGameGenieDecoder.cs
+++ b/BizHawk.Client.Common/cheats/NesGameGenieDecoder.cs
@@ -34,7 +34,7 @@ namespace BizHawk.Client.Common.cheats
 
 			if (code.Length != 6 && code.Length != 8)
 			{
-				return new InvalidResult("Game Genie codes need to be six or eight characters in length.");
+				return new InvalidCheatCode("Game Genie codes need to be six or eight characters in length.");
 			}
 
 			var result = new DecodeResult { Size = WatchSize.Byte };
diff --git a/BizHawk.Client.EmuHawk/tools/GameShark.cs b/BizHawk.Client.EmuHawk/tools/GameShark.cs
index 9f54568708..0b94dc43c5 100644
--- a/BizHawk.Client.EmuHawk/tools/GameShark.cs
+++ b/BizHawk.Client.EmuHawk/tools/GameShark.cs
@@ -103,33 +103,27 @@ namespace BizHawk.Client.EmuHawk
 			txtDescription.Clear();
 		}
 
-		private void GameBoy(string cheat)
+		private void GameBoy(string code)
 		{
 			// Game Genie
-			if (cheat.LastIndexOf("-") == 7 && cheat.IndexOf("-") == 3)
+			if (code.LastIndexOf("-") == 7 && code.IndexOf("-") == 3)
 			{
-				var decoder = new GbGgGameGenieDecoder(cheat);
-				var watch = Watch.GenerateWatch(MemoryDomains["System Bus"], decoder.Address, WatchSize.Word, Common.DisplayType.Hex, false, txtDescription.Text);
-				Global.CheatList.Add(decoder.Compare.HasValue
-					? new Cheat(watch, decoder.Value, decoder.Compare)
-					: new Cheat(watch, decoder.Value));
-			}
-			else if (cheat.Contains("-") && cheat.LastIndexOf("-") != 7 && cheat.IndexOf("-") != 3)
-			{
-				MessageBox.Show("All GameBoy Game Genie Codes need to have a dash after the third character and seventh character.", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
-				return;
+				var result = GbGgGameGenieDecoder.Decode(code);
+				if (result.IsValid)
+				{
+					var description = Description(code);
+					Global.CheatList.Add(result.ToCheat(MemoryDomains.SystemBus, description));
+				}
+				else
+				{
+					InputError(result.Error);
+				}
 			}
 
 			// Game Shark codes
-			if (cheat.Length != 8 && cheat.Contains("-") == false)
+			if (code.Length == 8 && code.Contains("-") == false)
 			{
-				MessageBox.Show("All GameShark Codes need to be Eight characters in Length", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
-				return;
-			}
-
-			if (cheat.Length == 8 && cheat.Contains("-") == false)
-			{
-				var test = cheat.Remove(2, 6);
+				var test = code.Remove(2, 6);
 				switch (test)
 				{
 					case "00":
@@ -140,7 +134,7 @@ namespace BizHawk.Client.EmuHawk
 						return;
 				}
 
-				var decoder = new GbGameSharkDecoder(cheat);
+				var decoder = new GbGameSharkDecoder(code);
 				var watch = Watch.GenerateWatch(MemoryDomains["System Bus"], decoder.Address, WatchSize.Word, Common.DisplayType.Hex, false, txtDescription.Text);
 				Global.CheatList.Add(new Cheat(watch, decoder.Value));
 			}
@@ -322,34 +316,39 @@ namespace BizHawk.Client.EmuHawk
 		}
 
 		// This also handles Game Gear due to shared hardware. Go figure.
-		private void Sms(string cheat)
+		private void Sms(string code)
 		{
 			// Game Genie
-			if (cheat.LastIndexOf("-") == 7 && cheat.IndexOf("-") == 3)
+			if (code.LastIndexOf("-") == 7 && code.IndexOf("-") == 3)
 			{
-				var decoder = new GbGgGameGenieDecoder(cheat);
-				var watch = Watch.GenerateWatch(MemoryDomains["System Bus"], decoder.Address, WatchSize.Word, Common.DisplayType.Hex, false, txtDescription.Text);
-				Global.CheatList.Add(decoder.Compare.HasValue
-					? new Cheat(watch, decoder.Value, decoder.Compare)
-					: new Cheat(watch, decoder.Value));
+				var result = GbGgGameGenieDecoder.Decode(code);
+				if (result.IsValid)
+				{
+					var description = Description(code);
+					Global.CheatList.Add(result.ToCheat(MemoryDomains.SystemBus, description));
+				}
+				else
+				{
+					InputError(result.Error);
+				}
 			}
 
 			// Action Replay
-			else if (cheat.IndexOf("-") == 3 && cheat.Length == 9)
+			else if (code.IndexOf("-") == 3 && code.Length == 9)
 			{
-				var decoder = new SmsActionReplayDecoder(cheat);
+				var decoder = new SmsActionReplayDecoder(code);
 				var watch = Watch.GenerateWatch(MemoryDomains["Main RAM"], decoder.Address, WatchSize.Byte, Common.DisplayType.Hex, false, txtDescription.Text);
 				Global.CheatList.Add(new Cheat(watch, decoder.Value));
 			}
 
 			// It's an Action Replay
-			else if (cheat.Length != 9 && cheat.LastIndexOf("-") != 7)
+			else if (code.Length != 9 && code.LastIndexOf("-") != 7)
 			{
 				InputError("All Master System Action Replay Codes need to be nine characters in length.");
 			}
 
 			// Game Genie
-			else if (cheat.LastIndexOf("-") != 7 && cheat.IndexOf("-") != 3)
+			else if (code.LastIndexOf("-") != 7 && code.IndexOf("-") != 3)
 			{
 				InputError("All Master System Game Genie Codes need to have a dash after the third character and seventh character.");
 			}