diff --git a/src/BizHawk.Client.Common/Api/Classes/GameInfoApi.cs b/src/BizHawk.Client.Common/Api/Classes/GameInfoApi.cs
index 9cb128c4e7..a74d9a527a 100644
--- a/src/BizHawk.Client.Common/Api/Classes/GameInfoApi.cs
+++ b/src/BizHawk.Client.Common/Api/Classes/GameInfoApi.cs
@@ -25,7 +25,7 @@ namespace BizHawk.Client.Common
 
 		public string GetBoardType() => BoardInfo?.BoardName ?? "";
 
-		public Dictionary<string, string> GetOptions()
+		public IReadOnlyDictionary<string, string> GetOptions()
 		{
 			var options = new Dictionary<string, string>();
 			if (_game == null) return options;
diff --git a/src/BizHawk.Client.Common/Api/Classes/JoypadApi.cs b/src/BizHawk.Client.Common/Api/Classes/JoypadApi.cs
index 33663b4d81..4a0d2cb2a6 100644
--- a/src/BizHawk.Client.Common/Api/Classes/JoypadApi.cs
+++ b/src/BizHawk.Client.Common/Api/Classes/JoypadApi.cs
@@ -20,17 +20,17 @@ namespace BizHawk.Client.Common
 			_movieSession = movieSession;
 		}
 
-		public IDictionary<string, object> Get(int? controller = null)
+		public IReadOnlyDictionary<string, object> Get(int? controller = null)
 		{
 			return _inputManager.AutofireStickyXorAdapter.ToDictionary(controller);
 		}
 
-		public IDictionary<string, object> GetWithMovie(int? controller = null)
+		public IReadOnlyDictionary<string, object> GetWithMovie(int? controller = null)
 		{
 			return _inputManager.ControllerOutput.ToDictionary(controller);
 		}
 
-		public IDictionary<string, object> GetImmediate(int? controller = null)
+		public IReadOnlyDictionary<string, object> GetImmediate(int? controller = null)
 		{
 			return _inputManager.ActiveController.ToDictionary(controller);
 		}
@@ -51,7 +51,7 @@ namespace BizHawk.Client.Common
 			foreach (var axis in controller.Definition.Axes.Keys) _inputManager.ButtonOverrideAdapter.SetAxis(axis, controller.AxisValue(axis));
 		}
 
-		public void Set(IDictionary<string, bool> buttons, int? controller = null)
+		public void Set(IReadOnlyDictionary<string, bool> buttons, int? controller = null)
 		{
 			// If a controller is specified, we need to iterate over unique button names. If not, we iterate over
 			// ALL button names with P{controller} prefixes
@@ -88,7 +88,7 @@ namespace BizHawk.Client.Common
 			}
 		}
 
-		public void SetAnalog(IDictionary<string, int?> controls, object controller = null)
+		public void SetAnalog(IReadOnlyDictionary<string, int?> controls, object controller = null)
 		{
 			foreach (var kvp in controls) SetAnalog(kvp.Key, kvp.Value, controller);
 		}
diff --git a/src/BizHawk.Client.Common/Api/Classes/MovieApi.cs b/src/BizHawk.Client.Common/Api/Classes/MovieApi.cs
index dd1b83555d..6c905b95c8 100644
--- a/src/BizHawk.Client.Common/Api/Classes/MovieApi.cs
+++ b/src/BizHawk.Client.Common/Api/Classes/MovieApi.cs
@@ -23,7 +23,7 @@ namespace BizHawk.Client.Common
 
 		public bool StartsFromSaveram() => _movieSession.Movie.IsActive() && _movieSession.Movie.StartsFromSaveRam;
 
-		public IDictionary<string, object> GetInput(int frame, int? controller = null)
+		public IReadOnlyDictionary<string, object> GetInput(int frame, int? controller = null)
 		{
 			if (_movieSession.Movie.NotActive())
 			{
diff --git a/src/BizHawk.Client.Common/Api/Interfaces/IGameInfoApi.cs b/src/BizHawk.Client.Common/Api/Interfaces/IGameInfoApi.cs
index 67c84c39cb..bbc7e92d96 100644
--- a/src/BizHawk.Client.Common/Api/Interfaces/IGameInfoApi.cs
+++ b/src/BizHawk.Client.Common/Api/Interfaces/IGameInfoApi.cs
@@ -10,6 +10,6 @@ namespace BizHawk.Client.Common
 		string GetStatus();
 		bool IsStatusBad();
 		string GetBoardType();
-		Dictionary<string, string> GetOptions();
+		IReadOnlyDictionary<string, string> GetOptions();
 	}
 }
diff --git a/src/BizHawk.Client.Common/Api/Interfaces/IJoypadApi.cs b/src/BizHawk.Client.Common/Api/Interfaces/IJoypadApi.cs
index b8523afe54..9fcc85b58c 100644
--- a/src/BizHawk.Client.Common/Api/Interfaces/IJoypadApi.cs
+++ b/src/BizHawk.Client.Common/Api/Interfaces/IJoypadApi.cs
@@ -4,13 +4,13 @@ namespace BizHawk.Client.Common
 {
 	public interface IJoypadApi : IExternalApi
 	{
-		IDictionary<string, object> Get(int? controller = null);
-		IDictionary<string, object> GetWithMovie(int? controller = null);
-		IDictionary<string, object> GetImmediate(int? controller = null);
+		IReadOnlyDictionary<string, object> Get(int? controller = null);
+		IReadOnlyDictionary<string, object> GetWithMovie(int? controller = null);
+		IReadOnlyDictionary<string, object> GetImmediate(int? controller = null);
 		void SetFromMnemonicStr(string inputLogEntry);
-		void Set(IDictionary<string, bool> buttons, int? controller = null);
+		void Set(IReadOnlyDictionary<string, bool> buttons, int? controller = null);
 		void Set(string button, bool? state = null, int? controller = null);
-		void SetAnalog(IDictionary<string, int?> controls, object controller = null);
+		void SetAnalog(IReadOnlyDictionary<string, int?> controls, object controller = null);
 		void SetAnalog(string control, int? value = null, object controller = null);
 	}
 }
diff --git a/src/BizHawk.Client.Common/Api/Interfaces/IMovieApi.cs b/src/BizHawk.Client.Common/Api/Interfaces/IMovieApi.cs
index dde5ae3c4d..bed563b7d7 100644
--- a/src/BizHawk.Client.Common/Api/Interfaces/IMovieApi.cs
+++ b/src/BizHawk.Client.Common/Api/Interfaces/IMovieApi.cs
@@ -7,7 +7,7 @@ namespace BizHawk.Client.Common
 		bool StartsFromSavestate();
 		bool StartsFromSaveram();
 		string Filename();
-		IDictionary<string, object> GetInput(int frame, int? controller = null);
+		IReadOnlyDictionary<string, object> GetInput(int frame, int? controller = null);
 		string GetInputAsMnemonic(int frame);
 		bool GetReadOnly();
 		ulong GetRerecordCount();
diff --git a/src/BizHawk.Client.Common/inputAdapters/InputManager.cs b/src/BizHawk.Client.Common/inputAdapters/InputManager.cs
index 9fe2f93986..00679d731d 100644
--- a/src/BizHawk.Client.Common/inputAdapters/InputManager.cs
+++ b/src/BizHawk.Client.Common/inputAdapters/InputManager.cs
@@ -32,7 +32,7 @@ namespace BizHawk.Client.Common
 		public StickyXorAdapter StickyXorAdapter { get; } = new StickyXorAdapter();
 
 		/// <summary>
-		/// Used to AND to another controller, used for <see cref="IJoypadApi.Set(IDictionary{string, bool}, int?)">JoypadApi.Set</see>
+		/// Used to AND to another controller, used for <see cref="IJoypadApi.Set(IReadOnlyDictionary{string, bool}, int?)">JoypadApi.Set</see>
 		/// </summary>
 		public OverrideAdapter ButtonOverrideAdapter { get; } = new OverrideAdapter();
 
diff --git a/src/BizHawk.Client.Common/lua/NLuaTableHelper.cs b/src/BizHawk.Client.Common/lua/NLuaTableHelper.cs
index da28d73cea..054c596693 100644
--- a/src/BizHawk.Client.Common/lua/NLuaTableHelper.cs
+++ b/src/BizHawk.Client.Common/lua/NLuaTableHelper.cs
@@ -22,7 +22,7 @@ namespace BizHawk.Client.Common
 
 		public LuaTable CreateTable() => _lua.NewTable();
 
-		public LuaTable DictToTable<T>(IDictionary<string, T> dictionary)
+		public LuaTable DictToTable<T>(IReadOnlyDictionary<string, T> dictionary)
 		{
 			var table = _lua.NewTable();
 
diff --git a/src/BizHawk.Emulation.Common/Extensions.cs b/src/BizHawk.Emulation.Common/Extensions.cs
index 1ba5ea40c9..59c35dab08 100644
--- a/src/BizHawk.Emulation.Common/Extensions.cs
+++ b/src/BizHawk.Emulation.Common/Extensions.cs
@@ -390,7 +390,7 @@ namespace BizHawk.Emulation.Common
 			return buttons;
 		}
 
-		public static IDictionary<string, object> ToDictionary(this IController controller, int? controllerNum = null)
+		public static IReadOnlyDictionary<string, object> ToDictionary(this IController controller, int? controllerNum = null)
 		{
 			var buttons = new Dictionary<string, object>();