diff --git a/dotnet/src/webdriver/DevTools/DevToolsEventData.cs b/dotnet/src/webdriver/DevTools/DevToolsEventData.cs
index 89cf25fec0624..9f27e3c88fe1e 100644
--- a/dotnet/src/webdriver/DevTools/DevToolsEventData.cs
+++ b/dotnet/src/webdriver/DevTools/DevToolsEventData.cs
@@ -34,7 +34,7 @@ public class DevToolsEventData
         /// <param name="eventArgsType">The type of the event args for the event to be raised.</param>
         /// <param name="invoker">The method that will be used to invoke the event.</param>
         /// <exception cref="ArgumentNullException">If<paramref name="eventArgsType"/> or <paramref name="invoker"/> is <see langword="null"/>.</exception>
-        public DevToolsEventData(Type eventArgsType, Action<object> invoker)
+        public DevToolsEventData(Type eventArgsType, Action<object?> invoker)
         {
             EventArgsType = eventArgsType ?? throw new ArgumentNullException(nameof(eventArgsType));
             EventInvoker = invoker ?? throw new ArgumentNullException(nameof(invoker));
@@ -48,6 +48,6 @@ public DevToolsEventData(Type eventArgsType, Action<object> invoker)
         /// <summary>
         /// The method to called to raise the event.
         /// </summary>
-        public Action<object> EventInvoker { get; }
+        public Action<object?> EventInvoker { get; }
     }
 }
diff --git a/third_party/dotnet/devtools/src/generator/CodeGen/Utility.cs b/third_party/dotnet/devtools/src/generator/CodeGen/Utility.cs
index eed62654e63fb..48baf369c88d7 100644
--- a/third_party/dotnet/devtools/src/generator/CodeGen/Utility.cs
+++ b/third_party/dotnet/devtools/src/generator/CodeGen/Utility.cs
@@ -58,7 +58,7 @@ public static string GetTypeMappingForType(TypeDefinition typeDefinition, Domain
                 {
                     var primitiveType = typeInfo.TypeName;
 
-                    if (typeDefinition.Optional && typeInfo.ByRef)
+                    if (typeDefinition.Optional)
                     {
                         primitiveType += "?";
                     }
@@ -70,8 +70,9 @@ public static string GetTypeMappingForType(TypeDefinition typeDefinition, Domain
 
                     return primitiveType;
                 }
+
                 mappedType = $"{typeInfo.Namespace}.{typeInfo.TypeName}";
-                if (typeDefinition.Optional && typeInfo.ByRef)
+                if (typeDefinition.Optional)
                 {
                     mappedType += "?";
                 }
@@ -79,7 +80,7 @@ public static string GetTypeMappingForType(TypeDefinition typeDefinition, Domain
             else if (knownTypes.TryGetValue($"{domainDefinition.Name}.{type}", out typeInfo))
             {
                 mappedType = typeInfo.TypeName;
-                if (typeInfo.ByRef && typeDefinition.Optional)
+                if (typeDefinition.Optional)
                 {
                     mappedType += "?";
                 }
@@ -89,15 +90,15 @@ public static string GetTypeMappingForType(TypeDefinition typeDefinition, Domain
                 switch (type)
                 {
                     case "number":
-                        mappedType = typeDefinition.Optional ? "double?" : "double";
+                        mappedType = "double";
                         break;
 
                     case "integer":
-                        mappedType = typeDefinition.Optional ? "long?" : "long";
+                        mappedType = "long";
                         break;
 
                     case "boolean":
-                        mappedType = typeDefinition.Optional ? "bool?" : "bool";
+                        mappedType = "bool";
                         break;
 
                     case "string":
@@ -115,12 +116,17 @@ public static string GetTypeMappingForType(TypeDefinition typeDefinition, Domain
 
                     case "array":
                         var items = typeDefinition.Items ?? throw new InvalidOperationException("Type definition was type array but has no Items");
-                        mappedType = GetTypeMappingForType(items, domainDefinition, knownTypes, true);
+                        mappedType = GetTypeMappingForType(items, domainDefinition, knownTypes, isArray: true);
                         break;
 
                     default:
                         throw new InvalidOperationException($"Unmapped data type: {type}");
                 }
+
+                if (typeDefinition.Optional)
+                {
+                    mappedType += "?";
+                }
             }
 
             if (isArray)
diff --git a/third_party/dotnet/devtools/src/generator/ProtocolDefinition/CommandDefinition.cs b/third_party/dotnet/devtools/src/generator/ProtocolDefinition/CommandDefinition.cs
index db554e452edb5..90db920a116c6 100644
--- a/third_party/dotnet/devtools/src/generator/ProtocolDefinition/CommandDefinition.cs
+++ b/third_party/dotnet/devtools/src/generator/ProtocolDefinition/CommandDefinition.cs
@@ -20,5 +20,8 @@ public sealed class CommandDefinition : ProtocolDefinitionItem
 
         [JsonIgnore]
         public bool NoParameters => Parameters == null || Parameters.Count == 0;
+
+        [JsonIgnore]
+        public bool NoReturn => Returns == null || Returns.Count == 0;
     }
 }
diff --git a/third_party/dotnet/devtools/src/generator/Templates/command.hbs b/third_party/dotnet/devtools/src/generator/Templates/command.hbs
index 19e14c320ddb5..be3f5f7a4d9ee 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/command.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/command.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System.Text.Json.Serialization;
@@ -28,7 +31,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         {{/if}}
         [JsonPropertyName("{{Name}}")]
         {{#if Optional}}[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]{{/if}}
-        public {{typemap ../context}} {{dehumanize Name}} { get; set; }
+        public {{typemap ../context}} {{dehumanize Name}} { get; set; }{{#unless Optional}} = default!;{{/unless}}
 
     {{/each}}
     }
@@ -50,7 +53,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         {{/if}}
         [JsonPropertyName("{{Name}}")]
         {{#if Optional}}[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]{{/if}}
-        public {{typemap ../context}} {{dehumanize Name}} { get; set; }
+        public {{typemap ../context}} {{dehumanize Name}} { get; set; }{{#unless Optional}} = default!;{{/unless}}
 
     {{/each}}
     }
diff --git a/third_party/dotnet/devtools/src/generator/Templates/domain.hbs b/third_party/dotnet/devtools/src/generator/Templates/domain.hbs
index 27c483f19d06e..bb6328b290576 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/domain.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/domain.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System;
@@ -38,22 +41,24 @@ namespace {{rootNamespace}}.{{domain.Name}}
         /// <summary>
         /// {{xml-code-comment Description 2}}
         /// </summary>
-        public event EventHandler<{{dehumanize Name}}EventArgs> {{dehumanize Name}};
+        public event EventHandler<{{dehumanize Name}}EventArgs>? {{dehumanize Name}};
 
     {{/each}}
+#nullable disable warnings
 
     {{#each domain.Commands}}
         /// <summary>
         /// {{xml-code-comment Description 2}}
         /// </summary>
-        public Task<{{dehumanize Name}}CommandResponse> {{dehumanize Name}}({{dehumanize Name}}CommandSettings command{{#if NoParameters}} = null{{/if}}, CancellationToken cancellationToken = default(CancellationToken), int? millisecondsTimeout = null, bool throwExceptionIfResponseNotReceived = true)
+        public Task<{{dehumanize Name}}CommandResponse{{#if NoReturn}}?{{/if}}> {{dehumanize Name}}({{dehumanize Name}}CommandSettings{{#if NoParameters}}?{{/if}} command{{#if NoParameters}} = null{{/if}}, CancellationToken cancellationToken = default(CancellationToken), int? millisecondsTimeout = null, bool throwExceptionIfResponseNotReceived = true)
         {
             return Session.SendCommand<{{dehumanize Name}}CommandSettings, {{dehumanize Name}}CommandResponse>(command{{#if NoParameters}} ?? new {{dehumanize Name}}CommandSettings(){{/if}}, cancellationToken, millisecondsTimeout, throwExceptionIfResponseNotReceived);
         }
 
     {{/each}}
+#nullable enable warnings
 
-        private void OnDevToolsEventReceived(object sender, DevToolsEventReceivedEventArgs e)
+        private void OnDevToolsEventReceived(object? sender, DevToolsEventReceivedEventArgs e)
         {
             if (e.DomainName == m_domainName)
             {
@@ -67,7 +72,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         }
 
     {{#each domain.Events}}
-        private void On{{dehumanize Name}}(object rawEventArgs)
+        private void On{{dehumanize Name}}(object? rawEventArgs)
         {
             if (rawEventArgs is {{dehumanize Name}}EventArgs e)
             {
diff --git a/third_party/dotnet/devtools/src/generator/Templates/event.hbs b/third_party/dotnet/devtools/src/generator/Templates/event.hbs
index 6b03b19e6cf8c..fb8a6f77a6bfb 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/event.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/event.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System;
@@ -21,7 +24,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         {{/if}}
         [JsonPropertyName("{{Name}}")]
         {{#if Optional}}[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]{{/if}}
-        public {{typemap ../context}} {{dehumanize Name}} { get; set; }
+        public {{typemap ../context}} {{dehumanize Name}} { get; set; }{{#unless Optional}} = default!;{{/unless}}
 
     {{/each}}
     }
diff --git a/third_party/dotnet/devtools/src/generator/Templates/type-enum.hbs b/third_party/dotnet/devtools/src/generator/Templates/type-enum.hbs
index a22cb967b80d3..8b6177192c155 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/type-enum.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/type-enum.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System.Runtime.Serialization;
diff --git a/third_party/dotnet/devtools/src/generator/Templates/type-hash.hbs b/third_party/dotnet/devtools/src/generator/Templates/type-hash.hbs
index f5ee9ea1d39df..e6c8ccb12cdc4 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/type-hash.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/type-hash.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System.Collections.Generic;
@@ -14,7 +17,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         ///</summary>
         [JsonPropertyName("{{Name}}")]
         {{#if Optional}}[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]{{/if}}
-        public {{typemap ../context}} {{dehumanize Name}} { get; set; }
+        public {{typemap ../context}} {{dehumanize Name}} { get; set; }{{#unless Optional}} = default!;{{/unless}}
 
     {{/each}}
     }
diff --git a/third_party/dotnet/devtools/src/generator/Templates/type-object.hbs b/third_party/dotnet/devtools/src/generator/Templates/type-object.hbs
index c3e84bc6527b4..cb1ac03d5085c 100644
--- a/third_party/dotnet/devtools/src/generator/Templates/type-object.hbs
+++ b/third_party/dotnet/devtools/src/generator/Templates/type-object.hbs
@@ -1,4 +1,7 @@
 // <auto-generated />
+
+#nullable enable
+
 namespace {{rootNamespace}}.{{domain.Name}}
 {
     using System.Text.Json.Serialization;
@@ -14,7 +17,7 @@ namespace {{rootNamespace}}.{{domain.Name}}
         ///</summary>
         [JsonPropertyName("{{Name}}")]
         {{#if Optional}}[JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]{{/if}}
-        public {{typemap ../context}} {{dehumanize Name}} { get; set; }
+        public {{typemap ../context}} {{dehumanize Name}} { get; set; }{{#unless Optional}} = default!;{{/unless}}
 
     {{/each}}
     }