diff --git a/dotnet/src/webdriver/SessionId.cs b/dotnet/src/webdriver/SessionId.cs
index 7cf90da7f69d1..8f605d4731775 100644
--- a/dotnet/src/webdriver/SessionId.cs
+++ b/dotnet/src/webdriver/SessionId.cs
@@ -17,6 +17,10 @@
 // under the License.
 // </copyright>
 
+using System;
+
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>
@@ -24,15 +28,16 @@ namespace OpenQA.Selenium
     /// </summary>
     public class SessionId
     {
-        private string sessionOpaqueKey;
+        private readonly string sessionOpaqueKey;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="SessionId"/> class
         /// </summary>
         /// <param name="opaqueKey">Key for the session in use</param>
+        /// <exception cref="ArgumentNullException">If <paramref name="opaqueKey"/> is <see langword="null"/>.</exception>
         public SessionId(string opaqueKey)
         {
-            this.sessionOpaqueKey = opaqueKey;
+            this.sessionOpaqueKey = opaqueKey ?? throw new ArgumentNullException(nameof(opaqueKey));
         }
 
         /// <summary>
@@ -54,20 +59,13 @@ public override int GetHashCode()
         }
 
         /// <summary>
-        /// Compares two Sessions
+        /// Indicates whether the current session ID value is the same as <paramref name="obj"/>.
         /// </summary>
-        /// <param name="obj">Session to compare</param>
-        /// <returns>True if they are equal or False if they are not</returns>
-        public override bool Equals(object obj)
+        /// <param name="obj">The session to compare to.</param>
+        /// <returns><see langword="true"/> if the values are equal; otherwise, <see langword="false"/>.</returns>
+        public override bool Equals(object? obj)
         {
-            bool objectsAreEqual = false;
-            SessionId other = obj as SessionId;
-            if (other != null)
-            {
-                objectsAreEqual = this.sessionOpaqueKey.Equals(other.sessionOpaqueKey);
-            }
-
-            return objectsAreEqual;
+            return obj is SessionId otherSession && this.sessionOpaqueKey.Equals(otherSession.sessionOpaqueKey);
         }
     }
 }
diff --git a/dotnet/src/webdriver/WebDriver.cs b/dotnet/src/webdriver/WebDriver.cs
index 9bcca8b28f8b3..672aa83b5b6e6 100644
--- a/dotnet/src/webdriver/WebDriver.cs
+++ b/dotnet/src/webdriver/WebDriver.cs
@@ -24,6 +24,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Threading.Tasks;
 
@@ -44,7 +45,7 @@ public class WebDriver : IWebDriver, ISearchContext, IJavaScriptExecutor, IFinds
         private IFileDetector fileDetector = new DefaultFileDetector();
         private NetworkManager network;
         private WebElementFactory elementFactory;
-        private SessionId sessionId;
+
         private List<string> registeredCommands = new List<string>();
 
         /// <summary>
@@ -191,10 +192,7 @@ public bool IsActionExecutor
         /// <summary>
         /// Gets the <see cref="SessionId"/> for the current session of this driver.
         /// </summary>
-        public SessionId SessionId
-        {
-            get { return this.sessionId; }
-        }
+        public SessionId SessionId { get; private set; }
 
         /// <summary>
         /// Gets or sets the <see cref="IFileDetector"/> responsible for detecting
@@ -612,7 +610,7 @@ protected virtual Response Execute(string driverCommandToExecute,
         /// <returns>A <see cref="Response"/> containing information about the success or failure of the command and any data returned by the command.</returns>
         protected virtual async Task<Response> ExecuteAsync(string driverCommandToExecute, Dictionary<string, object> parameters)
         {
-            Command commandToExecute = new Command(this.sessionId, driverCommandToExecute, parameters);
+            Command commandToExecute = new Command(SessionId, driverCommandToExecute, parameters);
 
             Response commandResponse;
 
@@ -641,6 +639,7 @@ protected virtual async Task<Response> ExecuteAsync(string driverCommandToExecut
         /// Starts a session with the driver
         /// </summary>
         /// <param name="capabilities">Capabilities of the browser</param>
+        [MemberNotNull(nameof(SessionId))]
         protected void StartSession(ICapabilities capabilities)
         {
             Dictionary<string, object> parameters = new Dictionary<string, object>();
@@ -679,7 +678,9 @@ protected void StartSession(ICapabilities capabilities)
 
             ReturnedCapabilities returnedCapabilities = new ReturnedCapabilities(rawCapabilities);
             this.capabilities = returnedCapabilities;
-            this.sessionId = new SessionId(response.SessionId);
+
+            string sessionId = response.SessionId ?? throw new WebDriverException($"The remote end did not respond with ID of a session when it was required. {response.Value}");
+            this.SessionId = new SessionId(sessionId);
         }
 
         /// <summary>
@@ -723,7 +724,7 @@ protected virtual void Dispose(bool disposing)
         {
             try
             {
-                if (this.sessionId is not null)
+                if (this.SessionId is not null)
                 {
                     this.Execute(DriverCommand.Quit, null);
                 }
@@ -739,7 +740,7 @@ protected virtual void Dispose(bool disposing)
             }
             finally
             {
-                this.sessionId = null;
+                this.SessionId = null;
             }
             this.executor.Dispose();
         }