diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/CancellationDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/CancellationDisposable.cs
index f4381903ef..353a263ab9 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/CancellationDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/CancellationDisposable.cs
@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 using System.Threading;
 
 namespace System.Reactive.Disposables
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/ContextDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/ContextDisposable.cs
index 4e5b0dac3e..49812ce7d5 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/ContextDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/ContextDisposable.cs
@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 using System.Reactive.Concurrency;
 using System.Threading;
 
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/MultipleAssignmentDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/MultipleAssignmentDisposable.cs
index 0e57f48b9a..335e501212 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/MultipleAssignmentDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/MultipleAssignmentDisposable.cs
@@ -2,9 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
-
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -12,7 +9,7 @@ namespace System.Reactive.Disposables
     /// </summary>
     public sealed class MultipleAssignmentDisposable : ICancelable
     {
-        private IDisposable _current;
+        private IDisposable? _current;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="MultipleAssignmentDisposable"/> class with no current underlying disposable.
@@ -30,7 +27,7 @@ public MultipleAssignmentDisposable()
         /// Gets or sets the underlying disposable. After disposal, the result of getting this property is undefined.
         /// </summary>
         /// <remarks>If the <see cref="MultipleAssignmentDisposable"/> has already been disposed, assignment to this property causes immediate disposal of the given disposable object.</remarks>
-        public IDisposable Disposable
+        public IDisposable? Disposable
         {
             get => Disposables.Disposable.GetValueOrDefault(ref _current);
             set => Disposables.Disposable.TrySetMultiple(ref _current, value);
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/RefCountDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/RefCountDisposable.cs
index 9369d75227..3ed932267d 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/RefCountDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/RefCountDisposable.cs
@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 using System.Threading;
 
 namespace System.Reactive.Disposables
@@ -14,7 +12,9 @@ namespace System.Reactive.Disposables
     public sealed class RefCountDisposable : ICancelable
     {
         private readonly bool _throwWhenDisposed;
-        private IDisposable _disposable;
+
+        private IDisposable? _disposable;
+
         /// <summary>
         /// Holds the number of active child disposables and the
         /// indicator bit (31) if the main _disposable has been marked
@@ -167,7 +167,7 @@ private void Release()
 
         private sealed class InnerDisposable : IDisposable
         {
-            private RefCountDisposable _parent;
+            private RefCountDisposable? _parent;
 
             public InnerDisposable(RefCountDisposable parent)
             {
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/ScheduledDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/ScheduledDisposable.cs
index 8196ea2a4e..f3819204f6 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/ScheduledDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/ScheduledDisposable.cs
@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 using System.Reactive.Concurrency;
 
 namespace System.Reactive.Disposables
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/SerialDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/SerialDisposable.cs
index 1d77b3f2bb..2c0c8a5fa9 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/SerialDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/SerialDisposable.cs
@@ -2,9 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
-
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -12,7 +9,7 @@ namespace System.Reactive.Disposables
     /// </summary>
     public sealed class SerialDisposable : ICancelable
     {
-        private IDisposable _current;
+        private IDisposable? _current;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="T:System.Reactive.Disposables.SerialDisposable"/> class.
@@ -30,7 +27,7 @@ public SerialDisposable()
         /// Gets or sets the underlying disposable.
         /// </summary>
         /// <remarks>If the SerialDisposable has already been disposed, assignment to this property causes immediate disposal of the given disposable object. Assigning this property disposes the previous disposable object.</remarks>
-        public IDisposable Disposable
+        public IDisposable? Disposable
         {
             get => Disposables.Disposable.GetValue(ref _current);
             set => Disposables.Disposable.TrySetSerial(ref _current, value);
diff --git a/Rx.NET/Source/src/System.Reactive/Disposables/SingleAssignmentDisposable.cs b/Rx.NET/Source/src/System.Reactive/Disposables/SingleAssignmentDisposable.cs
index ffcdec4e86..ffc4ee8681 100644
--- a/Rx.NET/Source/src/System.Reactive/Disposables/SingleAssignmentDisposable.cs
+++ b/Rx.NET/Source/src/System.Reactive/Disposables/SingleAssignmentDisposable.cs
@@ -2,9 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
-
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -13,7 +10,7 @@ namespace System.Reactive.Disposables
     /// </summary>
     public sealed class SingleAssignmentDisposable : ICancelable
     {
-        private IDisposable _current;
+        private IDisposable? _current;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="SingleAssignmentDisposable"/> class.
@@ -31,7 +28,7 @@ public SingleAssignmentDisposable()
         /// Gets or sets the underlying disposable. After disposal, the result of getting this property is undefined.
         /// </summary>
         /// <exception cref="InvalidOperationException">Thrown if the <see cref="SingleAssignmentDisposable"/> has already been assigned to.</exception>
-        public IDisposable Disposable
+        public IDisposable? Disposable
         {
             get => Disposables.Disposable.GetValueOrDefault(ref _current);
             set => Disposables.Disposable.SetSingle(ref _current, value);
diff --git a/Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Api/ApiApprovalTests.Core.verified.cs b/Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Api/ApiApprovalTests.Core.verified.cs
index 487ee6495a..3880022b41 100644
--- a/Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Api/ApiApprovalTests.Core.verified.cs
+++ b/Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Api/ApiApprovalTests.Core.verified.cs
@@ -598,7 +598,7 @@ public interface ICancelable : System.IDisposable
     public sealed class MultipleAssignmentDisposable : System.IDisposable, System.Reactive.Disposables.ICancelable
     {
         public MultipleAssignmentDisposable() { }
-        public System.IDisposable Disposable { get; set; }
+        public System.IDisposable? Disposable { get; set; }
         public bool IsDisposed { get; }
         public void Dispose() { }
     }
@@ -621,14 +621,14 @@ public void Dispose() { }
     public sealed class SerialDisposable : System.IDisposable, System.Reactive.Disposables.ICancelable
     {
         public SerialDisposable() { }
-        public System.IDisposable Disposable { get; set; }
+        public System.IDisposable? Disposable { get; set; }
         public bool IsDisposed { get; }
         public void Dispose() { }
     }
     public sealed class SingleAssignmentDisposable : System.IDisposable, System.Reactive.Disposables.ICancelable
     {
         public SingleAssignmentDisposable() { }
-        public System.IDisposable Disposable { get; set; }
+        public System.IDisposable? Disposable { get; set; }
         public bool IsDisposed { get; }
         public void Dispose() { }
     }