Inline GetIsDisposed.

Rx.NET/Source/src/System.Reactive/Disposables/Disposable.Utils.cs

@@ -125,19 +125,6 @@ namespace System.Reactive.Disposables
             }
         }
 
-        /// <summary>
-        /// Gets a value indicating whether <paramref name="fieldRef" /> has been disposed.
-        /// </summary>
-        /// <returns>true if <paramref name="fieldRef" /> has been disposed.</returns>
-        /// <returns>false if <paramref name="fieldRef" /> has not been disposed.</returns>
-        internal static bool GetIsDisposed([NotNullIfNotNull("fieldRef")] /*in*/ ref IDisposable? fieldRef)
-        {
-            // We use a sentinel value to indicate we've been disposed. This sentinel never leaks
-            // to the outside world (see the Disposable property getter), so no-one can ever assign
-            // this value to us manually.
-            return Volatile.Read(ref fieldRef) == BooleanDisposable.True;
-        }
-
         /// <summary>
         /// Disposes <paramref name="fieldRef" />. 
         /// </summary>

Rx.NET/Source/src/System.Reactive/Disposables/MultipleAssignmentDisposableValue.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
+using System.Threading;
+
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -14,7 +16,11 @@ namespace System.Reactive.Disposables
         /// <summary>
         /// Gets a value that indicates whether the object is disposed.
         /// </summary>
-        public bool IsDisposed => Disposables.Disposable.GetIsDisposed(ref _current);
+        public bool IsDisposed =>
+            // We use a sentinel value to indicate we've been disposed. This sentinel never leaks
+            // to the outside world (see the Disposable property getter), so no-one can ever assign
+            // this value to us manually.
+            Volatile.Read(ref _current) == BooleanDisposable.True;
 
         /// <summary>
         /// Gets or sets the underlying disposable. After disposal, the result of getting this property is undefined.

Rx.NET/Source/src/System.Reactive/Disposables/SerialDisposableValue.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
+using System.Threading;
+
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -14,7 +16,11 @@ namespace System.Reactive.Disposables
         /// <summary>
         /// Gets a value that indicates whether the object is disposed.
         /// </summary>
-        public bool IsDisposed => Disposables.Disposable.GetIsDisposed(ref _current);
+        public bool IsDisposed =>
+            // We use a sentinel value to indicate we've been disposed. This sentinel never leaks
+            // to the outside world (see the Disposable property getter), so no-one can ever assign
+            // this value to us manually.
+            Volatile.Read(ref _current) == BooleanDisposable.True;
 
         /// <summary>
         /// Gets or sets the underlying disposable.

Rx.NET/Source/src/System.Reactive/Disposables/SingleAssignmentDisposableValue.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
+using System.Threading;
+
 namespace System.Reactive.Disposables
 {
     /// <summary>
@@ -15,7 +17,11 @@ namespace System.Reactive.Disposables
         /// <summary>
         /// Gets a value that indicates whether the object is disposed.
         /// </summary>
-        public bool IsDisposed => Disposables.Disposable.GetIsDisposed(ref _current);
+        public bool IsDisposed =>
+            // We use a sentinel value to indicate we've been disposed. This sentinel never leaks
+            // to the outside world (see the Disposable property getter), so no-one can ever assign
+            // this value to us manually.
+            Volatile.Read(ref _current) == BooleanDisposable.True;
 
         /// <summary>
         /// Gets or sets the underlying disposable. After disposal, the result of getting this property is undefined.