Introduce SingleAssignmentDisposableValue.

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

@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT License.
+// See the LICENSE file in the project root for more information. 
+
+namespace System.Reactive.Disposables
+{
+    /// <summary>
+    /// Represents a disposable resource which only allows a single assignment of its underlying disposable resource.
+    /// If an underlying disposable resource has already been set, future attempts to set the underlying disposable resource will throw an <see cref="InvalidOperationException"/>.
+    /// </summary>
+    internal struct SingleAssignmentDisposableValue
+    {
+        private IDisposable? _current;
+
+        ///// <summary>
+        ///// Initializes a new instance of the <see cref="SingleAssignmentDisposable"/> class.
+        ///// </summary>
+        //public SingleAssignmentDisposableValue()
+        //{
+        //}
+
+        /// <summary>
+        /// Gets a value that indicates whether the object is disposed.
+        /// </summary>
+        public bool IsDisposed => Disposables.Disposable.GetIsDisposed(ref _current);
+
+        /// <summary>
+        /// 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
+        {
+            get => Disposables.Disposable.GetValueOrDefault(ref _current);
+            set => Disposables.Disposable.SetSingle(ref _current, value);
+        }
+
+        /// <summary>
+        /// Disposes the underlying disposable.
+        /// </summary>
+        public void Dispose()
+        {
+            Disposables.Disposable.Dispose(ref _current);
+        }
+    }
+}