1 files changed, 13 insertions, 0 deletions
diff --git a/src/mscorlib/shared/System/IObserver.cs b/src/mscorlib/shared/System/IObserver.cs
new file mode 100644
index 0000000000..39e123de8d
--- /dev/null
+++ b/src/mscorlib/shared/System/IObserver.cs
@@ -0,0 +1,13 @@
+// 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
+{
+    public interface IObserver<in T>
+    {
+        void OnNext(T value);
+        void OnError(Exception error);
+        void OnCompleted();
+    }
+}