Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/mscorlib/system/iobserver.cs

@@ -0,0 +1,29 @@
+// ==++==
+// 
+//   Copyright (c) Microsoft Corporation.  All rights reserved.
+// 
+// ==--==
+/*============================================================
+**
+** Class:  IObserver<T>
+** 
+** <OWNER>kimhamil</OWNER>
+**
+**
+** Purpose: Interface for exposing an Observer in the 
+** Observer pattern
+**
+**
+===========================================================*/
+
+using System;
+
+namespace System
+{
+    public interface IObserver<in T>
+    {
+        void OnNext(T value);
+        void OnError(Exception error);
+        void OnCompleted();
+    }
+}