mono/netcore/System.Private.CoreLib/shared/System/IObserver.cs

// 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();
    }
}