/// <summary> /// Determines whether the specified <paramref name="key"/> is an element of the <paramref name="cache"/>. /// </summary> /// <typeparam name="TKey">The type of the key.</typeparam> /// <typeparam name="TValue">The type of the value.</typeparam> /// <param name="cache">The cache to use.</param> /// <param name="key">The key to check.</param> /// <param name="scheduler">The scheduler to run check on.</param> /// <returns></returns> /// <exception cref="System.ArgumentNullException"></exception> public static IObservable <bool> Contains <TKey, TValue>(this IObservableCache <TKey, TValue> cache, TKey key, IScheduler scheduler = null) { if (cache == null) { throw new ArgumentNullException(nameof(cache)); } if (key == null) { throw new ArgumentNullException(nameof(key)); } return(cache.Contains(Observable.Return(key), scheduler)); }