/// <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));
}