WhereNotNull.cs

/// <summary>Filters the elements of an observable sequence based on a nullability.</summary>
/// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
/// <param name="source">An observable sequence whose elements to filter.</param>
/// <returns>An observable sequence that contains elements from the input sequence that are not <c>null</c>.</returns>
/// <exception cref="T:System.ArgumentNullException"><paramref name="source" /> is null.</exception>
[NotNull]
[ContractAnnotation("source:null => halt")]
public static IObservable<TSource> WhereNotNull<TSource>([NotNull] this IObservable<TSource> source) where TSource : class
{
    if (source == null)
        throw new ArgumentNullException(nameof(source));

    // ReSharper disable once ExceptionNotDocumentedOptional
    return source.Where(WhereNotNullPredicate);
}

/// <summary>Filters the elements of an observable sequence based on a nullability.</summary>
/// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
/// <param name="source">An observable sequence whose elements to filter.</param>
/// <returns>An observable sequence that contains elements from the input sequence that are not <c>null</c>.</returns>
/// <exception cref="T:System.ArgumentNullException"><paramref name="source" /> is null.</exception>
[NotNull]
[ContractAnnotation("source:null => halt")]
public static IObservable<TSource> WhereNotNull<TSource>([NotNull] this IObservable<TSource?> source) where TSource : struct
{
    if (source == null)
        throw new ArgumentNullException(nameof(source));

    // ReSharper disable once ExceptionNotDocumentedOptional
    return source.Where(WhereNotNullPredicate).Select(StructureValue);
}

static bool WhereNotNullPredicate<T>([CanBeNull] T item)
{
    return null != item;
}

static bool WhereNotNullPredicate<T>([CanBeNull] T? item) where T: struct
{
    return null != item;
}

static T StructureValue<T>([CanBeNull] T? item) where T : struct
{
    // ReSharper disable once PossibleInvalidOperationException
    return item.Value;
}