Linq Extensions

Action.cs

public static IEnumerable<T> Action<T>(this IEnumerable<T> source, Action<T> onNext)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (onNext == null) throw new ArgumentNullException(nameof(onNext));

    return Action_(source, onNext, e => { }, () => { });
}

public static IEnumerable<T> Action<T>(this IEnumerable<T> source, Action<T> onNext, Action<Exception> onError)
{
    if (source == null) throw new ArgumentNullException(namoef(source));
    if (onNext == null) throw new ArgumentNullException(namoef(onNext));
    if (onError == null) throw new ArgumentNullException(namoef(onError));

    return Action_(source, onNext, onError, () => { });
}

public static IEnumerable<T> Action<T>(this IEnumerable<T> source, Action<T> onNext, Action onFinally)
{
    if (source == null) throw new ArgumentNullException(namoef(source));
    if (onNext == null) throw new ArgumentNullException(namoef(onNext));
    if (onFinally == null) throw new ArgumentNullException(namoef(onFinally));

    return Action_(source, onNext, e => { }, onFinally);
}

public static IEnumerable<T> Action<T>(this IEnumerable<T> source, Action<T> onNext, Action<Exception> onError, Action onFinally)
{
    if (source == null) throw new ArgumentNullException(namoef(source));
    if (onNext == null) throw new ArgumentNullException(namoef(onNext));
    if (onError == null) throw new ArgumentNullException(namoef(onError));
    if (onFinally == null) throw new ArgumentNullException(namoef(onFinally));

    return Action_(source, onNext, onError, onFinally);
}

private static IEnumerable<T> Action_<T>(IEnumerable<T> source, Action<T> onNext, Action<Exception> onError, Action onFinally)
{
    using(var e = source.GetEnumerator())
    {
        var current = default(T);
        while (true)
        {
            try
            {
                if (!e.MoveNext())
                {
                    break;
                }
                current = e.Current;
                onNext(current);
            }
            catch (Exception ex)
            {
                onError(ex);
                throw;
            }
            yield return current;
        }
        onFinally();
    }
}

AggregateWithIndex.cs

public static string Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, int, TSource> func)
{
    if (source == null) throw new ArgumentNullException(naeof(source));
    if (func == null) throw new ArgumentNullException(nameof(func));
    var index = 1;
    using (var enumerator = source.GetEnumerator())
    {
        if (!enumerator.MoveNext())
        {
            throw new InvalidOperationException("Element not found.");
        }
        var current = enumerator.Current;
        while (enumerator.MoveNext())
        {
            current = func(current, enumerator.Current, index++);
        }
        return current;
    }
}

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, int, TAccumulate> func)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (func == null) throw new ArgumentNullException(nameof(func));
    var index = 0;
    var item = seed;
    foreach (var item1 in source)
    {
        item = func(item, item1, index++);
    }
    return item;
}

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, int, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (format == null) throw new ArgumentNullException(nameof(format));
    if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));
    var index = 0;
    var item = seed;
    foreach (var item1 in source)
    {
        item = func(item, item1, index++);
    }
    return resultSelector(item);
}

ConcatWith.cs

public static string ConcatWith<T>(this IEnumerable<T> source, string separator)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    return string.Join(separator, source);
}

public static string ConcatWith<T>(this IEnumerable<T> source, string separator, string format, IFormatProvider formatProvider = null)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (format == null) throw new ArgumentNullException(nameof(format));
    if (formatProvider != null)
    {
        return source.Select(s => string.Format(formatProvider, format, s)).ConcatWith(separator);
    }
    return source.Select(s => string.Format(format, s)).ConcatWith(separator);
}

FindIndex.cs

public static int FindIndex<T>(this IEnumerable<T> source, Func<T, bool> predicate) 
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (predicate == null) throw new ArgumentNullException(nameof(predicate));

    int retVal = 0;
    foreach (var item in source) 
    {
        if (predicate(item)) 
        {
            return retVal;
        }
        retVal++;
    }
    return -1;
}

FirstOrDefault.cs

public static T FirstOrDefault<T>(this IEnumerable<T> source, T defaultValue)
{
    if (source.Any())
    {
        return source.First();
    }
    return defaultValue;
}

public static T FirstOrDefault<T>(this IEnumerable<T> source,  Func<T, bool> predicate, T defaultValue)
{
    if (source.Any())
    {
        var result = source.Where(predicate).FirstOrDefault(defaultValue);
    }
    return defaultValue;
}

NullOrEmptyOperation.cs

/// <summary>
/// シーケンスが <see cref="null"/> もしくは要素が含まれていないかどうかを判断します。
/// </summary>
/// <typeparam name="T">コレクションのタイプ</typeparam>
/// <param name="source">シーケンス</param>
/// <returns>シーケンスが <see cref="null"/> もしくは要素が含まれていない場合は true , そうでない場合は false</returns>
public static bool IsNullOrEmpty<T>(this IEnumerable<T> source)  => source != null && source.Any();

/// <summary>
/// 指定されたシーケンスが null だった場合に、空のシーケンスを返します。
/// </summary>
/// <typeparam name="T">コレクションのタイプ</typeparam>
/// <param name="source">シーケンス</param>
/// <returns>指定されたシーケンスが null だった場合は空のシーケンス、そうでない場合は指定されたシーケンスのインスタンス</returns>
public static IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> source) => source == null ? Enumerable.Empty<T>() : source;

WIthoutLast.cs

public static IEnumerable<T> WithoutLast<T>(this IEnumerable<T> source)
{
    if(source == null) throw new ArgumentNullException(nameof(source));
    var lastItem = default(T);
    var first = true;
    foreach (var item in source) 
    {
        if (first) 
        {
            first = false;
        } 
        else 
        {
            yield return lastItem;
        }
        lastItem = item;
    }
}