下面是我们工作代码库中的一个有趣的例子。在作业线程上遍历一个昂贵的lazy-eval枚举对象，并通过一个可观察对象推回结果。

public static IObservable<T> ToAsyncObservable<T>(this IEnumerable<T> @this)
{
    return Observable.Create<T>(observer =>
    {
        var task = new Task(() =>
        {
            try
            {
                @this.Run(observer.OnNext);
                observer.OnCompleted();
            }
            catch (Exception e)
            {
                observer.OnError(e);
            }
        });

        task.Start();

        return () => { };
    });
}

愚蠢的示例:

new DirectoryInfo(@"c:\program files")
    .EnumerateFiles("*", SearchOption.AllDirectories)
    .ToAsyncObservable()
    .BufferWithTime(TimeSpan.FromSeconds(0.5))
    .ObserveOnDispatcher()
    .Subscribe(
        l => Console.WriteLine("{0} received", l.Count),
        () => Console.WriteLine("Done!"));

for (;;)
{
    Thread.Sleep(10);
    Dispatcher.PushFrame(new DispatcherFrame());
}

显然，这个扩展将是无用的，如果你不使用辉煌的响应式扩展!

感谢评论中的Richard，这个扩展方法是不必要的。RX已经有一个扩展方法“tooobservable”，它接受IScheduler。那就用这个吧!

这是我写的唯一一个我经常使用的扩展。 它使得用System.Net.Mail发送电子邮件更容易一些。

public static class MailExtension
{
    // GetEmailCreditial(out strServer) gets credentials from an XML file
    public static void Send(this MailMessage email)
    {
        string strServer = String.Empty;
        NetworkCredential credentials = GetEmailCreditial(out strServer);
        SmtpClient client = new SmtpClient(strServer) { Credentials = credentials };
        client.Send(email);
    }

    public static void Send(this IEnumerable<MailMessage> emails)
    {
        string strServer = String.Empty;
        NetworkCredential credentials = GetEmailCreditial(out strServer);
        SmtpClient client = new SmtpClient(strServer) { Credentials = credentials };
        foreach (MailMessage email in emails)
            client.Send(email);
    }
}

// Example of use: 
new MailMessage("info@myDomain.com","you@gmail.com","This is an important Subject", "Body goes here").Send();
//Assume email1,email2,email3 are MailMessage objects
new List<MailMessage>(){email1, email2, email}.Send();

在将表单输入放入数据库之前解析表单输入时，这些扩展方法对我来说非常有用

public static int? ToInt(this string input) 
{
    int val;
    if (int.TryParse(input, out val))
        return val;
    return null;
}

public static DateTime? ToDate(this string input)
{
    DateTime val;
    if (DateTime.TryParse(input, out val))
        return val;
    return null;
}

public static decimal? ToDecimal(this string input)
{
    decimal val;
    if (decimal.TryParse(input, out val))
        return val;
    return null;
}

日期时间扩展

例子:

DateTime firstDayOfMonth = DateTime.Now.First();
DateTime lastdayOfMonth = DateTime.Now.Last();
DateTime lastFridayInMonth = DateTime.Now.Last(DayOfWeek.Friday);
DateTime nextFriday = DateTime.Now.Next(DayOfWeek.Friday);
DateTime lunchTime = DateTime.Now.SetTime(11, 30);
DateTime noonOnFriday = DateTime.Now.Next(DayOfWeek.Friday).Noon();
DateTime secondMondayOfMonth = DateTime.Now.First(DayOfWeek.Monday).Next(DayOfWeek.Monday).Midnight();

当我迁移到c#时，我错过了Visual Basic的With语句，所以它是这样的:

public static void With<T>(this T obj, Action<T> act) { act(obj); }

下面是如何在c#中使用它:

someVeryVeryLonggggVariableName.With(x => {
    x.Int = 123;
    x.Str = "Hello";
    x.Str2 = " World!";
});

节省了大量的输入!

将其与:

someVeryVeryLonggggVariableName.Int = 123;
someVeryVeryLonggggVariableName.Str = "Hello";
someVeryVeryLonggggVariableName.Str2 = " World!";

加入codeplex项目

gitorious.org/cadenza是我所见过的一些最有用的扩展方法的完整库。