这个很有用:

    public static IEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IEnumerable<TFirst> first, IEnumerable<TSecond> second, Func<TFirst, TSecond, TResult> selector)
    {
        if (first == null)
            throw new ArgumentNullException("first");
        if (second == null)
            throw new ArgumentNullException("second");
        if (selector == null)
            throw new ArgumentNullException("selector");

        using (var enum1 = first.GetEnumerator())
        using (var enum2 = second.GetEnumerator())
        {
            while (enum1.MoveNext() && enum2.MoveNext())
            {
                yield return selector(enum1.Current, enum2.Current);
            }
        }
    }

它在。net 4.0中被添加到Enumerable类中，但是在3.5中使用它很方便。

例子:

var names = new[] { "Joe", "Jane", "Jack", "John" };
var ages = new[] { 42, 22, 18, 33 };

var persons = names.Zip(ages, (n, a) => new { Name = n, Age = a });

foreach (var p in persons)
{
    Console.WriteLine("{0} is {1} years old", p.Name, p.Age);
}

我想我以前在什么地方见过这个，但在这里找不到。MS在dictionary接口上有一个TryGetValue函数，但是它返回一个bool值，并在一个out参数中给出值，所以这里有一个更简单，更清晰的实现:

public static TVal GetValueOrDefault<TKey, TVal>(this IDictionary<TKey, TVal> d, TKey key) {
  if (d.ContainsKey(key))
    return d[key];
  return default(TVal);
}

另一个对我有用的是:

/// <summary>
/// Converts any type in to an Int32
/// </summary>
/// <typeparam name="T">Any Object</typeparam>
/// <param name="value">Value to convert</param>
/// <returns>The integer, 0 if unsuccessful</returns>
public static int ToInt32<T>(this T value)
{
  int result;
  if (int.TryParse(value.ToString(), out result))
  {
    return result;
  }
  return 0;
}

/// <summary>
/// Converts any type in to an Int32 but if null then returns the default
/// </summary>
/// <param name="value">Value to convert</param>
/// <typeparam name="T">Any Object</typeparam>
/// <param name="defaultValue">Default to use</param>
/// <returns>The defaultValue if unsuccessful</returns>
public static int ToInt32<T>(this T value, int defaultValue)
{
  int result;
  if (int.TryParse(value.ToString(), out result))
  {
    return result;
  }
  return defaultValue;
}

例子:

int number = "123".ToInt32();

or:

int badNumber = "a".ToInt32(100); // Returns 100 since a is nan

ThrowIfArgumentIsNull是做空检查的好方法，我们都应该这样做。

public static class Extensions
{
    public static void ThrowIfArgumentIsNull<T>(this T obj, string parameterName) where T : class
    {
        if (obj == null) throw new ArgumentNullException(parameterName + " not allowed to be null");
    }
}

下面是使用它的方法，它适用于您的命名空间中的所有类或任何您使用该命名空间的地方。

internal class Test
{
    public Test(string input1)
    {
        input1.ThrowIfArgumentIsNull("input1");
    }
}

在CodePlex项目上使用这段代码是可以的。

将double类型转换为使用指定区域性格式化的字符串:

public static class ExtensionMethods 
{
  public static string ToCurrency(this double value, string cultureName)
  {
    CultureInfo currentCulture = new CultureInfo(cultureName);
    return (string.Format(currentCulture, "{0:C}", value));
  }
}

例子:

double test = 154.20;
string testString = test.ToCurrency("en-US"); // $154.20

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();