使用断言检查开发人员的假设，使用异常检查环境的假设。

我已经在这里阅读了答案，我认为我应该添加一个重要的区别。使用断言有两种非常不同的方式。一种是作为临时的开发人员快捷方式，表示“这不应该真的发生，所以如果它发生了，就告诉我，这样我就可以决定怎么做”，有点像一个条件断点，用于你的程序能够继续的情况。另一种方法是在代码中假设有效的程序状态。

在第一种情况下，断言甚至不需要出现在最终代码中。您应该使用Debug。在开发期间断言，如果/当不再需要时，您可以删除它们。如果你想要保留它们或者忘记删除它们，没有问题，因为它们在发布汇编中不会有任何后果。

But in the second case, the assertions are part of the code. They, well, assert, that your assumptions are true, and also document them. In that case, you really want to leave them in the code. If the program is in an invalid state it should not be allowed to continue. If you couldn't afford the performance hit you wouldn't be using C#. On one hand it might be useful to be able to attach a debugger if it happens. On the other, you don't want the stack trace popping up on your users and perhaps more important you don't want them to be able to ignore it. Besides, if it's in a service it will always be ignored. Therefore in production the correct behavior would be to throw an Exception, and use the normal exception handling of your program, which might show the user a nice message and log the details.

跟踪。Assert有实现这一点的完美方法。它不会在生产环境中被删除，并且可以使用app.config配置不同的侦听器。 因此，对于开发来说，默认的处理程序就可以了，对于生产来说，您可以创建一个简单的TraceListener(如下所示)，它会抛出一个异常并在生产配置文件中激活它。

using System.Diagnostics;

public class ExceptionTraceListener : DefaultTraceListener
{
    [DebuggerStepThrough]
    public override void Fail(string message, string detailMessage)
    {
        throw new AssertException(message);
    }
}

public class AssertException : Exception
{
    public AssertException(string message) : base(message) { }
}

在产品配置文件中:

<system.diagnostics>
  <trace>
    <listeners>
      <remove name="Default"/>
      <add name="ExceptionListener" type="Namespace.ExceptionTraceListener,AssemblyName"/>
    </listeners>
  </trace>
 </system.diagnostics>

您应该使用Debug。断言来测试程序中的逻辑错误。编译器只能通知您语法错误。因此，您肯定应该使用Assert语句来测试逻辑错误。比如测试一个销售汽车的项目，只有蓝色的宝马可以得到15%的折扣。编译器不能告诉你你的程序在执行这个操作时逻辑上是否正确，但是assert语句可以。

摘自《实用程序员:从熟练工到高手》

Leave Assertions Turned On There is a common misunderstanding about assertions, promulgated by the people who write compilers and language environments. It goes something like this: Assertions add some overhead to code. Because they check for things that should never happen, they'll get triggered only by a bug in the code. Once the code has been tested and shipped, they are no longer needed, and should be turned off to make the code run faster. Assertions are a debugging facility. There are two patently wrong assumptions here. First, they assume that testing finds all the bugs. In reality, for any complex program you are unlikely to test even a miniscule percentage of the permutations your code will be put through (see Ruthless Testing). Second, the optimists are forgetting that your program runs in a dangerous world. During testing, rats probably won't gnaw through a communications cable, someone playing a game won't exhaust memory, and log files won't fill the hard drive. These things might happen when your program runs in a production environment. Your first line of defense is checking for any possible error, and your second is using assertions to try to detect those you've missed. Turning off assertions when you deliver a program to production is like crossing a high wire without a net because you once made it across in practice. There's dramatic value, but it's hard to get life insurance. Even if you do have performance issues, turn off only those assertions that really hit you.

所有的断言应该是代码，可以优化为:

Debug.Assert(true);

因为它检验的是你已经假设为真的东西。例如:

public static void ConsumeEnumeration<T>(this IEnumerable<T> source)
{
  if(source != null)
    using(var en = source.GetEnumerator())
      RunThroughEnumerator(en);
}
public static T GetFirstAndConsume<T>(this IEnumerable<T> source)
{
  if(source == null)
    throw new ArgumentNullException("source");
  using(var en = source.GetEnumerator())
  {
    if(!en.MoveNext())
      throw new InvalidOperationException("Empty sequence");
    T ret = en.Current;
    RunThroughEnumerator(en);
    return ret;
  }
}
private static void RunThroughEnumerator<T>(IEnumerator<T> en)
{
  Debug.Assert(en != null);
  while(en.MoveNext());
}

在上面，有三种不同的空参数方法。第一个接受它为允许的(它只是什么都不做)。第二个则抛出异常供调用代码处理(或不处理，导致错误消息)。第三种假设它不可能发生，并断言它是这样的。

在第一种情况下，没有问题。

在第二种情况下，调用代码有一个问题——它不应该用null调用GetFirstAndConsume，所以它会返回一个异常。

在第三种情况下，这段代码有一个问题，因为在调用它之前，它应该已经检查过en != null，所以它不是真的是一个错误。或者换句话说，它应该是理论上可以优化为Debug.Assert(true)的代码，sicne en != null应该始终为真!

就其价值而言……我发现我的公共方法倾向于使用if () {throw;}模式，以确保方法被正确调用。我的私有方法倾向于使用Debug.Assert()。

The idea is that with my private methods, I'm the one under control, so if I start calling one of my own private methods with parameters that are incorrect, then I've broken my own assumption somewhere--I should have never gotten into that state. In production, these private asserts should ideally be unnecessary work since I am supposed to be keeping my internal state valid and consistent. Contrast with parameters given to public methods, which could be called by anyone at runtime: I still need to enforce parameter constraints there by throwing exceptions.

此外，如果某些东西在运行时不起作用(网络错误、数据访问错误、从第三方服务检索的坏数据等)，我的私有方法仍然可以抛出异常。我的断言只是为了确保我没有破坏我自己关于对象状态的内部假设。