When should I use Debug.Assert()?

In Debugging Microsoft .NET 2.0 Applications John Robbins has a big section on assertions. His main points are:

1. Assert liberally. You can never have too many assertions.
2. Assertions don't replace exceptions. Exceptions cover the things your code demands; assertions cover the things it assumes.
3. A well-written assertion can tell you not just what happened and where (like an exception), but why.
4. An exception message can often be cryptic, requiring you to work backwards through the code to recreate the context that caused the error. An assertion can preserve the program's state at the time the error occurred.
5. Assertions double as documentation, telling other developers what implied assumptions your code depends on.
6. The dialog that appears when an assertion fails lets you attach a debugger to the process, so you can poke around the stack as if you had put a breakpoint there.

PS: If you liked Code Complete, I recommend following it up with this book. I bought it to learn about using WinDBG and dump files, but the first half is packed with tips to help avoid bugs in the first place.

Put Debug.Assert() everywhere in the code where you want have sanity checks to ensure invariants. When you compile a Release build (i.e., no DEBUG compiler constant), the calls to Debug.Assert() will be removed so they won't affect performance.

You should still throw exceptions before calling Debug.Assert(). The assert just makes sure that everything is as expected while you're still developing.

FWIW ... I find that my public methods tend to use the if () { throw; } pattern to ensure that the method is being called correctly. My private methods tend to use 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.

Additionally, my private methods can still throw exceptions if something doesn't work at runtime (network error, data access error, bad data retrieved from a third party service, etc.). My asserts are just there to make sure that I haven't broken my own internal assumptions about the state of the object.

From Code Complete

8 Defensive Programming

8.2 Assertions

An assertion is code that’s used during development—usually a routine or macro—that allows a program to check itself as it runs. When an assertion is true, that means everything is operating as expected. When it’s false, that means it has detected an unexpected error in the code. For example, if the system assumes that a customer-information file will never have more than 50,000 records, the program might contain an assertion that the number of records is lessthan or equal to 50,000. As long as the number of records is less than or equal to 50,000, the assertion will be silent. If it encounters more than 50,000 records, however, it will loudly “assert” that there is an error in the program.

Assertions are especially useful in large, complicated programs and in high reliability programs. They enable programmers to more quickly flush out mismatched interface assumptions, errors that creep in when code is modified, and so on.

An assertion usually takes two arguments: a boolean expression that describes the assumption that’s supposed to be true and a message to display if it isn’t.

(…)

Normally, you don’t want users to see assertion messages in production code; assertions are primarily for use during development and maintenance. Assertions are normally compiled into the code at development time and compiled out of the code for production. During development, assertions flush out contradictory assumptions, unexpected conditions, bad values passed to routines, and so on. During production, they are compiled out of the code so that the assertions don’t degrade system performance.

Use asserts to check developer assumptions and exceptions to check environmental assumptions.

If I were you I would do:

Debug.Assert(val != null);
if ( val == null )
    throw new exception();

Or to avoid repeated condition check

if ( val == null )
{
    Debug.Assert(false,"breakpoint if val== null");
    throw new exception();
}

If you want Asserts in your production code (i.e. Release builds), you can use Trace.Assert instead of Debug.Assert.

This of course adds overhead to your production executable.

Also if your application is running in user-interface mode, the Assertion dialog will be displayed by default, which may be a bit disconcerting for your users.

You can override this behaviour by removing the DefaultTraceListener: look at the documentation for Trace.Listeners in MSDN.

In summary,

• Use Debug.Assert liberally to help catch bugs in Debug builds.

• If you use Trace.Assert in user-interface mode, you probably want to remove the DefaultTraceListener to avoid disconcerting users.

• If the condition you're testing is something your app can't handle, you're probably better off throwing an exception, to ensure execution doesn't continue. Be aware that a user can choose to ignore an assertion.

Asserts are used to catch programmer (your) error, not user error. They should be used only when there is no chance a user could cause the assert to fire. If you're writing an API, for example, asserts should not be used to check that an argument is not null in any method an API user could call. But it could be used in a private method not exposed as part of your API to assert that YOUR code never passes a null argument when it isn't supposed to.

I usually favour exceptions over asserts when I'm not sure.

Mostly never in my book. In the vast majority of occasions if you want to check if everything is sane then throw if it isn't.

What I dislike is the fact that it makes a debug build functionally different to a release build. If a debug assert fails but the functionality works in release then how does that make any sense? It's even better when the asserter has long left the company and no-one knows that part of the code. Then you have to kill some of your time exploring the issue to see if it is really a problem or not. If it is a problem then why isn't the person throwing in the first place?

To me this suggests by using Debug.Asserts you're deferring the problem to someone else, deal with the problem yourself. If something is supposed to be the case and it isn't then throw.

I guess there are possibly performance critical scenarios where you want to optimise away your asserts and they're useful there, however I am yet to encounter such a scenario.

According to the IDesign Standard, you should

Assert every assumption. On average, every fifth line is an assertion.

using System.Diagnostics;

object GetObject()
{...}

object someObject = GetObject();
Debug.Assert(someObject != null);

As a disclaimer I should mention I have not found it practical to implement this IRL. But this is their standard.

Use assertions only in cases where you want the check removed for release builds. Remember, your assertions will not fire if you don't compile in debug mode.

Given your check-for-null example, if this is in an internal-only API, I might use an assertion. If it's in a public API, I would definitely use the explicit check and throw.

You should always use the second approach (throwing exceptions).

Also if you're in production (and have a release-build), it's better to throw an exception (and let the app crash in the worst-case) than working with invalid values and maybe destroy your customer's data (which may cost thousand of dollars).

You should use Debug.Assert to test for logical errors in your programs. The complier can only inform you of syntax errors. So you should definetely use Assert statements to test for logical errors. Like say testing a program that sells cars that only BMWs that are blue should get a 15% discount. The complier could tell you nothing about if your program is logically correct in performing this but an assert statement could.

I don't know how it is in C# and .NET, but in C will assert() only work if compiled with -DDEBUG - the enduser will never see an assert() if it's compiled without. It's for developer only. I use it really often, it's sometimes easier to track bugs.

I would not use them in production code. Throw exceptions, catch and log.

Also need to be careful in asp.net, as an assert can show up on the console and freeze the request(s).