C# - What does the Assert() method do? Is it still useful?

0 votes
asked Oct 2, 2008 by pokus

I am debugging with breakpoints and I realize the assert call? I thought it was only for unit tests. What does it do more than breakpoint? Since I can breakpoint, why should I use Assert?

9 Answers

0 votes
answered Oct 2, 2008 by patrick-desjardins

In a debug compilation, Assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.

If you compile in Release, all Debug.Assert's are automatically left out.

0 votes
answered Oct 2, 2008 by jeff-yates

Assert allows you to assert a condition (post or pre) applies in your code. It's a way of documenting your intentions and having the debugger inform you with a dialog if your intention is not met.

Unlike a breakpoint, the Assert goes with your code and can be used to add additional detail about your intention.

0 votes
answered Oct 2, 2008 by ryan

Assert can help you give separate messaging behavior between testing and release. For example,

Debug.Assert(x > 2)

will only trigger a break if you are running a "debug" build, not a release build. There's a full example of this behavior here

0 votes
answered Oct 2, 2008 by thelsdj

You should use it for times when you don't want to have to breakpoint every little line of code to check variables, but you do want to get some sort of feedback if certain situations are present, for example:

Debug.Assert(someObject != null, "someObject is null! this could totally be a bug!");
0 votes
answered Oct 2, 2008 by joe

Assert also gives you another opportunity to chuckle at Microsoft's UI design skills. I mean: a dialog with three buttons Abort, Retry, Ignore, and an explanation of how to interpret them in the title bar!

0 votes
answered Oct 2, 2008 by juan

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.

0 votes
answered Oct 2, 2008 by flory

The way I think of it is Debug.Assert is a way to establish a contract about how a method is supposed to be called, focusing on specifics about the values of a paramter (instead of just the type). For example, if you are not supposed to send a null in the second parameter you add the Assert around that parameter to tell the consumer not to do that.

It prevents someone from using your code in a boneheaded way. But it also allows that boneheaded way to go through to production and not give the nasty message to a customer (assuming you build a Release build).

0 votes
answered Oct 16, 2010 by knightlore

Assertions feature heavily in Design by Contract (DbC) which as I understand was introducted/endorsed by Meyer, Bertand. 1997. Object-Oriented Software Contruction.

An important feature is that they mustn't produce side-effects, for example you can handle an exception or take a different course of action with an if statement(defensive programming).

Assertions are used to check the pre/post conditions of the contract, the client/supplier relationship - the client must ensure that the pre-conditions of the supplier are met eg. sends £5 and the supplier must ensure the post-conditions are met eg. delivers 12 roses. (Just simple explanation of client/supplier - can accept less and deliver more, but about Assertions). C# also introduces Trace.Assert(), which can be used for release code.

To answer the question yes they still useful, but can add complexity+readability to code and time+difficultly to maintain. Should we still use them? Yes, Will we all use them? Probably not, or not to the extent of how Meyer describes.

(Even the OU Java course that I learnt this technique on only showed simple examples and the rest of there code didn't enforce the DbC assertion rules on most of code, but was assumed to be used to assure program correctness!)

0 votes
answered Sep 15, 2017 by serge-voloshenko

First of all Assert() method is available for Trace and Debug classes.
Debug.Assert() is executing only in Debug mode.
Trace.Assert() is executing in Debug and Release mode.

Here is an example:

        int i = 1 + 3;
        // Debug.Assert method in Debug mode fails, since i == 4
        Debug.Assert(i == 3);
        Debug.WriteLine(i == 3, "i is equal to 3");

        // Trace.Assert method in Release mode is not failing.
        Trace.Assert(i == 4);
        Trace.WriteLine(i == 4, "i is equla to 4");

        Console.WriteLine("Press a key to continue...");

Run this code in Debug mode and then in Release mode.

enter image description here

You will notice that during Debug mode your code Debug.Assert statement fails, you get a message box showing the current stack trace of the application. This is not happening in Release mode since Trace.Assert() condition is true (i == 4).

WriteLine() method simply gives you an option of logging the information to Visual Studio output. enter image description here

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