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Exercise - Logging and tracing

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Now that the application has started development, it's good to add more diagnostics to the logic to help developers as they add new features. We can use our new knowledge of debug diagnostics to accomplish this task.

Write to the debug console

Before we debug the application, let's add more debug diagnostics. Additional diagnostics will help diagnose the application while it's being run under debug.

At the top of the Program.cs file, we'll add a new using statement to bring in System.Diagnostics so we can use the Debug methods.

using System.Diagnostics;

Add a WriteLine statement at the start of the Fibonacci method to get clarity when you debug through the code.

Debug.WriteLine($"Entering {nameof(Fibonacci)} method");
Debug.WriteLine($"We are looking for the {n}th number");

At the end of our for loop, we could print out every value. We could also use a conditional print statement by using WriteIf or WriteLineIf to add a print line only when sum is 1 at the end of the for loop:

for (int i = 2; i <= n; i++)
{                  
    sum = n1 + n2;
    n1 = n2;
    n2 = sum;
    Debug.WriteLineIf(sum == 1, $"sum is 1, n1 is {n1}, n2 is {n2}");    
}

Debug the application and you should get the following output:

Entering Fibonacci method
We are looking for the 5th number
sum is 1, n1 is 1, n2 is 1

Check for conditions with Assert

In some situations, you might want to stop the entire running application when a certain condition isn't met. By using Debug.Assert, you can check for a condition and output additional information about the state of the application. Let's add a check right before the return statement to ensure n2 is 5.

// If n2 is 5 continue, else break.
Debug.Assert(n2 == 5, "The return value is not 5 and it should be.");
return n == 0 ? n1 : n2;

Our application logic is already correct, so let's update our Fibonacci(5); to Fibonacci(6);, which will have a different result.

Debug the application. When Debug.Assert is run in the code, the debugger stops the application so you can inspect variables, watch window, call stack, and more. It also outputs the message to the debug console.

---- DEBUG ASSERTION FAILED ----
---- Assert Short Message ----
The return value is not 5 and it should be.
---- Assert Long Message ----

   at Program.<<Main>$>g__Fibonacci|0_0(Int32 n) in C:\Users\Jon\Desktop\DotNetDebugging\Program.cs:line 23
   at Program.<Main>$(String[] args) in C:\Users\Jon\Desktop\DotNetDebugging\Program.cs:line 3

Stop debugging, and then run the application without debug by entering the following command in the terminal.

dotnet run

The application is terminated after the assertion has failed and information has been logged to the application output.

Process terminated. Assertion failed.
The return value is not 5 and it should be.
   at Program.<<Main>$>g__Fibonacci|0_0(Int32 n) in C:\Users\Jon\Desktop\DotNetDebugging\Program.cs:line 23
   at Program.<Main>$(String[] args) in C:\Users\Jon\Desktop\DotNetDebugging\Program.cs:line 3

Now, let's run the application in Release configuration with the following command in the terminal.

dotnet run --configuration Release

The application successfully runs to completion because we're no longer in the Debug configuration.

Congratulations, you've successfully and efficiently debugged code by using features of .NET, which include Debug.WriteLine and Debug.Assert. Well done!

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