Quickstart: Debug ASP.NET Core with the Visual Studio debugger

The Visual Studio debugger provides many powerful features to help you debug your apps. This topic provides a quick way to learn some of the basic features.

Create a new project

  1. Open Visual Studio.

    Press Esc to close the start window. Type Ctrl + Q to open the search box, type asp.net, choose Templates, then choose Create new ASP.NET Core Web Application. In the dialog box that appears, choose Create.

    From the top menu bar, choose File > New > Project. In the left pane of the New project dialog box, under Visual C#, choose Web, and then in the middle pane choose ASP.NET Core Web Application. Type a name like MyDbgApp and click OK.

    In the dialog box that appears, choose Web Application in the middle pane, and then click OK.

    Choose a Web application

    If you don't see the ASP.NET Core Web Application project template, go to Tools > Get Tools and Features..., which opens the Visual Studio Installer. Choose the ASP.NET and web development workload, then choose Modify.

    Visual Studio creates the project.

  2. In Solution Explorer, open About.cshtml.cs (under Pages/About.cshtml) and replace the following code

    public void OnGet()
    {
        Message = "Your application description page.";
    }
    

    with this code:

    public void OnGet()
    {
        LinkedList<int> result = doWork();
        Message = "Result of work: " + result.First.Value + ", " + result.First.Value;
    }
    
    private static LinkedList<int> doWork()
    {
        LinkedList<int> c1 = new LinkedList<int>();
    
        c1.AddLast(10);
        c1.AddLast(20);
    
        LinkedList<int> c2 = new LinkedList<int>(c1);
    
        return c2;
    
    }
    

Set a breakpoint

A breakpoint is a marker that indicates where Visual Studio should suspend your running code so you can take a look at the values of variables, or the behavior of memory, or whether or not a branch of code is getting run. It is the most basic feature in debugging.

  1. To set the breakpoint, click in the gutter to the left of the doWork function (or select the line of code and press F9).

    Set a breakpoint

    The breakpoint is set to the left of the opening brace ({).

  2. Now press F5 (or choose Debug > Start Debugging).

  3. When the web page loads, click the About link at the top of the web page.

    The debugger pauses where you set the breakpoint. The statement where the debugger and app execution is paused is indicated by the yellow arrow. The line with the opening brace ({) after the doWork function declaration has not yet executed.

    Hit a breakpoint

    Tip

    If you have a breakpoint in a loop or recursion, or if you have many breakpoints that you frequently step through, use a conditional breakpoint to make sure that your code is suspended ONLY when specific conditions are met. This saves time and can also make it easier to debug issues that are hard to reproduce.

There are different commands to instruct the debugger to continue. We show a useful code navigation command that is available starting in Visual Studio 2017.

While paused at the breakpoint, hover over the statement return c2 until the green Run to click button Run to Click appears, and then press the Run to click button.

Run to click

The app continues execution, and pauses on the line of code where you clicked the button.

Common keyboard commands used to step through code include F10 and F11. For more in-depth instructions, see First look at the debugger.

Inspect variables in a datatip

  1. In the current line of code (marked by the yellow execution pointer), hover over the c2 object with your mouse to show a datatip.

    View a datatip

    The datatip shows you the current value of the c2 variable and allows you to inspect its properties. When debugging, if you see a value you don't expect, you probably have a bug in the preceding or calling lines of code.

  2. Expand the datatip to look at the current property values of the c2 object.

  3. If you want to pin the datatip so that you can continue to see the value of c2 while you execute code, click the small pin icon. (You can move the pinned datatip to a convenient location.)

Edit code and continue debugging

If you identify a change that you want to test in your code while in the middle of a debugging session, you can do that, too.

  1. In the OnGet method, click the second instance of result.First.Value and change result.First.Value to result.Last.Value.

  2. Press F10 (or Debug > Step Over) a few times to advance the debugger and execute the edited code.

    Edit and continue

    F10 advances the debugger one statement at a time, but steps over functions instead of stepping into them (the code that you skip still executes).

For more information on using edit-and-continue and on feature limitations, see Edit and Continue.

Next steps

In this tutorial, you've learned how to start the debugger, step through code, and inspect variables. You may want to get a high-level look at debugger features along with links to more information.