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Start with the checks that apply to most projects
- Choose the intended startup target. In Solution Explorer, right-click the runnable project and select Set as Startup Project. In a multi-project C# solution, this is usually the project with the program entry point. A class library or DLL generally is not a standalone application; it may need to be launched by a calling application or through a project-specific launch action. See Microsoft’s C# run-program guide.
- Check that the solution builds. Visual Studio normally builds before it launches under the debugger. Open View > Output and View > Error List; investigate the first relevant build error, missing dependency, or target-framework incompatibility rather than focusing only on the final launch message.
- Select the Debug configuration. Use the configuration selector on the Visual Studio toolbar and choose Debug when diagnosing with the debugger. Debug and Release are separate configurations; Release is optimized for performance. Microsoft explains the distinction in its debugging overview.
- Start the selected target under the debugger. Press
F5, or choose Debug > Start Debugging. This launches the selected startup target with the debugger; it will not correct a wrong target or a failed build. See Microsoft’s debugger overview.
If Visual Studio stops at build or deployment
Read the specific error in Output or Error List first. Then check how Visual Studio is configured to respond when a build or deployment fails: open Tools > Options > Projects and Solutions > Build and Run. The available behavior can include building all projects or only the startup project and its dependencies, and the response to a failed build can affect whether launch proceeds. The exact options can vary by project type. Microsoft documents them in Configure build and run options in Visual Studio.
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Some C++ configurations can prompt, launch an older successful build, or avoid launching after build or deployment errors. If an older build launches, the running program may not match the source currently open in the editor. Do not treat a successful launch alone as proof that the latest source built. If the Output window does not provide enough detail, increase MSBuild output verbosity in the Build and Run options, then rebuild and inspect the new output.
If a web project will not launch with F5
For ASP.NET projects, confirm the selected launch profile and its URL, App URL or App SSL URL, environment variables, and Debug configuration. These settings are project-specific; a profile intended for one server or URL may not match the way you are trying to launch the app. Microsoft’s ASP.NET debugging guide covers the relevant project properties.
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A useful diagnostic is to start the web application independently and try Debug > Attach to Process. If attachment works and a breakpoint is hit while F5 still fails, the debugger can connect to the process, so inspect the project’s launch properties and profile. If your setup uses a HOSTS entry, check that configuration as well.
When the message says “Unable to start debugging on the Web server”
This message is generic, not a diagnosis. Read any detail appended to it before changing IIS or project settings. Microsoft lists possible causes such as a mismatch between the launch URL and server configuration, or changes to the IIS application pool. Apply a remedy only when the detailed error matches it; an unrelated IIS reset is not a general fix. See Microsoft’s web-server debugging error guide.
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If the application runs but breakpoints do not work
Separate a launch failure from an attachment or breakpoint problem. Choose Debug > Attach to Process (or press Ctrl+Alt+P), select the process running your application, and choose the appropriate code type if Visual Studio does not identify it automatically. Microsoft’s Attach to running processes guide describes the workflow.
For source-level breakpoints to bind, the source and symbols need to correspond to the build running in that process. If the app was built from different source or without suitable symbols, attachment may succeed while breakpoints remain unbound or do not behave as expected. A Debug build is the normal starting point when diagnosing source-level debugging.
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Check project-specific debug settings
Open the project’s Properties and inspect its debug or start action. The names and choices depend on the project type and runtime, so use the settings intended for that project rather than copying instructions for another one. In particular, legacy C# debug settings do not necessarily apply to .NET Core, ASP.NET, or other project types. Microsoft’s C# debug configuration reference describes settings for the C# configurations it covers.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use the symptom to choose the next check
| What happens | Check next |
|---|---|
| F5 targets the wrong app, or no runnable app starts | Set the executable or intended launch target as the startup project; verify whether the selected project is a library that needs a host. |
| Build errors appear or Visual Studio asks what to do | Inspect the first useful error in Output or Error List, then review Build and Run behavior and, if needed, MSBuild output verbosity. |
| An ASP.NET app fails to start under IIS or another web profile | Check the selected profile, URL settings, environment variables, and detailed web-server error; compare F5 with Attach to Process. |
| The app runs, but Visual Studio does not stop at breakpoints | Attach to the correct process, choose the right code type, and verify that source and symbols match the running build. |
These checks distinguish failures in the build, startup selection, web-server launch, and debugger attachment—the main paths that can all appear to a user as “can’t start debugging.”
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