How to generate the openapi.json with nswag

I am using nswag to generate the swagger. However the openapi json file is only in memory, you can only access it by starting the application and navigate to the link swagger/v1/swagger.json.

I want the json file to be generated and save physically in the project after every build. How should I achieved this?

i created a hosted service which executes my custom method to generate and save the api file in the project folder

        public static async Task GenerateOpenApiDoc(IServiceProvider services, string contentRootPath)
        {
#if DEBUG
            int oldvalue = Interlocked.CompareExchange(ref _executed, 1, 0);
            if (oldvalue == 1)
            {
                return;
            }

            string GetProjectDir()
            {
                string contentRoot = contentRootPath;
                string assemblyName = Assembly.GetEntryAssembly().GetName().Name;
                string projName = $"{assemblyName}.csproj";
                while (true)
                {
                    if (string.IsNullOrEmpty(contentRoot))
                    {
                        return null;
                    }

                    if (!Directory.Exists(contentRoot))
                    {
                        return null;
                    }

                    string projPath = Path.Combine(contentRoot, projName);
                    if (File.Exists(projPath))
                    {
                        return Path.GetDirectoryName(projPath);
                    }
                    contentRoot = Directory.GetParent(contentRoot)?.FullName;
                }
            }

            await Task.Run(async () =>
            {
                using (var serviceScope = services.GetService<IServiceScopeFactory>()!.CreateScope())
                {
                    IServiceProvider serviceProvider = serviceScope.ServiceProvider;
                    var options = serviceProvider.GetService<IOptions<SwaggerOptions>>();
                    string documentName = options.Value.Name;
                    var documentProvider = serviceProvider.GetService<NSwag.Generation.IOpenApiDocumentGenerator>() ?? throw new InvalidOperationException("The NSwag DI services are not registered: Call AddSwaggerDocument() in ConfigureServices().");
                    NSwag.OpenApiDocument apidoc = await documentProvider.GenerateAsync(documentName);
                    string spec = apidoc.ToJson();
                    string path = GetProjectDir();
                    string assemblyName = Assembly.GetEntryAssembly().GetName().Name;
                    if (string.IsNullOrEmpty(path))
                    {
                        throw new InvalidOperationException($"Failed to get project directory path for {assemblyName}");
                    }
                    string filePath = Path.Combine(path, $"{assemblyName}.json");
                    File.WriteAllText(filePath, spec);
                }
            });

#else
            await Task.CompletedTask;
#endif
        }