Migrating an existing ASP.NET application to ASP.NET Core can be a significant undertaking, but it offers numerous benefits, including improved performance, cross-platform support, and modern development practices. This guide will walk you through the key steps involved in migrating your application, along with sample code and best practices.

1. Assessing the Existing Application

Before starting the migration process, it's essential to assess your existing application. Consider the following:

• Dependencies: Identify third-party libraries and dependencies that may not be compatible with ASP.NET Core.
• Architecture: Review the architecture of your application, including the use of MVC, Web API, or Web Forms.
• Features: List the features of your application that need to be preserved or improved during the migration.

2. Setting Up the New ASP.NET Core Project

Create a new ASP.NET Core project using the .NET CLI or Visual Studio. You can choose between different templates, such as Web Application (Model-View-Controller) or Web API.

Creating a New ASP.NET Core Project

        
            dotnet new mvc -n MyNewApp
            

This command creates a new ASP.NET Core MVC project named MyNewApp.

3. Migrating the Project Structure

ASP.NET Core has a different project structure compared to traditional ASP.NET applications. You will need to migrate your existing files and folders into the new project structure:

• Controllers: Move your existing controllers to the Controllers folder in the new project.
• Views: Move your Razor views to the Views folder.
• Models: Move your model classes to the Models folder.
• Static Files: Place any static files (CSS, JavaScript, images) in the wwwroot folder.

4. Updating Dependencies

Update your project dependencies to use ASP.NET Core-compatible libraries. You can do this by editing the MyNewApp.csproj file and adding the necessary NuGet packages.

Sample Code for Updating Dependencies

        
            <Project Sdk="Microsoft.NET.Sdk.Web">
                <PropertyGroup>
                    <TargetFramework>net5.0</TargetFramework>
                </PropertyGroup>
                <ItemGroup>
                    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.2.0" />
                    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="5.0.0" />
                </ItemGroup>
            </Project>
            

This example shows how to add references to ASP.NET Core MVC and Entity Framework Core in the project file.

5. Updating Configuration

ASP.NET Core uses a different configuration system compared to traditional ASP.NET applications. You will need to migrate your configuration settings from web.config to the new appsettings.json file.

Sample Code for appsettings.json

        
            {
                "ConnectionStrings": {
                    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=MyNewApp;Trusted_Connection=True;MultipleActiveResultSets=true"
                },
                "Logging": {
                    "LogLevel": {
                        "Default": "Information",
                        "Microsoft": "Warning"
                    }
                }
            }
            

This example shows how to define connection strings and logging settings in the appsettings.json file.

6. Updating Startup Configuration

In ASP.NET Core, the Startup.cs file is used to configure services and the application pipeline. You will need to set up middleware, services, and routing in this file.

Sample Code for Startup.cs

        
            using Microsoft.AspNetCore.Builder;
            using Microsoft.AspNetCore.Hosting;
            using Microsoft.Extensions.DependencyInjection;
            using Microsoft.Extensions.Hosting;

            public class Startup
            {
                public void ConfigureServices(IServiceCollection services)
                {
                    services.AddControllersWithViews();
                }

                public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
                {
                    if (env.IsDevelopment())
                    {
                        app.UseDeveloperExceptionPage();
                    }
                    else
                    {
                        app.UseExceptionHandler("/Home/Error");
                        app.UseHsts();
                    }

                    app.UseHttpsRedirection();
                    app.UseStaticFiles();

                    app.UseRouting();

                    app.UseAuthorization();

                    app.UseEndpoints(endpoints =>
                    {
                        endpoints.MapControllerRoute(
                            name: "default",
                            pattern: "{controller=Home}/{action=Index}/{id?}");
                    });
                }
            }
            

This example demonstrates how to configure services and middleware in the Startup.cs file, including setting up routing and exception handling.

7. Testing the Migrated Application

After completing the migration, thoroughly test your application to ensure that all features work as expected. Pay special attention to:

• Routing and URL handling
• Data access and database migrations
• Authentication and authorization mechanisms
• Static file serving and client-side resources

8. Conclusion

Migrating an existing ASP.NET application to ASP.NET Core can be a complex process, but by following the steps outlined in this guide, you can successfully transition to a modern framework that offers enhanced performance and flexibility. Remember to assess your application, update dependencies, and thoroughly test the migrated application to ensure a smooth transition.