What are the key differences between .NET Framework, .NET Core, and .NET 5+?

The .NET ecosystem has evolved significantly, with different versions serving different purposes and platforms.

Historical Timeline:

• .NET Framework (2002): Original .NET platform for Windows
• .NET Core (2016): Cross-platform, open-source rewrite
• .NET 5+ (2020): Unified platform combining Framework and Core

Key Differences:

.NET Framework:

// .NET Framework - Windows only
// Uses System.Web for web applications
// Requires .NET Framework runtime installed
// Larger package size
// Limited cross-platform support

// Example: ASP.NET Web Forms (Framework only)
public partial class Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Framework-specific code
    }
}

.NET Core:

// .NET Core - Cross-platform
// Modern, lightweight, fast
// Self-contained deployments
// Better performance
// Docker-friendly

// Example: ASP.NET Core Web API
[ApiController]
[Route("api/[controller]")]
public class WeatherController : ControllerBase
{
    [HttpGet]
    public IEnumerable<WeatherForecast> Get()
    {
        // Core-specific code
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = Random.Shared.Next(-20, 55)
        });
    }
}

.NET 5+ (Unified):

// .NET 5+ - Best of both worlds
// Single platform for all scenarios
// Improved performance
// Modern language features
// Long-term support versions

// Example: Modern .NET 6+ Web API
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");
app.Run();

Migration Path:

// .NET Framework → .NET Core → .NET 5+
// 1. Update project file format
// 2. Replace Framework-specific APIs
// 3. Update dependencies
// 4. Test cross-platform compatibility

// Old .NET Framework project file
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
  </PropertyGroup>
</Project>

// New .NET 5+ project file
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>
</Project>

When to Use Each:

.NET Framework:

• Legacy Windows applications
• When you need Windows-specific features
• Existing applications that are difficult to migrate
• When you need specific Framework-only libraries

.NET Core:

• New cross-platform applications
• Microservices and containers
• High-performance scenarios
• Cloud-native applications

.NET 5+:

• All new development (recommended)
• Modern applications
• When you want the latest features
• Long-term support and updates

Performance Comparison:

// Benchmark results (approximate)
// .NET Framework: Baseline
// .NET Core: 2-3x faster
// .NET 5+: 3-4x faster
// .NET 6+: 4-5x faster

// Example: JSON serialization performance
var data = new { Name = "John", Age = 30 };
var json = JsonSerializer.Serialize(data); // Much faster in .NET 5+

Package Size Comparison:

.NET Framework: ~50MB (runtime)
.NET Core: ~30MB (runtime)
.NET 5+: ~25MB (runtime)
Self-contained: ~100MB+ (includes runtime)

API Differences:

// .NET Framework
using System.Web;
using System.Web.Mvc;

// .NET Core/5+
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.DependencyInjection;

// Configuration differences
// Framework: web.config, app.config
// Core/5+: appsettings.json, environment variables

Best Practices:

1. Use .NET 5+ for all new development
2. Migrate gradually from Framework to .NET 5+
3. Test thoroughly when migrating
4. Use self-contained deployments for containers
5. Leverage cross-platform benefits when possible
6. Keep dependencies updated for security and performance