The State of .NET Core 3.1: Migration Guide for Enterprise

.NET Core 3.1 is the most significant Long Term Support (LTS) release for the platform to date. For enterprise teams still running on .NET Framework 4.8 or older .NET Core 2.x versions, 3.1 represents the standard for the next three years. This deep dive covers the architectural changes, migration strategies for large monolithic applications, and the performance characteristics that make 3.1 a critical upgrade.

Why .NET Core 3.1 Matters for Enterprise

Unlike previous versions, .NET Core 3.1 completes the “API porting” journey. With the Windows Compatibility Pack, over 20,000 APIs from the .NET Framework are now available, making the migration of legacy WCF, WinForms, and WPF applications feasible.

graph TB
    subgraph "Legacy Framework"
        FW[".NET Framework 4.8"]
        WCF["WCF / SOAP"]
        WebForms["ASP.NET Web Forms"]
        Entity["EF 6"]
    end
    
    subgraph "Modern .NET Core 3.1"
        Core[".NET Core 3.1 LTS"]
        gRPC["gRPC Services"]
        Blazor["Blazor Server"]
        EFCore["EF Core 3.1"]
    end
    
    FW -->|Migrate Logic| Standard[".NET Standard 2.0/2.1"]
    Standard --> Core
    WCF -->|Replace with| gRPC
    WebForms -->|Rewrite in| Blazor
    Entity -->|Port to| EFCore
    
    style Core fill:#E1F5FE,stroke:#0277BD
    style Standard fill:#C8E6C9,stroke:#2E7D32

Breaking Changes in 3.1

The move to 3.1 introduces strictness that can break existing code, particularly in ASP.NET Core.

Synchronous I/O Disabled by Default

Kestrel now throws an exception if you attempt synchronous I/O. This affects many legacy libraries.

// ❌ Breaks in 3.1
public void Upload(HttpContext context)
{
    using var reader = new StreamReader(context.Request.Body);
    var body = reader.ReadToEnd(); // InvalidOperationException
}

// ✅ Correct Async Pattern
public async Task UploadAsync(HttpContext context)
{
    using var reader = new StreamReader(context.Request.Body);
    var body = await reader.ReadToEndAsync();
}

// ⚠️ Temporary Workaround (Not Recommended)
services.Configure<KestrelServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});

Migration Strategy: The Strangler Fig Pattern

For massive monoliths, a “big bang” migration is high risk. Use the Strangler Fig pattern with YARP (Yet Another Reverse Proxy).

• **Phase 1**: Place YARP in front of legacy .NET 4.8 app.
• **Phase 2**: Migrate high-value slices (e.g., /api/orders) to .NET Core 3.1.
• **Phase 3**: Configure YARP to route /api/orders to new service, everything else to legacy.

// Startup.cs in Proxy Layer
public void Configure(IApplicationBuilder app)
{
    app.UseEndpoints(endpoints =>
    {
        // New .NET Core 3.1 Service
        endpoints.MapReverseProxy(proxyPipeline =>
        {
            proxyPipeline.UseLoadBalancing();
        });
    });
}

// appsettings.json
"ReverseProxy": {
  "Routes": {
    "orders-route": {
      "ClusterId": "orders-cluster",
      "Match": { "Path": "/api/orders/{**catch-all}" }
    },
    "legacy-route": {
      "ClusterId": "legacy-cluster",
      "Match": { "Path": "{**catch-all}" }
    }
  }
}

Performance Benchmarks

.NET Core 3.1 is significantly faster than Framework 4.8. Our internal benchmarks on a TechEmpower-style JSON serialization test showed:

Key Takeaways

• .NET Core 3.1 LTS is the target platform for all new dev.
• Legacy API migration is enabled by .NET Standard 2.0 compatibility.
• Beware of synchronous I/O exceptions in Kestrel.
• Use YARP for incremental migration of large systems.
• Performance gains alone often justify the migration effort.