How to Design Backward Compatible APIs in .NET, Real Lessons and Tips from Production

Designing APIs for Backward Compatibility: Lessons from Production

Backward compatibility in API design is one of those things you don’t care about until you break someone’s integration and get a Slack message at 3AM. I’ve worn that badge of shame, so let’s talk about what actually works for keeping your APIs stable and evolving, using C# and .NET as the battleground.

Why Backward Compatibility Hurts (and Why It’s Worth the Pain)

Shipping an API is easy. Keeping it working as requirements change, edge cases pop up, and new product managers ask for “just one more field” is where things get real. Breaking changes aren’t just annoying, they’re expensive. I’ve seen entire teams blocked because a “minor” rename in a serialized DTO took down half the frontend.

You can’t freeze your API forever, but you can design it to survive change. That means making trade-offs between “clean” design and practical, sometimes messy, solutions that keep old clients humming along.

Real Example: Evolving an Orders API Without Breaking Everyone

A while back, we shipped an Orders API for a SaaS platform. Life was good, until we needed to add new order statuses and tweak response shapes. Here’s a simplified version of our original C# model:

public class OrderDto
{
    public Guid Id { get; set; }
    public string Status { get; set; } // "Pending", "Completed"
    public decimal Amount { get; set; }
}

Product wanted to add “Cancelled” and “Refunded” statuses, plus an optional discount. So what did we do wrong at first? We renamed fields, made some required, and trusted our tests. Production promptly reminded us that old clients weren’t ready for those changes.

Principles That Saved Us

1. Never Break Existing Contracts Silently

If you must change a field, add a new one instead of renaming or removing the old. Keep the old field, mark it as obsolete in the code, but don’t yank it from the wire contract until all consumers migrate. In C#, add [Obsolete] to warn developers:

[Obsolete("Use NewStatus instead.")]
public string Status { get; set; }
public string NewStatus { get; set; }

2. Version Your API, But Don’t Abuse Versioning

Versioning is a last resort, not a license to break everything. We used URL versioning (/api/v1/orders) and only bumped versions when we had to remove or fundamentally change behaviors. For additive changes (new fields, enum values), we kept the same version and documented changes carefully.

3. Be Generous in What You Accept, Strict in What You Send

Old clients might send data you no longer use. In our controllers, we tolerated extra fields in input models, but always validated and sanitized output. This avoided “unknown field” errors when older SDKs talked to newer APIs.

4. Communicate Deprecations Early and Often

We made mistakes by deprecating fields silently. Now, every breaking or significant change gets a changelog entry, an email to integrators, and a sunset period. It’s less glamorous than a code refactor, but it makes for fewer angry calls.

Dealing with Enum Changes and Status Fields

Adding new enum values is a classic way to break clients that use strict deserialization. In .NET, we learned to document all possible values and encourage clients to use strings, not enums, at the boundary layer. If you must use enums, always provide a fallback for unknown values.

Example defensive parsing in C#:

public enum OrderStatus { Pending, Completed, Cancelled, Refunded }

public OrderStatus ParseStatus(string status)
{
    return Enum.TryParse<OrderStatus>(status, true, out var parsed)
        ? parsed
        : OrderStatus.Pending; // Default or handle as needed
}

Edge Cases: Optional Fields and Nullability

When adding new fields, always make them optional for input and output. Resist the urge to add required fields unless you’re introducing a new API version. This keeps old clients from failing on deserialization.

public class OrderDto
{
    // ...
    public decimal? Discount { get; set; } // Optional new field
}

Actionable Takeaways

• When evolving APIs, add fields instead of renaming or removing them. Mark deprecated fields as obsolete, but don’t break contracts by surprise.

• Use versioning only when you need to remove or fundamentally change behavior. Otherwise, be additive and backward-compatible.

• Communicate every change, especially deprecations, to clients right away. Documentation beats guesswork.

• Treat enums and status codes defensively, clients will break if you’re not careful.

• Make new fields optional, and never force old clients to provide data they don’t know about.

How have you handled backward compatibility in your APIs, especially when requirements change mid-flight? Any horror stories, or clever patterns that actually worked? Drop your experiences below,let’s trade scars.