The `field` Keyword in C# 13 — Cleaner Auto-Property Logic with Compiler-Generated Backing Fields

The field Keyword in C# 13 — Cleaner Auto-Property Logic with Compiler-Generated Backing Fields

Level: Advanced

Tags: #csharp #dotnet #csharp13 #properties #autoproperty #fieldkeyword

In C# 13, the language introduces a new contextual keyword: field, which allows developers to interact directly with the compiler-synthesized backing field of an auto-implemented property.

This feature is part of C# 13’s preview features (GB), and available with .NET 9. You must enable it via:

<LangVersion>preview</LangVersion>

The field keyword enables more precise control over property accessors, while maintaining the concise syntax of auto-properties.

What Problem Does It Solve?

Previously, to enforce logic like null-checking, you had to convert an auto-property into a full property with an explicit backing field:

private string _name;

public string Name

    get => _name;
    set => _name = value ?? throw new ArgumentNullException(nameof(value));

This breaks encapsulation, adds noise, and requires declaring _name manually.

C# 13 Solution: Use field Instead

Now, you can write:

public string Name

    get;
    set => field = value ?? throw new ArgumentNullException(nameof(value));

This keeps the auto-property feel, but gives you a hook into the set accessor for validation, transformation, or other logic.

Example: Clean Business Rule Injection

public string Email

    get;
    set => field = value?.ToLowerInvariant()
        ?? throw new ArgumentNullException(nameof(value));

• field refers to the backing field generated by the compiler
• You don’t need to declare it manually
• It works just like _email would, but safer and cleaner

Summary of Behavior

Warnings & Ambiguities

• If you declare a field named field, you may hit shadowing issues inside property accessors.
• Disambiguate with:

  this.field       // property keyword
  @field           // actual field named "field"

Example

private string field; // user-declared field

public string Name

    get => this.field; // Refers to the compiler's backing field
    set => this.field = value;

To avoid confusion: avoid naming variables field in the same scope.

Why Use field?

Learn More

Final Thoughts

The field keyword in C# 13 is a small but powerful enhancement that makes auto-properties more flexible and less verbose. It strikes a balance between readability and expressive control, especially for library authors, architects, and developers writing business logic-heavy code.

⚠️ Use it thoughtfully — especially in teams where field name collisions may arise. But once mastered, it will make your property implementations simpler and more expressive.

Written by: [Cristian Sifuentes] – Property Design Specialist | Refactoring Evangelist | Modern .NET Architect

Have you tried field in C# 13 preview? Share your patterns, and let’s refine clean property logic together!