What's New in C# 13

C# 13 shipped with .NET 9 and continued the trend of improving expressiveness while also unlocking more advanced scenarios for ref struct, spans, and library authors.

Original Microsoft Learn reference: What's new in C# 13.

A practical theme for C# 13

Many C# 13 features are best understood as language support for cleaner modern APIs, especially where performance-sensitive code and expressive APIs meet.

Broadly useful additions

Some of the most practical C# 13 additions include:

• params collections, which extend params beyond array-only scenarios
• improved lock support with System.Threading.Lock
• implicit index access with ^ in object initializers
• partial properties and indexers

public static void PrintAll(params ReadOnlySpan<string> values)
{
    foreach (string value in values)
    {
        Console.WriteLine(value);
    }
}

This direction reflects a larger theme in C# 13: making high-performance and modern collection-based APIs feel more natural.

Advanced additions

Microsoft's C# 13 documentation also highlights:

• allows ref struct constraints
• ref struct interface support
• ref and unsafe usage in more async and iterator scenarios
• overload resolution priority for library authors
• the field keyword as a preview feature in that release cycle

These features are more specialized, but they show how the language keeps expanding support for safe high-performance code.

A practical way to adopt C# 13

Most application developers should adopt the broadly useful features first and treat the advanced low-level additions as tools for specific scenarios, not default habits.

That usually means:

• pay attention first to features that simplify public APIs or common application code
• adopt the more specialized low-level features only when your code genuinely works in those domains

Summary

• C# 13 continued the trend toward expressive modern APIs
• some features help ordinary application code, while others target advanced library or performance scenarios
• params collections are one of the clearest examples of practical everyday value
• low-level language power should be adopted when needed, not just because it exists
• the best adoption strategy is selective rather than feature-collection driven

Practice

Review one existing helper API and ask whether a new params collection or partial member feature would actually improve it, or whether the older form is already clear enough.

As a second exercise, explain why a team might deliberately adopt only one or two C# 13 features instead of trying to use everything new immediately.