Facilitating the spread of knowledge and innovation in professional software development



Choose your language

InfoQ Homepage News Performance of Structs in C# 7.2

Performance of Structs in C# 7.2

This item in japanese


The C# compiler, under some circumstances involving readonly, creates defensive copies of a struct. While this issue is well known and documented, it’s worth revisiting as it’s tied to several features of C# 7.2. The in and ref readonly keywords make occurrences of the issue more frequent, while readonly structs offer a way to fix it.

Structs in C# are generally used for high-performance purposes to avoid the cost of memory allocation/dealocation. However, the potential pitfalls have restrained their use. C# 7.2 adds an improvement, readonly structs, to deal with the most common occurrences of the issue.

The C# compiler will create copies of a struct if:

  •  The struct signature is not readonly.
  •  The struct variable has the readonly modifier.
  •  A method (including properties) is called.
public struct SomeStruct  
	private int _x;

	public int X { get { return _x; } }

private readonly SomeStruct s = new SomeStruct(42);

s.X; // Compiler creates a defensive copy.

The same rules apply when x is an in-parameter, a ref readonly local variable or a result of a method invocation that returns a value by readonly reference.

public void BadFunction(in SomeStruct s)
  s.X; // Compiler creates a defensive copy.


C# 7.2 adds the possibility to declare a struct readonly, providing a solution to avoid defensive copies. Structs declared as readonly can’t have property setters and prevents assignation of the struct members.

The defensive copies issue can be detected through static analysis. ErrorProne.NET is inspired from ErrorProne, a static analysis tool for Java. The .NET port is a set of Roslyn analyzers with a focus on correctness and performance. A subset of analyzers focuses on structs and is available in a Nuget package.

We need your feedback

How might we improve InfoQ for you

Thank you for being an InfoQ reader.

Each year, we seek feedback from our readers to help us improve InfoQ. Would you mind spending 2 minutes to share your feedback in our short survey? Your feedback will directly help us continually evolve how we support you.

Take the Survey

Rate this Article


Hello stranger!

You need to Register an InfoQ account or or login to post comments. But there's so much more behind being registered.

Get the most out of the InfoQ experience.

Allowed html: a,b,br,blockquote,i,li,pre,u,ul,p

Community comments

Allowed html: a,b,br,blockquote,i,li,pre,u,ul,p

Allowed html: a,b,br,blockquote,i,li,pre,u,ul,p


Is your profile up-to-date? Please take a moment to review and update.

Note: If updating/changing your email, a validation request will be sent

Company name:
Company role:
Company size:
You will be sent an email to validate the new email address. This pop-up will close itself in a few moments.