A C# record is a data-centric type that usually doesn’t contain behaviors. C# 9 introduced the keyword record to quickly declare a class primarily designed for data representation. In C# 10 and upper, the new syntax record class is equivalent to C# 9 record and the syntax record struct declares a structure-based record.
This tutorial dives into C# records’ syntax, applications and benefits.
Declaring a Record Type
In C# 9, the Primary Constructor syntax was introduced as a concise means to declare a record’s constructor and properties. This syntax avoids several lines of boilerplate code:
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record Person(string FirstName, string LastName); ... var person = new Person("Bill", "Gates"); Assert.IsTrue(person.FirstName == "Bill"); Assert.IsTrue(person.LastName == "Gates"); |
The Primary Constructor syntax is optional and a C# record can be declared without it:
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record Person { public Person(string firstName, string lastName) { this.FirstName = firstName; this.LastName = lastName; } public string FirstName { get; } public string LastName { get; } } |
The Primary Constructor syntax also applies to record struct:
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record struct Point(int X, int Y); |
Comparing Records with Classes in C#: Value-Based Semantics
Up until this point, we haven’t discussed anything that distinguishes record class from regular class. The key is that – unlike a C# class – a C# record relies on Value-Based Semantics. Such semantic results from two characteristics: Immutability and Value-Based Equality.
Immutability
By default, a record class declared with the Primary Constructor syntax is immutable. This means that its instances’ state cannot be modified after creation, ensuring data integrity and thread safety:
Here the tooltip reverse-engineers the property FirstName { get; init; } generated through the Primary Constructor record syntax. Keep in mind that this compiler behavior is different for the record struct syntax that we will explain later.
Value-based equality
Value-based equality semantic means that two C# record instances are considered equal when their data matches. This stands in contrast to reference types like classes, where comparison is based on reference equality. Here is an example that demonstrates record’s Value-based equality:
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var record1 = new Person("Bill", "Gates"); var record2 = new Person("Bill", "Gates"); Assert.IsFalse(ReferenceEquals(record1, record2)); // Different objects ... Assert.IsTrue(record1 == record2); // ... but considered as equal because <strong>their data matches</strong> |
To achieve Value-based equality on record class the compiler overrides the virtual methods Object.Equals(Object) and Object.GetHashCode(). The compiler also overrides the operators == and !=.
Practical Application of Value-Based Semantics
As a .NET developer, you rely implicitly on value-based semantics daily. Indeed the class System.String adheres to Value-Based Semantics.
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var string1 = "Bill Gates"; var string2 = string1.Substring(0, 5) + string1.Substring(5); // Clone string1 into another string object // string Value-Base Equality Assert.IsFalse(ReferenceEquals(string1, string2)); // Different string objects ... Assert.IsTrue(string1 == string2); // ... considered as equal because their data matches // string Immutability var string3 = string1.Replace("Gates", "Clinton"); // When modifying a string, a new string object is created Assert.IsFalse(ReferenceEquals(string1, string3)); Assert.IsTrue(string1 == "Bill Gates"); // string1 hasn't been modified |
Non-destructive record mutation with the keyword ‘with’
The code above demonstrates that modifying a string object leads to a new string object being created to hold the modified state. This behavior is named non-destructive mutation because the original string object doesn’t get modified. Non-destructive mutation is available for C# record through the keyword with:
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var record1 = new Person("Bill", "Gates"); var record3 = record1 with { LastName = "Clinton" }; Assert.IsFalse(ReferenceEquals(record1, record3)); // When modifying a string, a new string object is created Assert.IsTrue(record1 == new Person("Bill", "Gates")); // record1 hasn't been changed |
When decompiling this C# code above with a .NET decompiler, we can see that the compiler generates a method named <Clone>$() for our record class. The nondestructive mutation is achieved by first cloning the record object and then assigning the property LastName. Normally this property cannot be assigned because the record is immutable. However special IL code generated by the C# compiler relying on IsExternalInit can assign it.
StringBuilder is to mitigate this impact. Similarly, non-destructive mutation of records can yield similar degraded performance outcomes.Declaring mutable record class
Let’s mention that it is possible to declare mutable record classes when not relying on the Primary Constructor syntax:
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var record1 = new Person { FirstName = "Bill", LastName = "Gates" }; record1.LastName = "Clinton"; Assert.IsTrue(record1 == new Person { FirstName = "Bill", LastName = "Clinton" }); public record class Person { public string FirstName { get; set; } public string LastName { get; set; } } |
Formatting
Another difference between C# classes and C# records is that the compiler overrides the Object.ToString() method to present a record as a string in a well-formatted manner:
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var person = new Person("Bill", "Gates"); Assert.IsTrue(person.ToString() == "Person { FirstName = Bill, LastName = Gates }"); |
The same string representation is available at debug time:
C# record struct vs. C# record class
Unlike record class, by default a record struct is mutable. This means that a setter is generated for each property declared through the Primary Constructor syntax:
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var point = new Point(1, 2); point.X = 3; // Possible to change the state of the point Assert.IsTrue(point == new Point(3, 2)); // Regular Value-based equality for a structure record struct Point(int X, int Y); |
To get an immutable record struct the keyword readonly must be used:
Another record struct specificity is that a default parameterless constructor is provided: it sets each field to its default value:
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var point = new Point(); Assert.IsTrue(point == new Point(0, 0)); |
Record deconstruction
When using the Primary Constructor syntax, the compiler generates a Deconstruct() method with an out parameter for each positional parameter provided in the record declaration. Here is record deconstruction in action:
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var person = new Person("Bill", "Gates"); var (FirstName, LastName) = person; // Deconstruct the record person Assert.IsTrue(FirstName == "Bill"); Assert.IsTrue(LastName == "Gates"); |
Notice that deconstruction makes records work seamlessly with C# Pattern Matching:
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var person = new Person("Bill", "Gates"); Assert.IsTrue(WhoIsThis(person) == "Billionaire Entrepreneur"); static string WhoIsThis(Person person) => person switch { ("Bill", "Gates") => "Billionaire Entrepreneur", ("Bill", "Clinton") => "US president", (_,_) => "Who is this person?" }; |
Record and Inheritance
record class can deal with regular C# class inheritance. A C# record can inherit from another C# record. Nevertheless, a record cannot inherit from a class, nor can a class inherit from a record.
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public record Person { public string FirstName { get; init; } public string LastName { get; init; } } public sealed record Student : Person { public int ID { get; init; } } |
While this feature is beneficial, it can introduce complexities in certain situations when dealing with the with syntax and value-based equality.
Inheritance and value-based equality
Viewed from the perspective of the Person record, in the following code sample, both references hold identical values since they both have the same FirstName and LastName values.
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Person person1 = new Student { FirstName = "Bill", LastName = "Gates", ID = 123 }; Person person2 = new Person { FirstName = "Bill", LastName = "Gates" }; Assert.IsFalse(person1 == person2); |
Fortunately, the compiler-generated code ensures that these two objects are treated as distinct. Upon revisiting the code produced by the compiler, you’ll notice that the implementation of the Equals() method relies on the virtual property protected virtual Type EqualityContract => typeof(Person);. This property is used to verify that the two compared objects share the same type.
Inheritance and the keyword ‘with’
In the code sample below it is not clear that person2 is a Student since it is inferred from a Person reference using the with syntax. As we saw in the Nondestructive mutation section, under the hood the generated virtual <Clone>$() method is called by the compiler. This virtual method is overridden by Student and its implementation calls the Student copy constructor:
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Person person1 = new Student { FirstName = "Bill", LastName = "Gates", ID= 123 }; Person person2 = person1 with { LastName = "Clinton" }; Assert.IsTrue(person2 is Student); |
Declaring attributes
It is possible to declare attributes for any of the elements generated by the compiler based on the C# record definition. You can specify a target for any attribute you use on the positional properties of the record. In the following example, the System.Text.Json.Serialization.JsonPropertyNameAttribute is assigned to each property generated by the C# compiler, of the Person record. The property target is used to specify that the attribute applies to the compiler-generated property, while other targets such as field and param can be used to apply the attribute to the field or parameter, respectively.
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public record Person( [property: JsonPropertyName("firstName")] string FirstName, [property: JsonPropertyName("lastName")] string LastName); |
Generic Record
Let’s note that a record can be a generic class or structure, which offers flexibility. However, developers should keep in mind that EqualityComparer<T>.Default is employed for each property typed with T in the generated code to perform state comparisons. This can potentially result in confusing behavior like in the following short program
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var pair1 = new Pair<string>("1", "2"); var pair2 = new Pair<string>("1", "2"); Assert.IsTrue(pair1 == pair2); // equals with string var pair3 = new Pair<object>(new object(), new object()); var pair4 = new Pair<object>(new object(), new object()); Assert.IsFalse(pair3 == pair4); // different with object record Pair<T>(T Left, T Right); |
Situations where records shine
C# Records find practical applications in various contexts:
- Simplifying Code: Records can help reduce boilerplate code, making your codebase more concise and easier to maintain. They are particularly beneficial when working with DTOs (Data Transfer Objects) and POCOs (Plain Old CLR Objects).
- Domain Models: Records serve as effective tools for representing domain models in your application. You can define a record with properties that align with the data you’re modeling. Records offer the advantages of working with data in a strongly typed and immutable manner, and they provide built-in capabilities for equality checking, formatting, and nondestructive mutation.
- Configuration Settings: Records are well-suited for representing configuration settings within an application. By creating a record with properties corresponding to different settings, you can easily pass these settings between methods and services, maintaining their immutability throughout.
- Concurrent Access Scenario: Since C# record classes are immutable by default, they inherently avoid issues related to concurrent modifications because a record object cannot be modified. For the same reason, no synchronization effort is required.
- API Response Models: Records are valuable for representing response models in RESTful APIs. They enable you to define a record with properties that mirror the data returned by the API. Records simplify the deserialization process, providing a strongly typed object for working with the data effortlessly.
- Debugging and Logging: Records provide a clear and concise string representation of their data, making them valuable for debugging and logging. When you log or print a record, you get a well-formatted view of its contents.
Conclusion
In conclusion, understanding C# records is crucial for modern C# programming, offering a streamlined, efficient way to handle immutable data. Throughout this article, we’ve explored the intricacies of C# record syntax, compared them with traditional classes, and delved into their practical applications. The advantages of using records, such as improved readability and thread safety, make them an invaluable feature in your C# development toolkit. Whether you’re a seasoned developer or new to C#, embracing records can significantly enhance your coding practices.
As the C# language continues to evolve, staying updated with features like records is essential for writing clean, efficient, and maintainable code.