Records, Anonymous Types, and Non-Destructive Mutation

Records, Anonymous Types, and Non-Destructive Mutation


Today, I'm going to share some fascinating things you can do with records and anonymous types. I will introduce you to the concept of non-destructive mutation. And I will talk about when and why we might want to use this C# language feature.

What Is a Record?

With C# 9 we can use records that are a new reference type. C# 10 introduced record structs so that you can define records as value types. Records are distinct from classes in that record types use value-based equality.

Let's see how we would define a record:

public record Food(string Name, double Price);

This way of declaring a record is called a positional record. The constructor we have defined here is called the primary constructor.

The Name and Price properties are init only properties. This means they can only be set in the constructor or using a property initializer.

Since our properties are init only, is there any way to change their value?

Non-Destructive Mutation Using The With Expression

We said we can't modify the properties of our record, because the properties are init only. However, we can use the with expression (introduced in C# 9) to create a new instance of our record with modified values.

Let's see how we would use the with expression:

var banana = new Food("🍌", 1.95);

var bananaOnSale = banana with
    Price = 0.99

It's important to highlight two things here:

  • The original banana instance remains unchanged
  • The with expression creates a new record instance with only the Price property modified

I mentioned Anonymous Types in the title, so let me show you something interesting you can do with them.

Anonymous Types And Non-Destructive Mutation

Did you know that you can use the with expression with anonymous types?

Just a reminder that the with expression is available from C# 9 and later.

Let's create an anonymous type:

var apple = new
    Name = "🍎",
    Price = 1.21

This is how we can modify it using the with expression:

var orange = apple with
    Name = "🍊"

And again the same rules apply:

  • The original apple instance remains unchanged
  • The with expression creates a new anonymous type instance with only the Name property modified

I found this feature useful in LINQ method chains.

For example, loading an anonymous type from the database where some properties have a default value. You can then use this feature to calculate the values for these properties in memory.

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