Immutable data types after .NET 5 release

Just couple weeks ago, Microsoft released RC of .NET 5 which is (unfortunately) not going to be an LTS (Long Term Support) release but on the other hand, it’s coming with some great features in it (yep yep).

One of them comes as a part of the new release of C# 9.0 (part of the .NET 5 release) which is Immutable Objects and Properties (records and init-only properties). Quite a smart concept in my opinion …

Recap on immutable data type

The immutable data type is basically data type of the variable of which the value cannot be changed after creation.

How does it look in reality?

Well, once immutable data typed object is created then only way how to change its value is to create a new one with a copied value of the previous instance.

What are the current immutable (and mostly used) data types from .NET CLR?

Primitive types

  • Byte and SByte
  • Int16 and UInt16
  • Int32 and UInt32
  • Int64 and UInt64
  • IntPtr
  • Single
  • Double
  • Decimal

Others

  • All enumeration types (enum, Enum)
  • All delegate types
  • DateTime, TimeSpan and DateTimeOffset
  • DBNull
  • Guid
  • Nullable
  • String
  • Tuple<T>
  • Uri
  • Version
  • Void
  • Lookup<TKey, TElement>

As you can see, we have quite a few to choose from already. How this list is going to look like after .NET 5 full release in November 2020?

Well, it’s going to be a revolutionary change in my 2 cents.

Principally, any object using .NET 5 runtime (and C# 9.0) can be immutable and also implement its own immutable state – and that is HOT feature.

The syntax of the immutable properties looks like in this example:

public class ObjectName
{
    public string FirstProperty { get; init; }
    public string SecondProperty { get; init; }
}

On the other hand, the syntax of immutable object (called a record) looks this:

public data class ObjectName
{
    public string FirstProperty { get; init; }
    public string SecondProperty { get; init; }
}

As you can see, the syntax is very clear and intuitive to use.

More details about new C# 9.0 features can be found here https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#record-types.

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