Microsoft reveals C# 9.0 plans

The next version of Microsoft’s Java rival will offer new capabilities ranging from records to value-based equality

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Microsoft is plowing ahead with the development of C# 9.0, an upgrade to the company’s type-safe, object-oriented language that will include new capabilities such as records and value-based equality.

C# 9.0 is due to be part of the .NET 5 development platform, set to arrive in November. C# 8.0 arrived last September. The new features in C# 9.0, detailed at Build this week by C# lead designer Mads Torgersen, include the following:

Records, for declaring a whole object to be immutable and have it behave like a value. Records are intended to be seen more like data and less like objects.
with expressions, which use object initializer syntax to show what is different in a new object compared to an old one.
Improved pattern matching.
Value-based equality. All objects inherit a virtual Equals(object) method from the object class. This serves as a basis for the Object.Equals(object, object) static method when both parameters are non-null. Structs override this to have “value-based equality,” allowing comparisons of each field of the struct by calling Equals on them in a recursive manner. Records also do this. Thus, in accordance with “value-ness,” two record objects can be equal without being the same object.
Relational patterns, which are patterns corresponding to relational operators <, <=, and so on.
Logical patterns, which combine patterns with logical operators and, or, and not, spelled out as words to avoid confusion with operators used in expressions
Simple type patterns.
Init-only properties, introducing an init assessor that is a variant of the set assessor, for calling during object initialization. These properties address a limitation of object initializers, in which properties must be mutable for these initializers to work.
Improved target typing, which is a term describing when an expression gets its type from the context where it is being used. For example, null and lambda expressions are always targeted. With C# 9.0, some expressions that were not previously target-typed now can be guided by their context.
Target-typed new expressions, in which the type can be left out if there is a clear type that the expression is being assigned to.
Covariant returns, to express that a method override in a derived class has a more specific return type than the declaration in the base type.
Positional records, providing an approach to records where contents are given via constructor arguments and can be extracted with positional deconstruction.
Top-level programs, to address the issue of too much boilerplate code.