RFC FS-1353 - Parameterless struct constructors and struct initializers

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Summary

F# struct types can declare a public parameterless constructor (new() = ... or type S() = ...). Structs with a primary constructor can contain instance let, do and member val definitions, the F# counterpart of C# 10 struct field initializers. S() calls a public parameterless constructor and otherwise zero-initializes, as in C#. For a struct with such initializers, S() never zero-initializes; Unchecked.defaultof<S> still does. Non-public parameterless constructors of imported structs are ignored, as in C#. Gated by a preview feature; no FSharp.Core change.

Both suggestions are approved in principle: #362 (parameterless constructors in structs) and #1056 (do bindings in structs).

Current behaviour the RFC starts from

F# column checked with SDK 11.0.100-rc.1; C# column from the C# 10 proposal, with new S() and CS8958 checked on the same SDK.

Case	F# today	C#
new() = ... / type S() = in a struct	FS0870 / FS0081	allowed, must be public (CS8958)
let / do / member val in a primary-constructor struct	FS0901 / FS0035 / FS0901	field initializers allowed
S() on a C# struct with public S()	calls it	calls it
S() on an IL struct with a private or internal parameterless constructor	FS0801	zero-initializes
new 'T() for that IL struct	compiles, MissingMethodException at run time	same
Unchecked.defaultof, Array.zeroCreate, [<DefaultValue>] field, omitted [<Optional>] argument	zero, constructor not run	zero
<@ S() @>	NewObject with a constructor, DefaultValue without	–

Related

• Suggestions: Allow parameterless constructors in structs fslang-suggestions#362, Allow do bindings in structs fslang-suggestions#1056
• C# proposals: parameterless struct constructors (C# 10), auto-default structs (C# 11)

🤖 Generated with Claude Code

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If it makes sens to describe only the difference with classes that may be shorter