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Primary constructors

[!INCLUDESpecletdisclaimer]

Summary

Classes and structs can have a parameter list, and their base class specification can have an argument list. Primary constructor parameters are in scope throughout the class or struct declaration, and if they are captured by a function member or anonymous function, they are appropriately stored (e.g. as unspeakable private fields of the declared class or struct).

The proposal "retcons" the primary constructors already available on records in terms of this more general feature with some additional members synthesized.

Motivation

The ability of a class or struct in C# to have more than one constructor provides for generality, but at the expense of some tedium in the declaration syntax, because the constructor input and the class state need to be cleanly separated.

Primary constructors put the parameters of one constructor in scope for the whole class or struct to be used for initialization or directly as object state. The trade-off is that any other constructors must call through the primary constructor.

public class C(bool b, int i, string s) : B(b) // b passed to base constructor
{
    public int I { get; set; } = i; // i used for initialization
    public string S // s used directly in function members
    {
        get => s;
        set => s = value ?? throw new NullArgumentException(nameof(X));
    }
    public C(string s) : this(true, 0, s) { } // must call this(...)
}

Detailed design

This describes the generalized design across records and non-records, and then details how the existing primary constructors for records are specified by adding a set of synthesized members in the presence of a primary constructor.

Syntax

Class and struct declarations are augmented to allow a parameter list on the type name, an argument list on the base class, and a body consisting of just a ;:

class_declaration
  : attributes? class_modifier* 'partial'? class_designator identifier type_parameter_list?
  parameter_list? class_base? type_parameter_constraints_clause* class_body
  ;
  
class_designator
  : 'record' 'class'?
  | 'class'
  
class_base
  : ':' class_type argument_list?
  | ':' interface_type_list
  | ':' class_type  argument_list? ',' interface_type_list
  ;  
  
class_body
  : '{' class_member_declaration* '}' ';'?
  | ';'
  ;
  
struct_declaration
  : attributes? struct_modifier* 'partial'? 'record'? 'struct' identifier type_parameter_list?
    parameter_list? struct_interfaces? type_parameter_constraints_clause* struct_body
  ;

struct_body
  : '{' struct_member_declaration* '}' ';'?
  | ';'
  ;
  
interface_declaration
  : attributes? interface_modifier* 'partial'? 'interface'
    identifier variant_type_parameter_list? interface_base?
    type_parameter_constraints_clause* interface_body
  ;  
    
interface_body
  : '{' interface_member_declaration* '}' ';'?
  | ';'
  ;

enum_declaration
  : attributes? enum_modifier* 'enum' identifier enum_base? enum_body
  ;

enum_body
  : '{' enum_member_declarations? '}' ';'?
  | '{' enum_member_declarations ',' '}' ';'?
  | ';'
  ;

Note: These productions replace record_declaration in Records and record_struct_declaration in Record structs, which both become obsolete.

It is an error for a class_base to have an argument_list if the enclosing class_declaration does not contain a parameter_list. At most one partial type declaration of a partial class or struct may provide a parameter_list. The parameters in the parameter_list of a record declaration must all be value parameters.

Note, according to this proposal class_body, struct_body, interface_body and enum_body are allowed to consist of just a ;.

A class or struct with a parameter_list has an implicit public constructor whose signature corresponds to the value parameters of the type declaration. This is called the primary constructor for the type, and causes the implicitly declared parameterless constructor, if present, to be suppressed. It is an error to have a primary constructor and a constructor with the same signature already present in the type declaration.

Lookup

The lookup of simple names is augmented to handle primary constructor parameters. The changes are highlighted in bold in the following excerpt:

  • Otherwise, for each instance type T (§14.3.2), starting with the instance type of the immediately enclosing type declaration and continuing with the instance type of each enclosing class or struct declaration (if any):
    • If e is zero and the declaration of T includes a type parameter with name I, then the simple_name refers to that type parameter.
    • Otherwise, if the declaration of T includes a primary constructor parameter I and the reference occurs within the argument_list of T's class_base or within an initializer of a field, property or event of T, the result is the primary constructor parameter I
    • Otherwise, if a member lookup (§11.5) of I in T with e type arguments produces a match:
      • If T is the instance type of the immediately enclosing class or struct type and the lookup identifies one or more methods, the result is a method group with an associated instance expression of this. If a type argument list was specified, it is used in calling a generic method (§11.7.8.2).
      • Otherwise, if T is the instance type of the immediately enclosing class or struct type, if the lookup identifies an instance member, and if the reference occurs within the block of an instance constructor, an instance method, or an instance accessor (§11.2.1), the result is the same as a member access (§11.7.6) of the form this.I. This can only happen when e is zero.
      • Otherwise, the result is the same as a member access (§11.7.6) of the form T.I or T.I<A₁, ..., Aₑ>.
    • Otherwise, if the declaration of T includes a primary constructor parameter I, the result is the primary constructor parameter I.

The first addition corresponds to the change incurred by primary constructors on records, and ensures that primary constructor parameters are found before any corresponding fields within initializers and base class arguments. It extends this rule to static initializers as well. However, since records always have an instance member with the same name as the parameter, the extension can only lead to a change in an error message. Illegal access to a parameter vs. illegal access to an instance member.

The second addition allows primary constructor parameters to be found elsewhere within the type body, but only if not shadowed by members.

It is an error to reference a primary constructor parameter if the reference does not occur within one of the following:

  • a nameof argument
  • an initializer of an instance field, property or event of the declaring type (type declaring primary constructor with the parameter).
  • the argument_list of class_base of the declaring type.
  • the body of an instance method (note that instance constructors are excluded) of the declaring type.
  • the body of an instance accessor of the declaring type.

In other words, primary constructor parameters are in scope throughout the declaring type body. They shadow members of the declaring type within an initializer of a field, property or event of the declaring type, or within the argument_list of class_base of the declaring type. They are shadowed by members of the declaring type everywhere else.

Thus, in the following declaration:

class C(int i)
{
    protected int i = i; // references parameter
    public int I => i; // references field
}

The initializer for the field i references the parameter i, whereas the body of the property I references the field i.

Warn on shadowing by a member from base

Compiler will produce a warning on usage of an identifier when a base member shadows a primary constructor parameter if that primary constructor parameter was not passed to the base type via its constructor.

A primary constructor parameter is considered to be passed to the base type via its constructor when all the following conditions are true for an argument in class_base:

  • The argument represents an implicit or explicit identity conversion of a primary constructor parameter;
  • The argument is not part of an expanded params argument;

Semantics

A primary constructor leads to the generation of an instance constructor on the enclosing type with the given parameters. If the class_base has an argument list, the generated instance constructor will have a base initializer with the same argument list.

Primary constructor parameters in class/struct declarations can be declared ref, in or out. Declaring ref or out parameters remains illegal in primary constructors of record declaration.

All instance member initializers in the class body will become assignments in the generated constructor.

If a primary constructor parameter is referenced from within an instance member, and the reference is not inside of a nameof argument, it is captured into the state of the enclosing type, so that it remains accessible after the termination of the constructor. A likely implementation strategy is via a private field using a mangled name. In a readonly struct the capture fields will be readonly. Therefore, access to captured parameters of a readonly struct will have similar restrictions as access to readonly fields. Access to captured parameters within a readonly member will have similar restrictions as access to instance fields in the same context.

Capturing is not allowed for parameters that have ref-like type, and capturing is not allowed for ref, in or out parameters. This is similar to a limitation for capturing in lambdas.

If a primary constructor parameter is only referenced from within instance member initializers, those can directly reference the parameter of the generated constructor, as they are executed as part of it.

Primary Constructor will do the following sequence of operations:

  1. Parameter values are stored in capture fields, if any.
  2. Instance initializers are executed
  3. Base constructor initializer is called

Parameter references in any user code are replaced with corresponding capture field references.

For instance this declaration:

public class C(bool b, int i, string s) : B(b) // b passed to base constructor
{
    public int I { get; set; } = i; // i used for initialization
    public string S // s used directly in function members
    {
        get => s;
        set => s = value ?? throw new NullArgumentException(nameof(X));
    }
    public C(string s) : this(true, 0, s) { } // must call this(...)
}

Generates code similar to the following:

public class C : B
{
    public int I { get; set; }
    public string S
    {
        get => __s;
        set => __s = value ?? throw new NullArgumentException(nameof(X));
    }
    public C(string s) : this(0, s) { ... } // must call this(...)
    
    // generated members
    private string __s; // for capture of s
    public C(bool b, int i, string s)
    {
        __s = s; // capture s
        I = i; // run I's initializer
        B(b) // run B's constructor
    }
}

It is an error for a non-primary constructor declaration to have the same parameter list as the primary constructor. All non-primary constructor declarations must use a this initializer, so that the primary constructor is ultimately called.

Records produce a warning if a primary constructor parameter isn't read within the (possibly generated) instance initializers or base initializer. Similar warnings will be reported for primary constructor parameters in classes and structures:

  • for a by-value parameter, if the parameter is not captured and is not read within any instance initializers or base initializer.
  • for an in parameter, if the parameter is not read within any instance initializers or base initializer.
  • for a ref parameter, if the parameter is not read or written to within any instance initializers or base initializer.

Identical simple names and type names

There is a special language rule for scenarios often referred to as "Color Color" scenarios - Identical simple names and type names.

In a member access of the form E.I, if E is a single identifier, and if the meaning of E as a simple_name (§11.7.4) is a constant, field, property, local variable, or parameter with the same type as the meaning of E as a type_name (§7.8.1), then both possible meanings of E are permitted. The member lookup of E.I is never ambiguous, since I shall necessarily be a member of the type E in both cases. In other words, the rule simply permits access to the static members and nested types of E where a compile-time error would otherwise have occurred.

With respect to primary constructors, the rule affects whether an identifier within an instance member should be treated as a type reference, or as a primary constructor parameter reference, which, in turn, captures the parameter into the the state of the enclosing type. Even though "the member lookup of E.I is never ambiguous", when lookup yields a member group, in some cases it is impossible to determine whether a member access refers to a static member or an instance member without fully resolving (binding) the member access. At the same time, capturing a primary constructor parameter changes properties of enclosing type in a way that affects semantic analysis. For example, the type might become unmanaged and fail certain constraints because of that. There are even scenarios for which binding can succeed either way, depending on whether the parameter is considered captured or not. For example:

struct S1(Color Color)
{
    public void Test()
    {
        Color.M1(this);
    }
}

class Color
{
    public void M1<T>(T x, int y = 0)
    {
        System.Console.WriteLine("instance");
    }
    
    public static void M1<T>(T x) where T : unmanaged
    {
        System.Console.WriteLine("static");
    }
}

If we treat receiver Color as a value, we capture the parameter and 'S1' becomes managed. Then the static method becomes inapplicable due to the constraint and we would call instance method. However, if we treat the receiver as a type, we don't capture the parameter and 'S1' remains unmanaged, then both methods are applicable, but the static method is "better" because it doesn't have an optional parameter. Neither choice leads to an error, but each would result in distinct behavior.

Given this, compiler will produce an ambiguity error for a member access E.I when all the following conditions are met:

  • Member lookup of E.I yields a member group containing instance and static members at the same time. Extension methods applicable to the receiver type are treated as instance methods for the purpose of this check.
  • If E is treated as a simple name, rather than a type name, it would refer to a primary constructor parameter and would capture the parameter into the state of the enclosing type.

Double storage warnings

If a primary constructor parameter is passed to the base and also captured, there's a high risk that it is inadvertently stored twice in the object.

Compiler will produce a warning for in or by value argument in a class_base argument_list when all the following conditions are true:

  • The argument represents an implicit or explicit identity conversion of a primary constructor parameter;
  • The argument is not part of an expanded params argument;
  • The primary constructor parameter is captured into the state of the enclosing type.

Compiler will produce a warning for a variable_initializer when all the following conditions are true:

  • The variable initializer represents an implicit or explicit identity conversion of a primary constructor parameter;
  • The primary constructor parameter is captured into the state of the enclosing type.

For example:

public class Person(string name)
{
    public string Name { get; set; } = name;   // initialization
    public override string ToString() => name; // capture
}

Attributes targeting primary constructors

At https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-03-13.md we decided to embrace the #7047 proposal.

The "method" attribute target is allowed on a class_declaration/struct_declaration with parameter_list and results in the corresponding primary constructor having that attribute. Attributes with the method target on a class_declaration/struct_declaration without parameter_list are are ignored with a warning.

[method: FooAttr] // Good
public partial record Rec(
    [property: Foo] int X,
    [field: NonSerialized] int Y
);
[method: BarAttr] // warning CS0657: 'method' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'type'. All attributes in this block will be ignored.
public partial record Rec
{
    public void Frobnicate()
    {
        ...
    }
}
[method: Attr] // Good
public record MyUnit1();
[method: Attr] // warning CS0657: 'method' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'type'. All attributes in this block will be ignored.
public record MyUnit2;

Primary constructors on records

With this proposal, records no longer need to separately specify a primary constructor mechanism. Instead, record (class and struct) declarations that have primary constructors would follow the general rules, with these simple additions:

  • For each primary constructor parameter, if a member with the same name already exists, it must be an instance property or field. If not, a public init-only auto-property of the same name is synthesized with a property initializer assigning from the parameter.
  • A deconstructor is synthesized with out parameters to match the primary constructor parameters.
  • If an explicit constructor declaration is a "copy constructor" - a constructor that takes a single parameter of the enclosing type - it is not required to call a this initializer, and will not execute the member initializers present in the record declaration.

Drawbacks

  • The allocation size of constructed objects is less obvious, as the compiler determines whether to allocate a field for a primary constructor parameter based on the full text of the class. This risk is similar to the implicit capture of variables by lambda expressions.
  • A common temptation (or accidental pattern) might be to capture the "same" parameter at multiple levels of inheritance as it is passed up the constructor chain instead of explicitly allotting it a protected field at the base class, leading to duplicated allocations for the same data in objects. This is very similar to today's risk of overriding auto-properties with auto-properties.
  • As proposed here, there is no place for additional logic that might usually be expressed in constructor bodies. The "primary constructor bodies" extension below addresses that.
  • As proposed, execution order semantics are subtly different from within ordinary constructors, delaying member initializers to after the base calls. This could probably be remedied, but at the cost of some of the extension proposals (notably "primary constructor bodies").
  • The proposal only works for scenarios where a single constructor can be designated primary.
  • There is no way to express separate accessibility of the class and the primary constructor. An example is when public constructors all delegate to one private "build-it-all" constructor. If necessary, syntax could be proposed for that later.

Alternatives

No capture

A much simpler version of the feature would prohibit primary constructor parameters from occurring in member bodies. Referencing them would be an error. Fields would have to be explicitly declared if storage is desired beyond the initialization code.

public class C(string s)
{
    public S1 => s; // Nope!
    public S2 { get; } = s; // Still allowed
}

This could still be evolved to the full proposal at a later time, and would avoid a number of decisions and complexities, at the cost of removing less boilerplate initially, and probably also seeming unintuitive.

Explicit generated fields

An alternative approach is for primary constructor parameters to always and visibly generate a field of the same name. Instead of closing over the parameters in the same manner as local and anonymous functions, there would explicitly be a generated member declaration, similar to the public properties generated for primary construcor parameters in records. Just like for records, if a suitable member already exists, one would not be generated.

If the generated field is private it could still be elided when it is not used as a field in member bodies. In classes, however, a private field would often not be the right choice, because of the state duplication it could cause in derived classes. An option here would be to instead generating a protected field in classes, encouraging reuse of storage across inheritance layers. However, then we would not be able to elide the declaration, and would incur allocation cost for every primary constructor parameter.

This would align non-record primary constructors more closely with record ones, in that members are always (at least conceptually) generated, albeit different kinds of members with different accessibilities. But it would also lead to surprising differences from how parameters and locals are captured elsewhere in C#. If we were ever to allow local classes, for example, they would capture enclosing parameters and locals implicitly. Visibly generating shadowing fields for them would not seem to be a reasonable behavior.

Another problem often raised with this approach is that many developers have different naming conventions for parameters and fields. Which should be used for the primary constructor parameter? Either choice would lead to inconsistency with the rest of the code.

Finally, visibly generating member declarations is really the name of the game for records, but much more surprising and "out of character" for non-record classes and structs. All in all, those are the reasons why the main proposal opts for implicit capture, with sensible behavior (consistent with records) for explicit member declarations when they are desired.

Remove instance members from initializer scope

The lookup rules above are intended to allow for the current behavior of primary constructor parameters in records when a corresponding member is manually declared, and to explain the behavior of the generated member when it is not. This requires lookup to differ between "initialization scope" (this/base initializers, member initializers) and "body scope" (member bodies), which the above proposal achieves by changing when primary constructor parameters are looked for, depending on where the reference occurs.

An observation is that referencing an instance member with a simple name in initializer scope always leads to an error. Instead of merely shadowing instance members in those places, could we simply take them out of scope? That way, there wouldn't be this weird conditional ordering of scopes.

This alternative is probably possible, but it would have some consequences that are somewhat far-reaching and potentially undesirable. First of all, if we remove instance members from initializer scope then a simple name that does correspond to an instance member and not to a primary constructor parameter could accidentally bind to something outside of the type declaration! This seems like it would rarely be intentional, and an error would be better.

Furthermore, static members are fine to reference in initialization scope. So we would have to distinguish between static and instance members in lookup, something we don't do today. (We do distinguish in overload resolution but that is not in play here). So that would have to also be changed, leading to yet more situations where e.g. in static contexts something would bind "further out" rather than error because it found an instance member.

All in all this "simplification" would lead to quite a downstream complication that no-one asked for.

Possible extensions

These are variations or additions to the core proposal that may be considered in conjunction with it, or at a later stage if deemed useful.

Primary constructor parameter access within constructors

The rules above make it an error to reference a primary constructor parameter within another constructor. This could be allowed within the body of other constructors, though, since the primary constructor runs first. However it would need to remain disallowed within the argument list of the this initializer.

public class C(bool b, int i, string s) : B(b)
{
    public C(string s) : this(b, s) // b still disallowed
    { 
        i++; // could be allowed
    }
}

Such access would still incur capture, as that would be the only way the constructor body could get at the variable after the primary constructor has already run.

The prohibition on primary constructor parameters in the this-initializer's arguments could be weakened to allow them, but make them not definitely assigned, but that does not seem useful.

Allow constructors without a this initializer

Constructors without a this initializer (i.e. with an implicit or explicit base initializer) could be allowed. Such a constructor would not run instance field, property and event initializers, as those would be considered to be part of the primary constructor only.

In the presence of such base-calling constructors, there are a couple of options for how primary constructor parameter capture is handled. The simplest is to completely disallow capture in this situation. Primary constructor parameters would be for initialization only when such constructors exist.

Alternatively, if combined with the previously described option to allow access to primary constructor parameters within constructors, the parameters could enter the constructor body as not definitely assigned, and ones that are captured would need to be definitely assigned by the end of the constructor body. They would essentially be implicit out parameters. That way, captured primary constructor parameters would always have a sensible (i.e. explicitly assigned) value by the time they are consumed by other function members.

An attraction of this extension (in either form) is that it fully generalizes the current exemption for "copy constructors" in records, without leading to situations where uninitialized primary constructor parameters are observed. Essentially, constructors that initialize the object in alternative ways are fine. The capture-related restrictions would not be a breaking change for existing manually defined copy constructors in records, because records never capture their primary constructor parameters (they generate fields instead).

public class C(bool b, int i, string s) : B(b)
{
    public int I { get; set; } = i; // i used for initialization
    public string S // s used directly in function members
    {
        get => s;
        set => s = value ?? throw new NullArgumentException(nameof(X));
    }
    public C(string s2) : base(true) // cannot use `string s` because it would shadow
    { 
        s = s2; // must initialize s because it is captured by S
    }
    protected C(C original) : base(original) // copy constructor
    {
        this.s = original.s; // assignment to b and i not required because not captured
    }
}

Primary constructor bodies

Constructors themselves often contain parameter validation logic or other nontrivial initialization code that cannot be expressed as initializers.

Primary constructors could be extended to allow statement blocks to appear directly in the class body. Those statements would be inserted in the generated constructor at the point where they appear between initializing assignments, and would thus be executed interspersed with initializers. For instance:

public class C(int i, string s) : B(s)
{
    {
        if (i < 0) throw new ArgumentOutOfRangeException(nameof(i));
    }
	int[] a = new int[i];
    public int S => s;
}

A lot of this scenario might be adequately be covered if we were to introduce "final initializers" which run after the constructors and any object/collection initializers have completed. However, argument validation is one thing that would ideally happen as early as possible.

Primary constructor bodies could also provide a place for allowing an access modifier for the primary constructor, allowing it to deviate from the accessibility of the enclosing type.

Combined parameter and member declarations

A possible and often mentioned addition could be to allow primary constructor parameters to be annotated so that they would also declare a member on the type. Most commonly it is proposed to allow an access specifier on the parameters to trigger the member generation:

public class C(bool b, protected int i, string s) : B(b) // i is a field as well as a parameter
{
    void M()
    {
        ... i ... // refers to the field i
        ... s ... // closes over the parameter s
    }
}

There are some problems:

  • What if a property is desired, not a field? Having { get; set; } syntax inline in a parameter list does not seem appetizing.
  • What if different naming conventions are used for parameters and fields? Then this feature would be useless.

This is a potential future addition that can be adopted or not. The current proposal leaves the possibility open.

Open questions

Field targeting attributes for captured primary constructor parameters

Should we allow field targeting attributes for captured primary constructor parameters?

class C1([field: Test] int x) // Parameter is captured, the attribute goes to the capture field
{
    public int X => x;
}
class C2([field: Test] int x) // Parameter is not captured, the attribute is ignored with a warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'param'. All attributes in this block will be ignored.
{
    public int X = x;
}

Right now the attributes are ignored with the warning regardless of whether the parameter is captured.

Note that for records, field targeted attributes are allowed when a property is synthesized for it. The attributes go on the backing field then.

record R1([field: Test]int X); // Ok, the attribute goes on the backing field
record R2([field: Test]int X) // warning CS0657: 'field' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'param'. All attributes in this block will be ignored.
{
    public int X = X;
}

Conclusion:

Not allowed (https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-03.md#attributes-on-captured-parameters).

Warn on shadowing by a member from base

Should we report a warning when a member from base is shadowing a primary constructor parameter inside a member (see #7109 (reply in thread))?

Conclusion:

An alternative design is approved - https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-08.md#primary-constructors

Capturing instance of the enclosing type in a closure

When a parameter captured into the state of the enclosing type is also referenced in a lambda inside an instance initializer or a base initializer, the lambda and the state of the enclosing type should refer to the same location for the parameter. For example:

partial class C1
{
    public System.Func<int> F1 = Execute1(() => p1++);
}

partial class C1 (int p1)
{
    public int M1() { return p1++; }
    static System.Func<int> Execute1(System.Func<int> f)
    {
        _ = f();
        return f;
    }
}

Since naive implementation of capturing a parameter into the state of the type simply captures the parameter in a private instance field, the lambda needs to refer to the same field. As a result, it needs to be able to access the instance of the type. This requires capturing this into a closure before the base constructor is invoked. That, in turn, results in safe, but an unverifiable IL. Is this acceptable?

Alternatively we could:

  • Disallow lambdas like that;
  • Or, instead, capture parameters like that in an instance of a separate class (yet another closure), and share that instance between the closure and the instance of the enclosing type. Thus eliminating the need to capture this in a closure.

Conclusion:

We are comfortable with capturing this into a closure before the base constructor is invoked (https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-02-15.md). The runtime team didn't find the IL pattern problematic as well.

Assigning to this within a struct

C# allows to assign to this within a struct. If the struct captures a primary constructor parameter, the assignment is going to overwrite its value, which might be not obvious to the user. Do we want to report a warning for assignments like this?

struct S(int x)
{
    int X => x;
    
    void M(S s)
    {
        this = s; // 'x' is overwritten
    }
}

Conclusion:

Allowed, no warning (https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-02-15.md).

Double storage warning for initialization plus capture

We have a warning if a primary constructor parameter is passed to the base and also captured, because there's a high risk that it is inadvertently stored twice in the object.

It seems that there's a similar risk if a parameter is used to initialize a member, and is also captured. Here's a small example:

public class Person(string name)
{
    public string Name { get; set; } = name;   // initialization
    public override string ToString() => name; // capture
}

For a given instance of Person, changes to Name would not be reflected in the output of ToString, which is probably unintended on the developer's part.

Should we introduce a double storage warning for this situation?

This is how it would work:

The compiler will produce a warning for a variable_initializer when all the following conditions are true:

  • The variable initializer represents an implicit or explicit identity conversion of a primary constructor parameter;
  • The primary constructor parameter is captured into the state of the enclosing type.

Conclusion:

Approved, see https://github.com/dotnet/csharplang/blob/main/meetings/2023/LDM-2023-05-15.md#primary-constructors

LDM meetings