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Parameterless struct constructors

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Summary

Support parameterless constructors and instance field initializers for struct types.

Motivation

Explicit parameterless constructors would give more control over minimally constructed instances of the struct type. Instance field initializers would allow simplified initialization across multiple constructors. Together these would close an obvious gap between struct and class declarations.

Support for field initializers would also allow initialization of fields in record struct declarations without explicitly implementing the primary constructor.

record struct Person(string Name)
{
    public object Id { get; init; } = GetNextId();
}

If struct field initializers are supported for constructors with parameters, it seems natural to extend that to parameterless constructors as well.

record struct Person()
{
    public string Name { get; init; }
    public object Id { get; init; } = GetNextId();
}

Proposal

Instance field initializers

Instance field declarations for a struct may include initializers.

As with class field initializers §14.5.6.3:

A variable initializer for an instance field cannot reference the instance being created.

An error is reported if a struct has field initializers and no declared instance constructors since the field initializers will not be run.

struct S { int F = 42; } // error: 'struct' with field initializers must include an explicitly declared constructor

Constructors

A struct may declare a parameterless instance constructor.

A parameterless instance constructor is valid for all struct kinds including struct, readonly struct, ref struct, and record struct.

If no parameterless instance constructor is declared, the struct (see §15.4.9) ...

implicitly has a parameterless instance constructor which always returns the value that results from setting all value type fields to their default value and all reference type fields to null.

Modifiers

A parameterless instance struct constructor must be declared public.

struct S0 { }                   // ok
struct S1 { public S1() { } }   // ok
struct S2 { internal S2() { } } // error: parameterless constructor must be 'public'

Non-public constructors are ignored when importing types from metadata.

Constructors can be declared extern or unsafe. Constructors cannot be partial.

Executing field initializers

Instance variable initializers (§14.11.3) is modified as follows:

When a class instance constructor has no constructor initializer, or it has a constructor initializer of the form base(...), that constructor implicitly performs the initializations specified by the variable_initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor.

When a struct instance constructor has no constructor initializer, that constructor implicitly performs the initializations specified by the variable_initializers of the instance fields declared in its struct. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor.

When a struct instance constructor has a this() constructor initializer that represents the default parameterless constructor, the declared constructor implicitly clears all instance fields and performs the initializations specified by the variable_initializers of the instance fields declared in its struct. Immediately upon entry to the constructor, all value type fields are set to their default value and all reference type fields are set to null. Immediately after that, a sequence of assignments corresponding to the variable_initializers are executed.

Definite assignment

Instance fields (other than fixed fields) must be definitely assigned in struct instance constructors that do not have a this() initializer (see §15.4.9).

struct S0 // ok
{
    int x;
    object y;
}

struct S1 // error: 'struct' with field initializers must include an explicitly declared constructor
{
    int x = 1;
    object y;
}

struct S2
{
    int x = 1;
    object y;
    public S2() { } // error: field 'y' must be assigned
}

struct S3 // ok
{
    int x = 1;
    object y;
    public S3() { y = 2; }
}

No base() initializer

A base() initializer is disallowed in struct constructors.

The compiler will not emit a call to the base System.ValueType constructor from struct instance constructors.

record struct

An error is reported if a record struct has field initializers and does not contain a primary constructor nor any instance constructors since the field initializers will not be run.

record struct R0;                  // ok
record struct R1 { int F = 42; }   // error: 'struct' with field initializers must include an explicitly declared constructor
record struct R2() { int F = 42; } // ok
record struct R3(int F);           // ok

A record struct with an empty parameter list will have a parameterless primary constructor.

record struct R3();                // primary .ctor: public R3() { }
record struct R4() { int F = 42; } // primary .ctor: public R4() { F = 42; }

An explicit parameterless constructor in a record struct must have a this initializer that calls the primary constructor or an explicitly declared constructor.

record struct R5(int F)
{
    public R5() { }                  // error: must have 'this' initializer that calls explicit .ctor
    public R5(object o) : this() { } // ok
    public int F =  F;
}

Fields

The implicitly-defined parameterless constructor will zero fields rather than calling any parameterless constructors for the field types. No warnings are reported that field constructors are ignored. No change from C#9.

struct S0
{
    public S0() { }
}

struct S1
{
    S0 F; // S0 constructor ignored
}

struct S<T> where T : struct
{
    T F; // constructor (if any) ignored
}

default expression

default ignores the parameterless constructor and generates a zeroed instance. No change from C#9.

// struct S { public S() { } }

_ = default(S); // constructor ignored, no warning

new()

Object creation invokes the parameterless constructor if public; otherwise the instance is zeroed. No change from C#9.

// public struct PublicConstructor { public PublicConstructor() { } }
// public struct PrivateConstructor { private PrivateConstructor() { } }

_ = new PublicConstructor();  // call PublicConstructor::.ctor()
_ = new PrivateConstructor(); // initobj PrivateConstructor

A warning wave may report a warning for use of new() with a struct type that has constructors but no parameterless constructor. No warning will be reported when using substituting such a struct type for a type parameter with a new() or struct constraint.

struct S { public S(int i) { } }
static T CreateNew<T>() where T : new() => new T();

_ = new S();        // warning: no constructor called
_ = CreateNew<S>(); // ok

Uninitialized values

A local or field of a struct type that is not explicitly initialized is zeroed. The compiler reports a definite assignment error for an uninitialized struct that is not empty. No change from C#9.

NoConstructor s1;
PublicConstructor s2;
s1.ToString(); // error: use of unassigned local (unless type is empty)
s2.ToString(); // error: use of unassigned local (unless type is empty)

Array allocation

Array allocation ignores any parameterless constructor and generates zeroed elements. No change from C#9.

// struct S { public S() { } }

var a = new S[1]; // constructor ignored, no warning

Parameter default value new()

A parameter default value of new() binds to the parameterless constructor if public (and reports an error that the value is not constant); otherwise the instance is zeroed. No change from C#9.

// public struct PublicConstructor { public PublicConstructor() { } }
// public struct PrivateConstructor { private PrivateConstructor() { } }

static void F1(PublicConstructor s1 = new()) { }  // error: default value must be constant
static void F2(PrivateConstructor s2 = new()) { } // ok: initobj

Type parameter constraints: new() and struct

The new() and struct type parameter constraints require the parameterless constructor to be public if defined (see Satisfying constraints - §8.4.5).

The compiler assumes all structs satisfy new() and struct constraints. No change from C#9.

// public struct PublicConstructor { public PublicConstructor() { } }
// public struct InternalConstructor { internal InternalConstructor() { } }

static T CreateNew<T>() where T : new() => new T();
static T CreateStruct<T>() where T : struct => new T();

_ = CreateNew<PublicConstructor>();      // ok
_ = CreateStruct<PublicConstructor>();   // ok

_ = CreateNew<InternalConstructor>();    // compiles; may fail at runtime
_ = CreateStruct<InternalConstructor>(); // compiles; may fail at runtime

new T() is emitted as a call to System.Activator.CreateInstance<T>(), and the compiler assumes the implementation of CreateInstance<T>() invokes the public parameterless constructor if defined.

With .NET Framework, Activator.CreateInstance<T>() invokes the parameterless constructor if the constraint is where T : new() but appears to ignore the parameterless constructor if the constraint is where T : struct.

Optional parameters

Constructors with optional parameters are not considered parameterless constructors. No change from C#9.

struct S1 { public S1(string s = "") { } }
struct S2 { public S2(params object[] args) { } }

_ = new S1(); // ok: ignores constructor
_ = new S2(); // ok: ignores constructor

Metadata

Explicit parameterless struct instance constructors will be emitted to metadata.

Public parameterless struct instance constructors will be imported from metadata; non-public struct instance constructors will be ignored. No change from C#9.

See also

Design meetings