AutoConstructor

C# source generator that generates a constructor from readonly fields/properties in a class or struct.

Installation

• Grab the latest package on NuGet.

Requirements

Basic usage

The following code:

[AutoConstructor]
public partial class MyClass
{
    private readonly MyDbContext _context;
    private readonly IHttpClientFactory _clientFactory;
    private readonly IService _service;

    [AutoConstructorInject("options?.Value", "options", typeof(IOptions<ApplicationOptions>))]
    private readonly ApplicationOptions _options;
}

will generate:

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by the AutoConstructor source generator.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------
partial class MyClass
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("AutoConstructor", "5.0.0.0")]
    public MyClass(global::MyDbContext context, global::System.Net.Http.IHttpClientFactory clientFactory, global::IService service, global::Microsoft.Extensions.Options.IOptions<global::ApplicationOptions> options)
    {
        this._context = context;
        this._clientFactory = clientFactory;
        this._service = service;
        this._options = options?.Value;
    }
}

A sample containing more cases is available at the end of this README.

How to use

For any class where the generator will be used:

• Mark the class or struct as partial
• Use AutoConstructorAttribute on the class or struct

By default, all readonly non-static fields without initialization will be used. They will be injected with the same name without any leading _.

Fields marked with AutoConstructorIgnoreAttribute will be ignored.

Use AutoConstructorInjectAttribute to customize the behavior, usually when the injected parameter and the fields do not have the same type. It takes three optional parameters:

• initializer: a string that will be used to initialize the field (by example myService.GetData()), default to the parameterName if null or empty.
• parameterName: the name of the parameter to used in the constructor (by example myService), default to the field name trimmed if null or empty.
• injectedType: the type of the parameter to used in the constructor (by example IMyService), default to the field type if null.

If no parameters are provided, the behavior will be the same as without the attribute. Using the attribute on a field that would not be injected otherwise won't make the field injectable.

When using AutoConstructorInjectAttribute, the parameter name can be shared across multiple fields, and even use a parameter from another field not annotated with AutoConstructorInjectAttribute, but type must match.

Constructor accessibility

Constructor accessibility can be changed using the optional parameter accessibility on AutoConstructorAttribute (like [AutoConstructor("internal")]). The default is public and it can be set to one of the following values:

• public
• private
• protected
• internal
• protected internal
• private protected

Initializer method

It is possible to add a method call at the end of the constructor. To do this, the attribute AutoConstructorInitializer can be added to a parameterless method that returns void. This will generate a call to the method at the end.

[AutoConstructor]
internal partial class Test
{
    private readonly int _t;

    [AutoConstructorInitializer]
    public void Initializer()
    {
    }
}

will generate

public Test(int t)
{
    this._t = t;

    this.Initializer();
}

Configuring base call

It is possible to configure which base constructor is called when a type has a non-object base type and has its constructor generated. By default, a call to base is emitted only when the is only one constructor on the base type. This behavior can be changed by adding a [AutoConstructorDefaultBase] on a constructor in the base type to indicate that it must be chosen as the base call.

If the base type is also generated, the addDefaultBaseAttribute parameter on AutoConstructorAttribute can be used to generate the attribute with the generated constructor.

internal class BaseClass
{
    private readonly int _t;

    [AutoConstructorDefaultBase]
    public BaseClass(int t1, int t3)
    {
        this._t = t1 + t3;
    }

    public BaseClass(int t)
    {
        this._t = t;
    }

    public BaseClass()
    {
    }
}

[AutoConstructor]
internal partial class Test : BaseClass
{
    private readonly int _t2;
}

will generate

partial class Test
{
    public Test(int t2, int t1, int t3) : base(t1, t3)
    {
        this._t2 = t2;
    }
}

Base constructor parameter matching

When inheriting a class, a call to the base constructor will be generated. By default, any parameter with the same name and the same type in both the parent and the child class will be matched together.

// This (same name and type)
public class ParentClass
{
    private readonly int value;

    public ParentClass(int value)
    {
        this.value = value;
    }
}

[AutoConstructor]
public partial class Test : ParentClass
{
    private readonly int value;
}

// Generates
partial class Test
{
    public Test(int service) : base(service)
    {
        this.service = service;
    }
}

// This (same name but not same type)
public class ParentClass
{
    private readonly long value;

    public ParentClass(long value)
    {
        this.value = value;
    }
}

[AutoConstructor]
public partial class Test : ParentClass
{
    private readonly int value;
}

// Generates
partial class Test
{
    public Test(int service, long b0__service) : base(b0__service)
    {
        this.service = service;
    }
}

If wanted, the matching on the type can be disable by setting the parameter matchBaseParameterOnName on AutoConstructorAttribute as true. ⚠️ This can lead to invalid code, since the type is no longer checked, anything can be used as a parameter with the same name. Use this only when necessary.

// This (same name but not same type with matchBaseParameterOnName true)
public class ParentClass
{
    private readonly long value;

    public ParentClass(long value)
    {
        this.value = value;
    }
}

[AutoConstructor(matchBaseParameterOnName: true)]
public partial class Test : ParentClass
{
    private readonly int value;
}

// Generates
partial class Test
{
    public Test(int service) : base(service)
    {
        this.service = service;
    }
}

Properties injection

Get-only properties (public int Property { get; }) are injected by the generator by default. Non get-only properties (public int Property { get; set;}) are injected only if marked with ([field: AutoConstructorInject]) attribute. The behavior of the injection can be modified using auto-implemented property field-targeted attributes on its backing field. The following code show an injected get-only property with a custom injecter:

[field: AutoConstructorInject(initializer: "injected.ToString()", injectedType: typeof(int), parameterName: "injected")]
public int Property { get; }

⚠️ The compiler support for auto-implemented property field-targeted attributes is not perfect, and Roslyn analyzers are not running on backings fields so some warnings may not be reported.

Configuration

Generating ArgumentNullException

By default, null checks with ArgumentNullException are not generated when needed.

To enable this behavior, set AutoConstructor_GenerateArgumentNullExceptionChecks to true in the project file:

<AutoConstructor_GenerateArgumentNullExceptionChecks>true</AutoConstructor_GenerateArgumentNullExceptionChecks>

Generating this() calls

By default, if a non-generated parameterless constructor is available on the class (other than the implicit one), a call to this() is generated with the generated constructor. To disable this behavior, set AutoConstructor_GenerateThisCalls to false in the project file:

<AutoConstructor_GenerateThisCalls>false</AutoConstructor_GenerateThisCalls>

This is also configurable at the attribute level with the DisableThisCall parameter on AutoConstructorAttribute (⚠ it is not possible force the generation at the attribute level if the generation is globally disabled).

Generating XML documentation comment

By default, no XML documentation comment will be generated for the constructor. To enable this behavior, set AutoConstructor_GenerateConstructorDocumentation to true in the project file:

<AutoConstructor_GenerateConstructorDocumentation>true</AutoConstructor_GenerateConstructorDocumentation>

This will generate a default comment like this one, with each parameter reusing the corresponding field summary if available, and the parameter name otherwise:

/// <summary>
/// Initializes a new instance of the Test class.
/// </summary>
/// <param name=""t1"">Some field.</param>
/// <param name=""t2"">t2</param>

By using the AutoConstructor_ConstructorDocumentationComment property, you can configure the comment message:

<AutoConstructor_ConstructorDocumentationComment>Some comment for the {0} class.</AutoConstructor_ConstructorDocumentationComment>

This will generate the following code:

/// <summary>
/// Some comment for the Test class.
/// </summary>
/// <param name=""t1"">Some field.</param>
/// <param name=""t2"">t2</param>

Generating a parameterless constructor

If needed, a parameterless constructor can also be generated alongside the generated constructor using the addParameterless option on AutoConstructor.

[AutoConstructor(addParameterless: true)]
public partial class Test
{
    public int Id { get; set; }
    public string Name { get; set; }
}

will generate

partial class Test
{
    #pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor.
    [global::System.ObsoleteAttribute("Not intended for direct usage.", true)]
    public Test()
    {
    }
}

This option can also be used without any fields or properties to inject, this will disable the warning that will normally be reported and generate a parameterless constructor.

:warning: With inheritance, to be able to generate a parameterless constructor, the base type of the target type must have parameterless constructor itself.

By default, this constructor is marked as [Obsolete] with a default message. This is configurable with :

<AutoConstructor_MarkParameterlessConstructorAsObsolete>false</AutoConstructor_MarkParameterlessConstructorAsObsolete>
<AutoConstructor_ParameterlessConstructorObsoleteMessage>Custom obsolete message</AutoConstructor_ParameterlessConstructorObsoleteMessage>

Samples describing some cases

Sample for fields

The following code

[AutoConstructor]
partial class Test
{
    private readonly string _name;

    // Won't be injected
    private readonly Uri _uri = new Uri("/non-modified", UriKind.Relative);

    // Won't be injected
    [AutoConstructorIgnore]
    private readonly DateTime _dateNotTaken;

    // Won't be injected because not readonly. Attribute would be taken into account if this were a property, not a field.
    [AutoConstructorInject]
    private int  _stuff;

    // Won't be injected
    private int? _toto;

    // Support for nullables
    private readonly DateTime? _date;

    // Support for generics
    private readonly List<DateTime> _items;

    // Inject with custom initializer
    [AutoConstructorInject("guid.ToString()", "guid", typeof(Guid))]
    private readonly string _guidString;

    // Use existing parameter defined with AutoConstructorInject
    [AutoConstructorInject("guid.ToString().Length", "guid", typeof(Guid))]
    private readonly int _guidLength;

    // Use existing parameter from a basic injection
    [AutoConstructorInject("name.ToUpper()", "name", typeof(string))]
    private readonly string _nameShared;
}

will generate

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by the AutoConstructor source generator.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------
partial class Test
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("AutoConstructor", "5.0.0.0")]
    public Test(string name, global::System.DateTime? date, global::System.Collections.Generic.List<global::System.DateTime> items, global::System.Guid guid)
    {
        this._name = name ?? throw new global::System.ArgumentNullException(nameof(name));
        this._date = date;
        this._items = items ?? throw new global::System.ArgumentNullException(nameof(items));
        this._guidString = guid.ToString() ?? throw new global::System.ArgumentNullException(nameof(guid));
        this._guidLength = guid.ToString().Length;
        this._nameShared = name.ToUpper() ?? throw new global::System.ArgumentNullException(nameof(name));
    }
}

Sample for get-only properties

The following code

[AutoConstructor]
public partial class Test
{
    [field: AutoConstructorInject]
    public int Injected { get; }

    public int AlsoInjectedEvenWhenMissingAttribute { get; }

    /// <summary>
    /// Some property.
    /// </summary>
    [field: AutoConstructorInject]
    public int InjectedWithDocumentation { get; }

    [field: AutoConstructorInject]
    public int InjectedBecauseExplicitInjection { get; set; }

    [field: AutoConstructorInject]
    public static int NotInjectedBecauseStatic { get; }

    [field: AutoConstructorInject]
    public int NotInjectedBecauseInitialized { get; } = 2;

    [field: AutoConstructorIgnore]
    public int NotInjectedBecauseHasIgnoreAttribute { get; }

    [field: AutoConstructorInject(initializer: "injected.ToString()", injectedType: typeof(int), parameterName: "injected")]
    public string InjectedWithoutCreatingAParam { get; }
}

will generate

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by the AutoConstructor source generator.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------
partial class Test
{
    /// <summary>
    /// Initializes a new instance of the Test class.
    /// </summary>
    /// <param name="injected">injected</param>
    /// <param name="alsoInjectedEvenWhenMissingAttribute">alsoInjectedEvenWhenMissingAttribute</param>
    /// <param name="injectedWithDocumentation">Some property.</param>
    /// <param name="injectedBecauseExplicitInjection">injectedBecauseExplicitInjection</param>
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("AutoConstructor", "5.0.0.0")]
    public Test(int injected, int alsoInjectedEvenWhenMissingAttribute, int injectedWithDocumentation, int injectedBecauseExplicitInjection)
    {
        this.Injected = injected;
        this.AlsoInjectedEvenWhenMissingAttribute = alsoInjectedEvenWhenMissingAttribute;
        this.InjectedWithDocumentation = injectedWithDocumentation;
        this.InjectedBecauseExplicitInjection = injectedBecauseExplicitInjection;
        this.InjectedWithoutCreatingAParam = injected.ToString() ?? throw new global::System.ArgumentNullException(nameof(injected));
    }
}

Diagnostics

ACONS01

The AutoConstructor attribute is used on a class that is not partial.

ACONS02

The AutoConstructor attribute is used on a class without fields to inject (without specifying addParameterless as true).

ACONS03

The AutoConstructorIgnore attribute is used on a field that won't already be processed.

ACONS04

The AutoConstructorInject attribute is used on a field that won't already be processed.

ACONS05

The AutoConstructorIgnore or AutoConstructorInject are used on a class without the AutoConstructor attribute.

ACONS06

A type specified in AutoConstructorInject attribute does not match the type of another parameter with the same name.

In the following sample, both fields will be injected with guid as parameter name, but one of type string and the other of type Guid, preventing the generator from running.

public partial class Test
{
    [AutoConstructorInject("guid.ToString()", "guid", typeof(Guid))]
    private readonly string _guid2;
    private readonly string _guid;
}

ACONS07

The accessibility defined in the AutoConstructor attribute is not an allowed value.

ACONS08

AutoConstructorInitializer attribute used on multiple methods inside type.

ACONS09

AutoConstructorInitializer attribute used on a method not returning void.

ACONS10

AutoConstructorInitializer attribute used on a method with parameters.

ACONS11

AutoConstructorDefaultBase attribute used on multiple constructors inside type.

ACONS12

The addParameterless option is used on a type whose base type does not have a parameterless constructor.

ACONS99

AutoConstructor_DisableNullChecking is obsolete.