Treat constructor arguments as required properties

[krwq]

Compiler required keyword semantics

private class Test
{
    public required string Name { get; } // error

    public Test(string name)
    {
        Name = name;
    }
}

don't allow to compile above code because property doesn't have a setter - this makes sense because argument needs to be passed in the constructor anyway making required effectively redundant.

On the other hand when deserializing JSON Name does not have to be passed in the payload even though the only way to construct is through constructor - we will use default value (i.e. if value was int we'd pass 0 or null for string for you automatically):

var obj = System.Text.Json.JsonSerializer.Deserialize<Test>("{}"); // no error

public sealed class Test
{
    public string Name { get; }

    public Test(string name)
    {
        Name = name;
    }
}

Consider changing semantics here and make Name required instead making above code create error. Possibly there should be an option switch to opt-in to suggested behavior given this change will be breaking.

Example API suggestion (possibly consider enum and more generic name in case we need more related options in the future):

public partial class JsonSerializerOptions
{
    public bool RequireConstructorParameters { get; set; }
}

Related: #78098

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Compiler required keyword semantics

private class Test
{
    public required string Name { get; } // error

    public Test(string name)
    {
        Name = name;
    }
}

don't allow to compile above code because property doesn't have a setter - this makes sense because argument needs to be passed in the constructor anyway making required effectively redundant.

On the other hand when deserializing JSON Name does not have to be passed in the payload even though the only way to construct is through constructor - we will use default value (i.e. if value was int we'd pass 0 or null for string for you automatically):

var obj = System.Text.Json.JsonSerializer.Deserialize<Test>("{}"); // no error

public sealed class Test
{
    public string Name { get; }

    public Test(string name)
    {
        Name = name;
    }
}

Consider changing semantics here and make Name required instead making above code create error. Possibly there should be an option switch to opt-in to suggested behavior given this change will be breaking.

Example API suggestion (possibly consider enum and more generic name in case we need more related options in the future):

public partial class JsonSerializerOptions
{
    public RequireConstructorParameters { get; set; }
}

Related: #78098

Author:	krwq
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json
Milestone:	-

[krwq]

alternative design for that would be to provide Modifier for contract resolver but it's currently tricky to implement as a separate library because we don't have parametrized ctor APIs available yet (so basically user has to repeat JsonConstructor logic).

[eiriktsarpalis]

Proposal looks good to me. We should also include provision for configuring requiredness on the type level:

public sealed class Test
{
    [JsonConstructor(RequireParameters = true)]
    public Test(int p1, int p2, int p3, int p4, ...)
    {
    }
}

and at the individual parameter level:

public sealed class Test
{
    public Test(int p1, [JsonRequired] int p2, int p3, int p4, ...)
    {
    }
}

[krwq]

Extra overload of JsonConstructorAttribute sound good but for JsonRequired - we put [JsonConverter(...)] and [JsonNumberHandling(...)] directly on the property so I'm not sure why we'd do it differently for [JsonRequired]...

[krwq]

marking as 8.0 because we should look at this in conjunction with other 8.0 items.