Can a C# attribute be serialized into JSON?

Question

I've made a class from the Attribute type

   public class DemoAttribute : Attribute {
    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public string Label { get; private set; }

    public DemoAttribute(string label = null) {
        this.Label = label;
    }
}

When I try to serialize it with System.Text.Json

 var demo = new DemoAttribute("test");
 var json = JsonSerializer.Serialize(demo);

I get an InvalidOperationException:

Method may only be called on a Type for which Type.IsGenericParameter is true.

Can I serialize an Attribute without first copying it's properties to a 'regular' class with the same propeties?

Edit/Addition I'm using a far more extensive attribute with metadata on a property like (on the front end) what's the label, help-text, icon, validation rules, placeholder, etc. With reflection I get the attribute for the properties and I want to serialize it (the properties of the attribute) so I can send it to the front end.

Answer 1

Attribute has TypeId property which by default contains type of the attribute (see the remarks in docs), which fails during serialization when using System.Text.Json . You can override this property and ignore it:

public class DemoAttribute : Attribute
{
    [JsonIgnore]
    public override object TypeId { get; }

    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public string Label { get; private set; }

    public DemoAttribute(string label = null)
    {
        this.Label = label;
    }
}