您可以为类型写入custom converter或使用.NET 7 approach with customizing contract：

var foos = new[] { new Foo { Age = 1, Name = "Should not show" }, new Foo { Age = 19, Name = "Name" } };

var result = JsonSerializer.Serialize(foos, new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers = { SkipUnderagedName }
    }
});

Console.WriteLine(result); // prints "// [{"Age":1},{"Name":"Name","Age":19}]"

static void SkipUnderagedName(JsonTypeInfo ti)
{
    if (ti.Type.IsAssignableTo(typeof(Foo)))
    {
        var prop = ti.Properties.FirstOrDefault(p => p.Name == nameof(Foo.Name));
        if (prop is not null)
        {
            prop.ShouldSerialize = (obj, _) =>
            {
                var foo = obj as Foo;
                return foo?.Age >= 18;
            };
        }
    }
}

既然你不需要任何自定义的序列化器，你可以试试这段代码。如果年龄小于19岁，它返回Name属性值等于null。当Name为null时，序列化器忽略这个属性。

public class Foo
{
    private string _name;

    [System.Text.Json.Serialization.JsonPropertyName("Name")]
    [System.Text.Json.Serialization.JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public string _Name
    {
        get { return Age > 18 ? _name : null; }
        set { _name = value; }
    }

    [System.Text.Json.Serialization.JsonIgnore]
    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }
    public int Age { get; set; }
}