[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
class DontSendInPublicApiAttribute : Attribute { }

public static void RemoveSecretData(object obj)
{
    // Retrieve all public instance properties defined for the object's type and marked with [DontSendInPublicApi]
    var propertiesToHide = obj.GetType()
        .GetProperties(BindingFlags.Instance | BindingFlags.Public)
        .Where(p => p.GetCustomAttribute<DontSendInPublicApiAttribute>() != null);

    foreach (var prop in propertiesToHide)
    {
        // Set all of these properties in the given object to their default values.
        // VALUE TYPES (ints, chars, doubles, etc.) will be set to default(TheTypeOfValue), by calling Activator.CreateInstance(TheTypeOfValue).
        // REFERENCE TYPES will simply be set to null.
        var propertyType = prop.PropertyType;
        if (propertyType.IsValueType)
            prop.SetValue(obj, Activator.CreateInstance(prop.PropertyType));
        else
            prop.SetValue(obj, null);
    }
}

class Person
{
    public string Name { get; set; }
    public int Age { get; set; }

    [DontSendInPublicApi]
    public string Occupation { get; set; }
    [DontSendInPublicApi]
    public int Salary { get; set; }
}

var person = new Person() { Name = "John", Age = 29, Occupation = "Engineer", Salary = 200000 };
RemoveSecretData(person);