BuilderExample.cs

//externally exposed data object
public partial class Person
{
	public string FirstName { get; private set; }
	public string LastName { get; private set; }
}


public partial class Person
{
	internal struct Builder
	{
		private Person _instance;

		public Builder(Person instance)
		{
			_instance = instance;
		}

		public string FirstName
		{
			get => _instance.FirstName;
			set => _instance.FirstName = value;
		}

		public string LastName
		{
			get => _instance.LastName;
			set => _instance.LastName = value;
		}
	}
}


void EditPerson(Person p)
{
	//using the builder

	Person.Builder builder(p);

	builder.FirstName = "foo";
	builder.LastName = "bar";
}