Support for appending to an existing target object

[DedAnton]

Discussed in #150

^{Originally posted by TallBaldGeek August 15, 2023}
I was testing out NextGenMapper to see if it could replace some reflection-based mapping that we're currently doing in our application. It looks like a promising fit, but we have many use cases where we need to map several source objects to a single target object.

For example, if the target comprised of data from multiple sources as follows:

record SourcePhone(string PhoneNumber, string PhoneType);
record SourceName(string FirstName, string LastName);
record CustomerDto(string PhoneNumber, string PhoneType, string FirstName, string LastName);

It would be helpful to be able to build a complete CustomerDto by combining data from several sources:

var phone = new SourcePhone("5556667777", "Mobile");
var name = new SourceName("First", "Last");
var customer = phone.Map<CustomerDto>();
customer.MapAppend(name);

//or possibly in a fluent syntax?
var customer = phone.Map<CustomerDto>().Append(name);

This feature would be similar to Mapperly's support for using an existing target object.

[DedAnton]

another one variant

var customer = new CustomerDto("", "", "", "");
phone.Map(ref customer);
name.Map(ref customer);

record SourcePhone(string PhoneNumber, string PhoneType);
record SourceName(string FirstName, string LastName);
record CustomerDto(string PhoneNumber, string PhoneType, string FirstName, string LastName);

internal static partial class Mapper
{
    internal static void Map(this SourcePhone source, ref CustomerDto target)
    {
        target = target with
        {
            PhoneNumber = source.PhoneNumber,
            PhoneType = source.PhoneType
        };
    }

    internal static void Map(this SourceName source, ref CustomerDto target)
    {
        target = target with
        {
            FirstName = source.FirstName,
            LastName = source.LastName
        };
    }
}