As.cs

void Main()
{
	Mapper.Initialize(cfg =>
	{
		cfg.CreateMap<Source, Destination>();
		cfg.CreateMap<Source, IDestination>().As<Destination>();
	});

	Mapper.AssertConfigurationIsValid();
	
	Mapper.Map<IDestination>(new Source { Value = Guid.NewGuid().ToString(), TypedValue = Guid.NewGuid() }).Dump();
}

class Source
{
	public string Value;
	public Guid? TypedValue;
}

class Destination : IDestination
{
	public Guid? Value { get; set; }
	public string TypedValue { get; set; }
}

interface IDestination
{
	Guid? Value { get; set; }
	string TypedValue { get; set; }
}