Automapper reusing the same instance

gistfile1.txt

void Main()
{
  Mapper.CreateMap<SourceTest, DestTest>()
	    .ForMember(x=>x.CommonIgnoredValue, opt=>opt.Ignore())	
	    .IgnoreAllNonExisting();
	
	var source = new SourceTest{ CommonValue = "FromSource", CommonIgnoredValue = "Ignored Value from Source"};
	var dest = new DestTest { CommonValue = "FromDest", CommonIgnoredValue = "Ignored Value from Dest", DestOnlyValue = "Value Only In Dest" };
	
	Mapper.Map(source, dest);
	
	// Outputs:
        //          CommonValue = FromSource	
	//          CommonIgnoredValue = Ignored Value from Dest
        //	    DestValueOnly = Value only in Dest
	Console.WriteLine(dest);	
}

public class SourceTest
{
	public string CommonValue {get; set;}
	public string CommonIgnoredValue {get; set;}
}

public class DestTest
{
	public string CommonValue {get; set;}
	public string CommonIgnoredValue {get; set;}
	public string DestOnlyValue {get; set;}
}

public static class AutoMapperExtensions
{
   // Ignore any unmapped items that are in the destination.
   public static IMappingExpression<TSource, TDestination> IgnoreAllNonExisting<TSource, TDestination>(this IMappingExpression<TSource, TDestination> expression)
   {
       var sourceType = typeof(TSource);
       var destinationType = typeof(TDestination);
       var existingMaps = Mapper.GetAllTypeMaps().First(x => x.SourceType.Equals(sourceType) && x.DestinationType.Equals(destinationType));
       foreach (var property in existingMaps.GetUnmappedPropertyNames())
       {
           expression.ForMember(property, opt => opt.Ignore());
       }
       return expression;
   }
}