The best way to update a field.

Case1:

var testEntities = context.CreateQuery<new_testentity>().Select(r => new { 
    r.Id,
    r.testfield
});

foreach (var record in testEntities)
{
    var testEntity = context.GetAttachedEntities().First();
    testEntity.testfield = "updated";
    context.UpdateObject(testEntity);
    context.SaveChanges();
}

Case2:

var testEntities = context.CreateQuery<new_testentity>();
foreach (var record in testEntities)
{
    context.Detach(record);
    var testEntity = new testEntity();
    testEntity.testfield = "updated";
    testEntity.Id = record.Id;
    context.Attach(testEntity);
    context.UpdateObject(testEntity);
    context.SaveChanges();
}

Case3:

var testEntities = context.CreateQuery<new_testentity>().ToList();
context.Detach(testEntities);
foreach (var record in testEntities)
{
    var testEntity = new testEntity();
    testEntity.testfield = "updated";
    testEntity.Id = record.Id;
    context.Attach(testEntity);
    context.UpdateObject(testEntity);
    context.SaveChanges();
}

Taichi,Sato