datacontext会自动跟踪实体状态,当我们调用savechanges保存实体到数据库时,会用到跟踪状态.
但是某些情况下,实体 不在datacontext中,他们没有连接关系,比如:一个datacontext查询,别一个datacontext保存,这种情况下保存的datacontext不知道查询出来的这些实体的状态.
添加新的新实体到datacontext有两种方法
using(var context = new BookStore()) { Author author = new Author() { //code here }; context.Authors.Add(author); Console.WriteLine("Entity State: {0}", context.Entry(author).State); context.SaveChanges(); }
或
using(var context = new BookStore()) { Author author = new Author() { //code here }; context.Entry(author).State = EntityState.Added; Console.WriteLine("Entity State: {0}", context.Entry(author).State); context.SaveChanges(); }
添加已存在的实体到datacontext也有两种写法
using(var context = new BookStore()) { context.Authors.Attach(existingAuthor); Console.WriteLine("Entity State: {0}", context.Entry(existingAuthor).State); context.SaveChanges(); }
或
using(var context = new BookStore()) { context.Entry(existingAuthor).State = EntityState.Unchanged;//这里也可以修改为modified ,看情况 Console.WriteLine("Entity State: {0}", context.Entry(existingAuthor).State); context.SaveChanges(); }
ef检测是否存在数据库的方法
public void InsertOrUpdate(Author author) { using (var context = new BookStore()) { context.Entry(author).State = author.AuthorId == 0 ? EntityState.Added : EntityState.Modified; context.SaveChanges(); } }
原文：https://www.cnblogs.com/nocanstillbb/p/11494829.html