如Renat所说，删除:db.Books.Attach(book);

另外，将结果查询改为使用“AsNoTracking”，因为该查询会抛出实体框架的模型状态。它认为“结果”是现在要追踪的书，而你不想要那个。

var result = db.Books.AsNoTracking().SingleOrDefault(b => b.BookNumber == bookNumber);

你应该删除db.Books.Attach(book);

你可以使用AddOrUpdate方法:

db.Books.AddOrUpdate(book); //requires using System.Data.Entity.Migrations;
db.SaveChanges();

当尝试使用Attach()和SaveChanges()组合更新记录时，我也有同样的问题，但我使用的是SQLite DB及其EF提供者(相同的代码在SQLServer DB中工作没有问题)。

I found out, when your DB column has GUID (or UniqueIdentity) in SQLite and your model is nvarchar, SQLIte EF treats it as Binary(i.e., byte[]) by default. So when SQLite EF provider tries to convert GUID into the model (string in my case) it will fail as it will convert to byte[]. The fix is to tell the SQLite EF to treat GUID as TEXT (and therefore conversion is into strings, not byte[]) by defining "BinaryGUID=false;" in the connectionstring (or metadata, if you're using database first) like so:

  <connectionStrings>
    <add name="Entities" connectionString="metadata=res://savetyping...=System.Data.SQLite.EF6;provider connection string=&quot;data source=C:\...\db.sqlite3;Version=3;BinaryGUID=false;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
  </connectionStrings>

链接到对我有效的解决方案: SQLite实体框架6提供者如何处理指南?

Not related to this specific example, but I came across a challenge when trying to use EF and a DateTime field as the concurrency check field. It appears the EF concurrency code doesn't honor the precision setting from the metadata (edmx) i.e. Type="DateTime" Precision="3". The database datetime field will store a millisecond component within the field (i.e. 2020-10-18 15:49:02.123). Even if you set the original value of the Entity to a DateTime that includes the millisecond component, the SQL EF generates is this:

UPDATE [dbo].[People]
SET [dateUpdated] = @0
WHERE (([PeopleID] = @1) AND ([dateUpdated] = @2))
-- @0: '10/19/2020 1:07:00 AM' (Type = DateTime2)
-- @1: '3182' (Type = Int32)
-- @2: '10/19/2020 1:06:10 AM' (Type = DateTime2)

正如您所看到的，@2是一个没有毫秒组件的STRING表示。这将导致更新失败。

因此，如果您打算使用DateTime字段作为并发键，那么在检索记录时必须从数据库字段中剥离毫秒/ tick，并且仅使用类似剥离的DateTime传递/更新字段。

    //strip milliseconds due to EF concurrency handling
    PeopleModel p = db.people.Where(x => x.PeopleID = id);
    if (p.dateUpdated.Millisecond > 0)
    {
        DateTime d = new DateTime(p.dateUpdated.Ticks / 10000000 * 10000000);
        object[] b = {p.PeopleID, d};
        int upd = db.Database.ExecuteSqlCommand("Update People set dateUpdated=@p1 where peopleId=@p0", b);
        if (upd == 1)
            p.dateUpdated = d;
        else
            return InternalServerError(new Exception("Unable to update dateUpdated"));
    }
return Ok(p);

当用新值更新字段时，也去掉毫秒

(param)int id, PeopleModel person;
People tbl = db.People.Where(x => x.PeopleID == id).FirstOrDefault();
db.Entry(tbl).OriginalValues["dateUpdated"] = person.dateUpdated;
//strip milliseconds from dateUpdated since EF doesn't preserve them
tbl.dateUpdated = new DateTime(DateTime.Now.Ticks / 10000000 * 10000000);

我一直在审查实体框架的源代码，并找到了一种方法来实际更新一个实体，如果你知道Key属性:

public void Update<T>(T item) where T: Entity
{
    // assume Entity base class have an Id property for all items
    var entity = _collection.Find(item.Id);
    if (entity == null)
    {
        return;
    }

    _context.Entry(entity).CurrentValues.SetValues(item);
}

否则，请检查AddOrUpdate实现以获得想法。

希望这对你有所帮助!