ASP.NET LINQ（C＃）更新语法

Question

I am using LINQ to Entities in an ASP.NET C# app. The below code is my attempt to update (or reset, really) a value 'AvgRideTime' in my sql db to null. However it seems to do nothing, no errors, just nothing. Any thoughts?

protected void ResetAvgRT_Click(object sender, EventArgs e)
        {
            using (RamRideOpsEntities myEntities = new RamRideOpsEntities())
            {
                var avgTime = (from a in myEntities.AdminOptions
                               select a.AvgRideTime).First();

                if (avgTime != null)
                {
                    avgTime = null;
                    myEntities.SaveChanges();
                }
            }
        }

Answer 1

var avgTime = myEntities.AdminOptions.First();

            if (avgTime != null)
            {
                avgTime.AvgRideTime = null;
                myEntities.SaveChanges();
            }

Try the above code.

Answer 2

avgTime这是对象的链接，当你设置avgTime = null时，avgTime没有引用对象，但myEntities.AdminOptions包含这个对象。

Answer 3

也许avgTime得到一个空值。

Answer 4

Try the following code

protected void ResetAvgRT_Click(object sender, EventArgs e)
{
    using (RamRideOpsEntities myEntities = new RamRideOpsEntities())
    {
        var avgTime = (from a in myEntities.AdminOptions
                           select a).First();
        if (a.avgTime != null)
        {
            a.avgTime = null;
            myEntities.Entry(a).State = EntityState.Modified;
            myEntities.SaveChanges();
        }
    }
}