SaveChanges not setting value, selecting existing value instead - asp.net-mvc

Question

When trying to commit a modified entry to the database, Entity Framework is selecting values for two properties as opposed to setting them.
The code to commit the entity to the database is as follows:
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Edit(Till till)
{
if (ModelState.IsValid)
{
string username = User.Identity.Name.Substring(User.Identity.Name.LastIndexOf('\\') + 1);
till.ModifiedDate = DateTime.Now;
till.ModifiedByUid_FK = db.Users.Where(m => m.Name.ToLower() == username.ToLower()).First().UserUid_PK;
db.Entry(till).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("Index/" + (int)Session["siteid"]);
}
ViewBag.SiteUid_FK = new SelectList(db.Sites, "SiteUid_PK", "Name", till.SiteUid_FK);
ViewBag.ModifiedByUid_FK = new SelectList(db.Users, "UserUid_PK", "EHLogin", till.ModifiedByUid_FK);
ViewBag.CreatedByUid_FK = new SelectList(db.Users, "UserUid_PK", "EHLogin", till.CreatedByUid_FK);
return View(till);
}
When viewed in the profiler, it looks like this:
exec sp_executesql N'update [dbo].[Till]
set [NetworkId] = #0, [MID] = #1, [SiteUid_FK] = #2, [Location] = null, [MasterTillNo] = #3,
[IsMaster] = #4, [IsBackOffice] = #5, [IsCurrent] = #6, [ModifiedByUid_FK] = #7,
[CreatedByUid_FK] = #8, [ProcessFiles] = #9
where ([TillUid_PK] = #10)
select [ModifiedDate], [CreatedDate]
from [dbo].[Till]
where ##ROWCOUNT > 0 and [TillUid_PK] = #10',N'#0 varchar(50),#1 int,#2 int,#3 int,#4 bit,#5 bit,#6
bit,#7 int,#8 int,#9 bit,#10 int',
#0='',#1='',#2=4,#3=42,#4=1,#5=0,#6=1,#7=4,#8=1,#9=1,#10=699
I'm guessing there's some property of the field in the database that EF uses to determine this functionality but I can't figure out what it is. If anyone has any information that'd be fantastic :)
Edit 1
Using Ibrahim's answer below, results in relatively similar SQL:
exec sp_executesql N'declare #p int
update [dbo].[Till]
set #p = 0
where ([TillUid_PK] = #0)
select [ModifiedDate], [CreatedDate]
from [dbo].[Till]
where ##ROWCOUNT > 0 and [TillUid_PK] = #0',N'#0 int',#0=687

Answer 1

This typically happens when properties have the data annotation
[DatabaseGenerated(DatabaseGeneratedOption.Computed)]
or the fluent mapping
.HasDatabaseGeneratedOption(DatabaseGeneratedOption.Computed)
or the StoreGeneratedPattern is Computed in the edmx file.
I'd guess that there is a trigger or a default value on ModifiedDate and CreatedDate on the columns in the database. Maybe it's even useless to set them in your application code.

Answer 2

I have had bad luck with the following also:
db.Entry(entity).State = EntityState.Modified;
So I suggest trying to do the following:
//Get the existing Till
var existingTill = db.Tills.Find(till.TillId);
//Save changes for only changed properties
db.Entry(existingTill).CurrentValues.SetValues(till);
db.SaveChanges();