View takes values from query string instead of model - asp.net-mvc-4

Question

I have an action in my controller:
public PartialViewResult MyAction(int? myId, int? myId2)
{
MyModel model = new MyModel() { MyId = 10, MyId2 = 20 }
return PartialView(model);
}
Here is my view:
#model StartSite.Models.Shared.MyModel
#using (Html.BeginForm())
{
#Html.HiddenFor(m => m.MyId)
#Html.HiddenFor(m => m.MyId2)
<p>
<input type="submit" value="Submin" />
</p>
}
Lets call MyAction with params myId=1&myId2=2. But the model is created with different values new MyModel() { MyId = 10, MyId2 = 20 }. And what should be rendered in view? As I expect it should be:
<input id="MyId" name="MyId" type="hidden" value="10">
<input id="MyId2" name="MyId2" type="hidden" value="20">
But in fact the result is:
<input id="MyId" name="MyId" type="hidden" value="1">
<input id="MyId2" name="MyId2" type="hidden" value="2">
As I guess the Html.HiddenFor takes values not from my model but from Reauest.QueryString which is myId=1&myId2=2 at the moment the view is rendered.
Why it happens? Is it expected behaviour?
UPDATE 1:
I've edited my question to be more clear.

Answer 1

to have access to the Model in the submit try with this
[HttpPost]
public virtual PartialViewResult MyAction(MyModel model)
{
//MyModel model = new MyModel();
// if (myId != null)
// model.MyId= myId;
// else if (myId2 != null)
// model.MyId2= myId2;
//now you have access to your model values
return PartialView(model);
}

Answer 2

This is expected behavior. Firstly, an Action without any Attributes is automatically a HttpGet. Next, your Action expects a value for 'myId'. This means, if the url calling your Action has a querystring that matches, it'll accept the value. Finally, the value your Action accepts is case-insensitive. Therefore, you do not need to manually set your model values. You can simply do this:
public virtual PartialViewResult MyAction(MyModel model)
{
return PartialView(model);
}
So when you go to your url, e.g. localhost/myaction?myId=2, model.MyId will be set to 2.
If you do NOT want your model set by a querystring, you must change your Action to not accept any values.