Session object changes when object is updated in C

问题:

I have this really weird problem and I'm sure I'm missing something obvious here. I have these two lines:

HttpContext.Current.Session[listModelType + "ListModel"] = listModel;
listModel.ProductRows = new Collection<ProductRow>(listModel.ProductRows.Where(r => r.ParentRowId == 0).ToList());

After the second line is executed my session object is updated as well (according to "Watch" in Visual Studio)

What am I missing here?

I have tried

int i = 0;
HttpContext.Current.Session["i"] = i;
i++;

and HttpContext.Current.Session["i"] remains 0.

回答1:

See value types and reference types.

The int is a value type so will be stored "as-is" on the moment of assignment; your listModel is a reference type so you store a reference to the object in your session, not the value of the object.

You'll have to create a new instance of listModel if you want the one in your session untouched.

回答2:

In your first example you are storing a reference to the object (The lists memory location). So if the list is updated it will reflect in the session. This is a reference type.

In the second example you are using a value type:

int i = 0;
HttpContext.Current.Session["i"] = i;
i++;

You declare i and set it to 0 (Value type)

You store the value 0 in the session. (Not the memory location of i)

You increment i but the session still has the value 0

回答3:

In the first example your session variable is pointing to a reference so it gets updated because the two references are pointing to the same value.

The second session variable is pointing to a primitive (value) type so they have separate copies of value.

回答4:

In the first example your session variable is pointing to a reference so it gets updated because the two references are pointing to the same value.

so before assigning to the session you convert it into Json and then assign

HttpContext.Current.Session[listModelType + "ListModel"] = JsonConvert.SerializeObject(listModel);

Note: JsonConvert is from name space Newtonsoft.Json namespace in c#

In second line if value changes in listModel object that does't reflect session. but when you want to retreive the value from session you should convert to object form Json

if (HttpContext.Current.Session[listModelType + "ListModel"] != null)
{

    listModel = JsonConvert.DeserializeObject<*CLASS name of lsitmodel*>((string)HttpContext.Current.Session[listModelType + "ListModel"]);

}

回答5:

The correct way should be:

  int i = 0;
  i++;
  HttpContext.Current.Session["i"] = i;

the HttpContext.Current.Session["i"] remains 1.

回答6:

In the first example your session variable is pointing to a reference so it gets updated because the two references are pointing to the same value.

so before assigning to the session you convert it into Json and then assign

   HttpContext.Current.Session[listModelType + "ListModel"] = JsonConvert.SerializeObject(listModel);

Note: JsonConvert is from name space Newtonsoft.Json namespace in c#

In second line if value changes in listModel object that does't reflect session. but when you want to retreive the value from session you should convert to object form Json

   if (HttpContext.Current.Session[listModelType + "ListModel"] != null) {

        listModel = JsonConvert.DeserializeObject<*CLASS name of lsitmodel*>((string)HttpContext.Current.Session[listModelType + "ListModel"]);

    }