Session object changes when object is updated in C#

Question

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.

Answer 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.