Question

I have two functions that execute two ajax calls. If I call one function and not the other
the code executes normally. But if I call both of them at the same time I get two Internal Server errors messages. One for each function I presume.

This is my code:

   $(document).ready(function(){
         CategoryChangeState(@Model.CatId , subcategoryId);
         SubategoryChangeState(@Model.SubcatId);
    })
    public ActionResult ReturnListOfSubcategories( FormCollection collection ) {
            string categoryId = collection["result"];

            var subcategories = ProductManagerHelperClass.ReturnSubcategories(categoryId);
            return Json(subcategories);
    }
    public ActionResult ReturnListOfBrands() {

          var brands = ProductManagerHelperClass.ReturnBrands();
          return Json(brands);
    }
function CategoryChangeState(value ,  editPage) {
        .....
        $.ajax({
            type: "POST",
            url: "/ProductManager/ReturnListOfSubcategories",
            data: { result: value },
            datatype: "json",
            error: function (jqXHR, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });
        ...
}

function SubategoryChangeState(value) {
         ....
         $.ajax({
            type: "POST",
            url: "/ProductManager/ReturnListOfBrands",
            datatype: "json",
            error: function (jqXHR, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });
       ....
}

Why am I getting these errors and how can I solve it?

EDIT

While debugging I discovered that in this portion of code:

 public static Dictionary<string , string> ReturnSubcategories(string categoryId)
    {
        int catId = int.Parse(categoryId);

        var subcategories = (from s in dataContext.SubCategories
                             where s.CatId == catId
                             select new
                                        {
                                            s.SubCatId,
                                            s.SubCatName
                                        }).ToDictionary(x => x.SubCatId.ToString(), x => x.SubCatName);

        return subcategories;
    } 

The linq query throws an exception:

InvalidOperationException ExecuteReader requires an open and available Connection. The connection's current state is closed.

Again this exception is only thrown if I call both functions

Answer 1

That’s because you use a static class, in which you (probably) make a connection to your database.
The following happens:

1. Thread one enters your static class, a connection is opened.
2. In the meanwhile thread two enters the same class, and uses the same open connection.
3. When thread one is done, it closes the open connection.
4. Thread two throws an error because there is no open connection.

This won’t happen if you don’t use a static class to instantiate database connections.