Unable to cast object of type 'System.Collections.Generic.List to type 'System.Data.DataSet'

Question

I am getting an error here in sorting the gridview. My datasource is a var results, which I am getting thru a Linq query

protected void Page_Load(object sender, EventArgs e)
{
    dt1 = obj1.Table1data().Tables[0];
    dt2 = obj1.Table2data().Tables[0];
    dt3 = obj1.Table3data().Tables[0];

    var results = (
        from table1 in dt1.AsEnumerable()
        join table2 in dt2.AsEnumerable() on (int)table1["id"] equals (int)table2["id"]
        join table3 in dt3.AsEnumerable() on (int)table1["id"] equals (int)table3["id"]

        select new
        {
            id = (int)table1["id"],
            S1= (int)table1["S1"],
            P1= (double)table1["P1"],
            P2= (int)table2["P2"],
            P3= (double)table2["P3"],
            P4 = (int)table3["P4"],
            P5= (double)table3["P5"],

        }).ToList();

    Session["ds"] = results;
    GridView1.DataSource = results;
    GridView1.DataBind();
}

protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
{
    DataSet dataSet = (DataSet)Session["ds"];
    DataTable dataTable = dataSet.Tables[0];

    if (dataTable != null)
    {
        DataView dataView = new DataView(dataTable);
        dataView.Sort = e.SortExpression + " " + ConvertSortDirectionToSql(e.SortDirection);

        GridView1.DataSource = dataView;
        GridView1.DataBind();
    }
}

// here in the GridView1_sorting at DataSet dataSet = (DataSet)Session["ds"], I am getting an error

error:

Unable to cast object of type System.Collections.Generic.List`1[<>f__AnonymousType5`8[System.Int32,System.String,System.Int32,System.Double,System.Int32,System.Double,System.Int32,System.Double]]' to type 'System.Data.DataSet'

2) Also another question, What is the datatype of the var results. Thanks Sun

Answer 1

Session["ds"] holds var results , and results is a List<'A> , where 'A is an anonymous type generated by the compiler. You cannot cast this to a DataSet . If you want to put this into session and retrieve it later, declare a proper class, and then you can easily put the list into and out of Session.

What I mean is that your query is building an an anonymous type because of the select statement

 select new 
 {

This is normally fine, but you're trying to use this result beyond the immediate local scope by putting it into session. You need to build a proper class to hold that data. Give it the right properties.

 public class MyData
 {
      // give it the appropriate properties you need
      public int ID { get; set; }
      public int S1 { get; set; }
      public double P1 { get; set; }
      public int P2 { get; set; }
      public double P3 { get; set; }
      public int P4 { get; set; }
      public double P5 { get; set; }
      // by the way... you should really come up with better names 
      // for these properties!
 }

And then make your query

 select new MyData
 {

And when you invoke ToList() and the result, you will have List<MyData> . So when you go to retrieve this from session, this is what you would cast it to.

 var list = (List<MyData>)Session["ds"];