How do i clear session?

Question

I'm working with entity framework and right now I'm saving two assignments and want to show them to my gridview.

Here I'm saving them to my database table and show them in the gridview:

The code here:

protected void ButtonAddAssignmentClick(object sender, EventArgs e)
{
    Session.Remove("DataTable");

//Add to DB and show in gridview
using (var db = new KnowItCvdbEntities())
  {
      SPWeb theSite = SPControl.GetContextWeb(Context);
      SPUser theUser = theSite.CurrentUser;
      string strUserName = theUser.LoginName;
       var theEmplAssignment = (
                                  from p
                                  in db.EMPLOYEES
                                  where p.username == strUserName
                                  select p).FirstOrDefault();

      _emp = theEmplAssignment;

      if (_emp != null)
      {
          //Create assignment
          var myAssignment = new EMPLOYEES_ASSIGNMENT
          {
              assignment_id = new Random().Next(),
              employee_id = _emp.employee_id,

              reference_name = TextBoxReference.Text,
              company_name = TextBoxCompanyName.Text,
              sector = TextBoxSector.Text,
              area = TextBoxArea.Text,
              from_date = TextBoxFromDate.Text,
              to_date = TextBoxToDate.Text,
              description = TextBoxDesc.Text,
          };

          //Create assignment tools
          for (int i = 0; i < ListBoxAssignmentTools.Items.Count; i++)
          {
              var myTool = new ASSIGNMENT_TOOLS
              {
                  assignment_tools_id = new Random().Next(),
                  assignment_id = myAssignment.assignment_id,
                  employee_id = myAssignment.employee_id,

                  tool_name = ListBoxAssignmentTools.Items[i].ToString()
              };

              myAssignment.ASSIGNMENT_TOOLS.Add(myTool);
          }

          //Create assignment technology
          for (int i = 0; i < ListBoxAssignmentTechnology.Items.Count; i++)
          {
              var myTech = new ASSIGNMENT_TECHNOLOGY
              {
                  assignment_technology_id = new Random().Next(),
                  assignment_id = myAssignment.assignment_id,
                  employee_id = myAssignment.employee_id,

                  technology_name = ListBoxAssignmentTechnology.Items[i].ToString()
              };

              myAssignment.ASSIGNMENT_TECHNOLOGY.Add(myTech);
          }

          //Add assignment to db                   
          _emp.EMPLOYEES_ASSIGNMENT.Add(myAssignment);
          db.SaveChanges();

          //Populate gridview  
          var dt = new DataTable();
          if (Session["DataTable"] != null)
          {
              dt = (DataTable)Session["DataTable"];
          }
          else
          {
              dt.Columns.Add("Company name");
              dt.Columns.Add("Sector");
              dt.Columns.Add("Area");
              dt.Columns.Add("From");
              dt.Columns.Add("To");
              dt.Columns.Add("Tools");
              dt.Columns.Add("Technology");
              dt.Columns.Add("Description");
              dt.Columns.Add("Reference");
              dt.Rows.Clear();
          }

          DataRow dr = dt.NewRow();
          dr["Company name"] = TextBoxCompanyName.Text;
          dr["Sector"] = TextBoxSector.Text;
          dr["Area"] = TextBoxArea.Text;
          dr["From"] = TextBoxFromDate.Text;
          dr["To"] = TextBoxToDate.Text;
          dr["Description"] = TextBoxDesc.Text;
          dr["Reference"] = TextBoxReference.Text;

          string sToolsValue = string.Empty;
          for (int i = 0; i < ListBoxAssignmentTools.Items.Count; i++)
          {
              sToolsValue += ListBoxAssignmentTools.Items[i] + " ";
          }
          dr["Tools"] = sToolsValue;

          string sTechValue = string.Empty;
          for (int i = 0; i < ListBoxAssignmentTechnology.Items.Count; i++)
          {
               sTechValue += ListBoxAssignmentTechnology.Items[i] + " ";
          }
          dr["Technology"] = sTechValue;

          dt.Rows.Add(dr);
          Session["DataTable"] = dt;

          //Add to gridview
          GridViewShowAssignments.DataSource = dt;
          GridViewShowAssignments.DataBind();

          TextBoxCompanyName.Text = string.Empty;
          TextBoxArea.Text = string.Empty;
          TextBoxSector.Text = string.Empty;
          TextBoxFromDate.Text = string.Empty;
          TextBoxToDate.Text = string.Empty;
          TextBoxDesc.Text = string.Empty;
          TextBoxReference.Text = string.Empty;
          ListBoxAssignmentTools.Items.Clear();
          ListBoxAssignmentTechnology.Items.Clear();
      }                
  }
}

And I'm using a method on page load that retrieves all assignments on the current logged in user and showing it on the gridview. But the gridview is populating duplicates! I suspect that I must clear my session on page load but I don't really know how to do it.

The method code:

//Get assignment from db and populate gridview
private void GetEmployeeAssignment(EMPLOYEE theEmpl)
{
    Session.Remove("DataTable");

    using (var db = new KnowItCvdbEntities())
    {
        if (_emp != null)
        {
            var assignmentList = from p in db.EMPLOYEES_ASSIGNMENT.AsEnumerable()
                                 join at in db.ASSIGNMENT_TOOLS.AsEnumerable() on p.assignment_id equals at.assignment_id
                                 join ate in db.ASSIGNMENT_TECHNOLOGY.AsEnumerable() on p.assignment_id equals ate.assignment_id
                                 where p.employee_id == theEmpl.employee_id
                                 select new EmployeeAssignmentInfo
                                 {
                                     CompanyName = p.company_name,
                                     AssignmentId = p.assignment_id,
                                     Area = p.area,
                                     From = p.from_date,
                                     To = p.to_date,
                                     Description = p.description,
                                     Sector = p.sector,
                                     Reference = p.reference_name,

                                     ToolName = at.tool_name,
                                     AssignmentToolsId = at.assignment_tools_id,
                                     TechnologyName = ate.technology_name,
                                     AssignmentTechnologyId = ate.assignment_technology_id
                                 };

            foreach (var vAssignment in assignmentList)
            {
                //Populate gridview  
                var dt = new DataTable();
                if (Session["DataTable"] != null)
                {
                    dt = (DataTable)Session["DataTable"];
                }
                else
                {
                    dt.Columns.Add("Company name");
                    dt.Columns.Add("Sector");
                    dt.Columns.Add("Area");
                    dt.Columns.Add("From");
                    dt.Columns.Add("To");
                    dt.Columns.Add("Tools");
                    dt.Columns.Add("Technology");
                    dt.Columns.Add("Description");
                    dt.Columns.Add("Reference");
                    dt.Rows.Clear();
                }

                DataRow dr = dt.NewRow();
                dr["Company name"] = vAssignment.CompanyName;
                dr["Sector"] = vAssignment.Sector;
                dr["Area"] = vAssignment.Area;
                dr["From"] = vAssignment.From;
                dr["To"] = vAssignment.To;
                dr["Description"] = vAssignment.Description;
                dr["Reference"] = vAssignment.Reference;
                dr["Tools"] = vAssignment.ToolName + " ";
                dr["Technology"] = vAssignment.TechnologyName + " ";

                dt.Rows.Add(dr);
                Session["DataTable"] = dt;

                GridViewShowAssignments.DataSource = dt;
                GridViewShowAssignments.DataBind();
            }
        }
        else
        {
            LabelPleaseRegister.Visible = true;
            LabelPleaseRegister.Text = "Please register your personal information";
            PanelRegisterCv.Visible = false;
            PanelRegisterPersonalInfo.Visible = false;
        }
    }
}

Page load:

        protected void Page_Load(object sender, EventArgs e)
    {
        SPWeb theSite = SPControl.GetContextWeb(Context);
        SPUser theUser = theSite.CurrentUser;
        string strUserName = theUser.LoginName;

        LabelUsername.Text = strUserName;

        if (!IsPostBack)
        {
            _emp = GetEmployee(strUserName);                

            GetEmployeeAssignment(_emp);
        }
    }

Answer 1

Just do

Session["DataTable"] = null; 

or

Session.Remove("DataTable")

Update your code like:

 //Get assignment from db and populate gridview
 private void GetEmployeeAssignment(EMPLOYEE theEmpl)
  {
Session["DataTable"]=null;

using (var db = new KnowItCvdbEntities())
{
    if (_emp != null)
    {
        var assignmentList = from p in db.EMPLOYEES_ASSIGNMENT.AsEnumerable()
                             join at in db.ASSIGNMENT_TOOLS.AsEnumerable() on p.assignment_id equals at.assignment_id
                             join ate in db.ASSIGNMENT_TECHNOLOGY.AsEnumerable() on p.assignment_id equals ate.assignment_id
                             where p.employee_id == theEmpl.employee_id
                             select new EmployeeAssignmentInfo
                             {
                                 CompanyName = p.company_name,
                                 AssignmentId = p.assignment_id,
                                 Area = p.area,
                                 From = p.from_date,
                                 To = p.to_date,
                                 Description = p.description,
                                 Sector = p.sector,
                                 Reference = p.reference_name,

                                 ToolName = at.tool_name,
                                 AssignmentToolsId = at.assignment_tools_id,
                                 TechnologyName = ate.technology_name,
                                 AssignmentTechnologyId = ate.assignment_technology_id
                             };
      var dt = new DataTable();

        foreach (var vAssignment in assignmentList)
        {
            //Populate gridview  

            if (Session["DataTable"] != null)
            {
                dt = (DataTable)Session["DataTable"];
            }
            else
            {
                dt.Columns.Add("Company name");
                dt.Columns.Add("Sector");
                dt.Columns.Add("Area");
                dt.Columns.Add("From");
                dt.Columns.Add("To");
                dt.Columns.Add("Tools");
                dt.Columns.Add("Technology");
                dt.Columns.Add("Description");
                dt.Columns.Add("Reference");
                dt.Rows.Clear();
            }

            DataRow dr = dt.NewRow();
            dr["Company name"] = vAssignment.CompanyName;
            dr["Sector"] = vAssignment.Sector;
            dr["Area"] = vAssignment.Area;
            dr["From"] = vAssignment.From;
            dr["To"] = vAssignment.To;
            dr["Description"] = vAssignment.Description;
            dr["Reference"] = vAssignment.Reference;
            dr["Tools"] = vAssignment.ToolName + " ";
            dr["Technology"] = vAssignment.TechnologyName + " ";

            dt.Rows.Add(dr);

        }

            Session["DataTable"] = dt;

            GridViewShowAssignments.DataSource = dt;
            GridViewShowAssignments.DataBind();


    }
    else
    {
        LabelPleaseRegister.Visible = true;
        LabelPleaseRegister.Text = "Please register your personal information";
        PanelRegisterCv.Visible = false;
        PanelRegisterPersonalInfo.Visible = false;
    }
}

}

Best Regards