在列表和会话中使用类asp.net c＃

Question

I'm trying to get data from the database and put it in a list. Then, i want to create dropdown list with the received data.

The db table:

ID
projectName

The code:

public void ProjectsList()
{
    List<ProjectsListClass> ProjectsList=new List<ProjectsListClass>(); //list of projects
    ProjectsListClass projectDetails=new ProjectsListClass();//1 project details

    //creating session of projects table
    using (SqlConnection conn = new SqlConnection(mainDBConn))//connect to the database table
    {
        SqlCommand sqlProjectsList = new SqlCommand("SELECT * FROM dbo.Projects ORDER BY projectName", conn);

        conn.Open();

        SqlDataReader rsProjectsList = sqlProjectsList.ExecuteReader();

        if (rsProjectsList.HasRows && rsProjectsList.Read())//there is a project, insert into the array
        {
            projectDetails.ProjectsListProjectID = Convert.ToInt16(rsProjectsList["ID"]);
            projectDetails.ProjectsListProjectName = rsProjectsList["projectName"].ToString();
            ProjectsList.Add(projectDetails);
        }
        Session["ProjectsList"] = ProjectsList;

        rsProjectsList.Close();
        conn.Close();
    }  
}

and in the other page:

    List<ProjectsListClass>projectsArray=new List<ProjectsListClass>();
    projectsArray=(List<ProjectsListClass>)Session["ProjectsList"];

    foreach(var project in projectsArray)
    {
        ListItem l = new ListItem(project.ProjectsListProjectID.ToString()
            , project.ProjectsListProjectName.ToString()
            , true);
        projectIDDropDownList.Items.Add(l);
    }

i get the error:

Object reference not set to an instance of an object.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 20: foreach(var project in projectsArray) - this one in red

Line 21: { Line 22: ListItem l = new ListItem(project.ProjectsListProjectID.ToString()

What should i do?

Answer 1

I think there's a few things you need to be mindful of.

Although I don't know you're particular requirements, generally speaking I don't believe storing the your projectlist in session is the best approach. Generally I would have a Project Class with a GetProjectList() method that return a List of Projects. You could then call this method from your "other" page and bind to your DropDownList.

Also, I believe you may have your projectDetails declaration in the wrong place. If you want to return a list of projects try something like this (I'm a VB developer but I've had a go at the C#).

 SqlCommand sqlProjectsList = new SqlCommand("SELECT * FROM dbo.Projects ORDER BY projectName", conn); conn.Open(); SqlDataReader rsProjectsList = sqlProjectsList.ExecuteReader(); if (rsProjectsList.HasRows)//there is a project, insert into the array { while rsProjectsList.Read() { ProjectsListClass projectDetails=new ProjectsListClass(); projectDetails.ProjectsListProjectID = Convert.ToInt16(rsProjectsList["ID"]); projectDetails.ProjectsListProjectName = rsProjectsList["projectName"].ToString(); ProjectsList.Add(projectDetails); } } rsProjectsList.Close(); conn.Close(); 

To bind to your dropdown list, try the following code,

 projectIDDropDownList.DataSource = Project.GetProjects(); projectIDDropDownList.DataBind();