C# ComboBox Selected Value returns Null, Item and Value both from Database

Question

I added Employee LastName and First name to a combobox which is working, Eg. LastName, FirstName.. The value should be set as EmpID so that when I'm selecting an Item, EmpId will be return as value

conn.Open(); 
using (SqlCommand cmd = new SqlCommand("Select (LastName + ', ' + FirstName) AS Employee,EmpID from Employee ", conn))
using (SqlDataReader rdr = cmd.ExecuteReader())
{
    while (rdr.Read())
    {
        ListItem ComboItem = new ListItem();
        ComboItem.Text = rdr["Employee"].ToString();
        //ComboItem.Value = rdr["EmpID"].ToString();

        ComboItem.Value = rdr["EmpID"].ToString() ;
        GrpEmpCBox.Items.Add(ComboItem);  
        //GrpEmpCBox.Items.Add(rdr["Employee"].ToString());
    }
}

But when I'm getting the value of it returns null.

private void btnRunReport_Click(object sender, EventArgs e)
{
    EmployeeTimecardReport rptEmpTimecard = new EmployeeTimecardReport();
    rptEmpTimecard.Employee = GrpEmpCBox.SelectedValue.ToString(); //<--- this returns null
}

is this possible? I'm trying not to use databinding

my combobox properties:

DataSource: none

DisplayMember: none

Value Member: none

DropDownStyle: DropDownList

Answer 1

Use the DisplayMember and ValueMember properties instead.

There are many ways you could implement this... here's one:

comboBox1.ValueMember = "Item1";     // the value of the selected item
comboBox1.DisplayMember = "Item2";   // the field to display to the user

var emps = new List<Tuple<int, string>>();

while (rdr.Read())
    emps.Add(Tuple.Create(Convert.ToInt32(rdr["EmpID"]), rdr["Employee"].ToString()));

comboBox1.DataSource = emps;

Then to display the value of the selected item:

var selectedValue = comboBox1.SelectedValue.ToString();

---

I'll add a second example to demonstrate what those properties do.

Say you have an Employee class you created.

public class Employee
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public DateTime HireDate { get; set; }
}

And then you pull back a bunch of data from the Employee table in the database...

var emps = new List<Employee>();

while (rdr.Read())
    emps.Add(new Employee(ID = Convert.ToInt32(rdr["EmpID"]),
                          Name = rdr["Employee"].ToString(),
                          Age = Convert.ToInt32(rdr["Age"]),
                          HireDate = Convert.ToDateTime(rdr["Hired"])));

comboBox1.DataSource = emps;

You can pick two properties from your class to (1) display to the user (DisplayMember) and (2) act as the underlying value (ValueMember).

comboBox1.ValueMember = "ID";
comboBox1.DisplayMember = "Name";

Now you can get those values using comboBox1.SelectedValue and comboBox1.Text, respectively. Or you can just get the entire Employee record, which is stored in comboBox1.SelectedItem.

Answer 2

This if you Used the DisplayMember and ValueMember properties and still get Null Error

You must set the DataSource property of your Combobox control if you want to use the DisplayMember and ValueMember properties. Code Example

List<(int,string)> F = new List<(int,string)>();
         
           
            DataTable d = ConnandDisConnMode.Program.DisConnectedMode();
            for (int i = 0; i < d.Rows.Count; i++)
            {
                F.Add(((int)d.Rows[i]["id"], d.Rows[i]["Name"].ToString()));
            }

            comboBox1.DisplayMember = "Text";
            comboBox1.ValueMember = "Value";
            foreach (var item in F)
            {
                ComboboxItem i = new ComboboxItem();
                i.Text = item.Item2;
                i.Value = item.Item1;
                comboBox1.Items.Add(i);
            }

This would make the Combobox.SelectedValue Return NUll You have to change The Code to be like this

comboBox1.DisplayMember = "Text";
            comboBox1.ValueMember = "Value";
            List<ComboboxItem> z = new List<ComboboxItem>();
            foreach (var item in F)
            {
                ComboboxItem i = new ComboboxItem();
                i.Text = item.Item2;
                i.Value = item.Item1;
                z.Add(i);
                //comboBox1.Items.Add(i);
            }
            comboBox1.DataSource = z;

Answer 3

In my case, this problem was caused by accidentally calling InitializeComponent() of the whole form twice, once before and once after having set the combo box's ValueMember and DisplayMember.