error Unable to cast object of type 'System.Data.DataColumn' to type 'System.IConvert

[sattu]

hi friends,
i have one class and i have created two properties

and filled this property through dataset

but when i created object o this class to access this property to fill textbox

i am geting this problem

"Unable to cast object of type 'System.Data.DataColumn' to type 'System.IConvertible"

Please give me suggestion

public class Class1
{
    SqlConnection cn = new SqlConnection(System.Configuration.ConfigurationManager.AppSettings["cnstr"]);
    SqlDataAdapter da = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand();
    DataSet ds = new DataSet();
    SqlDataReader dr;
    public Class1()
    {
        //
        // TODO: Add constructor logic here
        //
    }
       public void getdata1()
    {
        cmd.Connection = cn;
        cn.Open();
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = "SELECT EmployeeID, FirstName FROM    employees ORDER BY FirstName";
        da.SelectCommand = cmd;
        da.Fill(ds);
 
        this._CategoryId = Convert.ToInt16(ds.Tables[0].Columns["employeeid"]);
        this._firstname = ds.Tables[0].Columns["firstname"].ToString();
 
 
 
       private int _CategoryId;
 
    public int CategoryId
    {
        set { _CategoryId = value; }
        get { return _CategoryId; }
    }
    private string _firstname;
 
    public string firstname
    { 
        set { _firstname = value; }
        get { return _firstname; }
 
    }
    private string _lastname;
 
    public string lastname
    {   
        set { _lastname = value; }
        get { return _lastname; }
 
    }
 

this is my .cs file code

Class1 objcls = new Class1();
 this.objcls.getdata1();
 
        txtid.Text = objcls.CategoryId.ToString();
        txtname.Text = objcls.firstname.ToString();