Autopopulate text Value in textbox depending on selected value from dropdownlist By using C# coding

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C#

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SQL

Hi, I am New to this subject. i hope someone will help me to do this task
Autopopulate text in textbox depending on selected value from dropdownlist
Thanks in advance................................
I wrote this code for Drop Down and i want the auto generated value show in text box

eg.E153 is last no. and new employee is joined to organization. i want to add him,the EmpCode must be auto generated in test box as E154.........


SqlCommand cmd = new SqlCommand();
cmd.CommandType = CommandType.Text;
cmd.CommandText = " SELECT EmpType_code FROM [systbl_EmpType] order by EmpType_code";
cmd.Connection = con;
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(ds);
ddlType.DataSource = ds;
ddlType.DataTextField = "EmpType_code";
ddlType.DataValueField = "EmpType_code";
ddlType.DataBind();
ddlType.Items.Insert(0, new ListItem("Please select", "NA"));
return ds;
}

protected void btn1_Click(object sender, EventArgs e)
{
SqlCommand cmd = new SqlCommand("SELECT MAX(EmpCode) from prl_EmployeeMaster WHERE EmpCode LIKE '[ddlType.SelectedItem.Value]%'");
cmd.Connection = con;
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();

double count;
con.Open();
count= Convert.ToDouble(cmd.ExecuteScalar())+1;
da.Fill(ds);
txt1.Text = (ddlType.SelectedItem.Value) + count;
ddlType.DataSource = ds.Tables[0];
ddlType.DataBind();
cmd.ExecuteNonQuery();
con.Close();