在最后一行使用下面的代码

 TextBox1.Text = dt1.Rows [ 0 ] [ 0 ]。ToString（）; 

试试这个

  protected   void  DropDownList1_SelectedIndexChanged（ object  sender，EventArgs e） 
 {

 SqlConnection con1 =  new  SqlConnection（ 您的连接字符串）; 
 SqlCommand cmd1 =  new  SqlCommand（  select item_price from quantity where item_name = @ item，con1）; 
 cmd1.Parameters.Add（  @ item，DropDownList1.SelectedItem.Text）; 
 SqlDataAdapter sda1 =  new  SqlDataAdapter（cmd1）; 
 DataTable dt1 =  new  DataTable（）; 
 sda1.Fill（dt1）; 
 if （dt1.Rows.Count >   0 ） //  验证以避免索引异常。 
 TextBox1.Text = dt1.Rows [ 0 ] [  item_price]的ToString（）;  //  定义列名 
} 

你应该知道 SQL注入 [ ^ ] 硬编码 SQl Statemetns

I am using dropdownlist and that has item retriveing from database, my question is that item select any one item that item value will display at text box?

What I have tried:

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{

    SqlConnection con1 = new SqlConnection("Data Source=XENORIX8-PC;Initial Catalog=xenorix;User ID=******;Password=*****");
    SqlCommand cmd1 = new SqlCommand("select item_price from quantity where item_name='" + DropDownList1.SelectedItem.Text + "'", con1);
    SqlDataAdapter sda1 = new SqlDataAdapter(cmd1);
    DataTable dt1 = new DataTable();
    sda1.Fill(dt1);
    TextBox1.Text = dt1.Rows[0].ToString();
}

Is this correct or not?, If its wrong please send any clue or code

use below code in last line

TextBox1.Text = dt1.Rows[0][0].ToString();

Try this

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
       {

           SqlConnection con1 = new SqlConnection("Your Connection string");
           SqlCommand cmd1 = new SqlCommand("select item_price from quantity where item_name=@item", con1);
           cmd1.Parameters.Add("@item", DropDownList1.SelectedItem.Text);
           SqlDataAdapter sda1 = new SqlDataAdapter(cmd1);
           DataTable dt1 = new DataTable();
           sda1.Fill(dt1);
           if (dt1.Rows.Count > 0) // validate for avoiding index exception.
               TextBox1.Text = dt1.Rows[0]["item_price"].ToString(); // define the column name
       }

You should be aware of SQL Injection[^] when hardcoding the SQl Statemetns