string strValue = "";
for(int i = 0; i < this.listBox1.Items.Count; i++)
{
    strValue += this.listBox1.Items[i].ToString();
}
this.textBox1.Text = strValue;

------解决方案--------------------
C# code
string s = "";
for(int i=0;i<listBox1.Items.Count;i++)
{
  s += listBox1.Items[i].ToString();
}
textbox.Text = s;

------解决方案--------------------
C# code
SqlConnection con = new SqlConnection("Data Source=xxx;DataBase=ycw;User ID=sa;PWD=sa");
con.Open();
SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
cmd.CommandType = CommandType.Text;
cmd.CommandText = "insert into tablename(col) values(@value)";
cmd.Parameters.AddWithValue("@value", this.textbox.Text);
cmd.ExecuteNonQuery();
con.Close();