添加

command.Parameters.Clear();

private void button1_Click(object sender, EventArgs e)
        {
            command.Parameters.AddWithValue("@StudentID", textBox1.Text);
            command.Parameters.AddWithValue("@Name", textBox2.Text);
            command.Parameters.AddWithValue("@Age", textBox3.Text);
            command.Parameters.AddWithValue("@Gender", textBox4.Text);
            command.Parameters.AddWithValue("@Courseno",comboBox1.Text);
            command.CommandText = "INSERT into details"+ "(StudentID,Name,Age,Gender,Courseno)VALUES" + "(@StudentID,@Name,@Age,@Gender,@Courseno)";

            try
            {
                con.Open();

                int result = command.ExecuteNonQuery();

                if (result > 0)
                    MessageBox.Show("student successfully updated");
                else
                    MessageBox.Show("failed to update");
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {
                con.Close();
                command.Parameters.Clear()
            }

            ClearFields();
        }