如何从 Visual Studio 的表中删除特定记录？

Question

I have written and rewritten the code several times now to delete records upon entering the 'name' in a textbox & clicking a button that will remove the row pertaining to the name entered. However, when I run the program and click on the button, the message box pops up stating that the particular records have been deleted, but when the table is displayed in the data grid view, the records are still present (they haven't been deleted from the table) Please check my coding and tell me what's wrong in it, and how do I resolve it? thank you :) Also I have to say, I've only learned basic stuff in C# programming.. Here is my coding:

private void btnremove_Click(object sender, EventArgs e)
{
    con.Open();
    DialogResult ans = MessageBox.Show("Are you sure you want to remove the selected records?", 
        "Confirmation For Membership Cancellation", MessageBoxButtons.YesNo, 
        MessageBoxIcon.Question);
    if (ans==DialogResult.Yes)
    {
        string sqldelete = "DELETE FROM membersdetails WHERE Name='" + txtname.Text + "'";
        string deletesql = "DELETE FROM currentstatus WHERE Name_='" + txtname.Text + "'";
        com = new SqlCommand(sqldelete,con);
        com = new SqlCommand(deletesql, con);
        SqlDataAdapter sqldataadapter = new SqlDataAdapter(com);
        com.ExecuteNonQuery();
        MessageBox.Show("Records have been removed- Membership has been  cancelled", 
           "Membership Cancellation", MessageBoxButtons.OK, MessageBoxIcon.Information);
    }
    con.Close();
    com.Dispose();
}

Answer 1

There are some issues with your code, in its current state:

1. The block below, you are overwriting part of your deleting mechanism:

    com = new SqlCommand(sqldelete,con); com = new SqlCommand(deletesql, con); SqlDataAdapter sqldataadapter = new SqlDataAdapter(com); com.ExecuteNonQuery();

   You should be able to merge the sqldelete and deletesql string variables into 1 command, whihc can be then passed on to the DB Engine.

2. From a coding perspective, classes should observe the Single Responsibility principle, that is, classes worry about doing only 1 thing. In your current code, the UI needs to look into keeping the UI updated and also keeping the DB updated. To fix this, we usually create repositories whose task is to wrap over the DB operations. This also allows you test individual layers of your application, and also giving you more flexibility should you decide to make any changes.

3. As is, your code is open for SQL Injection attacks, consider using prepared statements to cater for this problem.

4. Other than that, I cannot see anything inherently wrong with your deletion code. However, it might be the case that your grid is not being refreshed (while you are thinking that it is). The quickest and best way to check this is to use something like SQL Management Studio (or Azure Data) to connect to your DB and query the table directly.

Answer 2

The ExecuteNonQuery() has to be executed after and before of the last new instance:

    com = new SqlCommand(sqldelete,con);
    com.ExecuteNonQuery();
    com = new SqlCommand(deletesql, con);
    com.ExecuteNonQuery();
    
    // Then, you can show the table
    string sqlSelect = "SELECT * FROM yourtable WHERE bla bla...";
    SqlDataAdapter sqldataadapter = new SqlDataAdapter(sqlSelect, con);
    DataTable tbl = new DataTable();
    sqldataadapter.Fill(tbl);
    yourDataGridView.DataSource = tbl;

Try this, I hope it could help.

Answer 3

The following is an example which may need some changes to fit into your code. The idea is to setup a DataGridView data source with a BindingSource that has required columns from a table for member details. Data operations are in a class to separate frontend operations from data operations. Note there is no need for a SqlDataAdapter which is overkill.

For the delete operation best to use a primary key as this is constant while a name may change at some point.

The following class is a mock up for read and delete operations, make sure you understand it's not a copy-n-paste solution so take time to read the code before trying it out.

Data class

using System.Collections.Generic;
using System.Data.SqlClient;

namespace YourNamespace
{
    public class MembershipOperations
    {
        /// <summary>
        /// Connection string to your database
        /// - change TODO to your environment
        /// - Integrated Security (check this too)
        /// </summary>
        private static readonly string _connectionString =
            "Data Source=****TODO****;" +
            "Initial Catalog=****TODO****;" +
            "Integrated Security=True";

        /// <summary>
        /// Delete a single member by primary key
        /// Recommend adding a try-catch to the open and ExecuteNonQuery lines
        /// </summary>
        /// <param name="memberIdentifier">Member primary key</param>
        /// <returns>
        /// Success of the operation
        /// </returns>
        public static bool RemoveSingleOrder(int memberIdentifier)
        {
            bool success = false;

            string deleteStatement = "DELETE FROM membersdetails  WHERE id = @id";

            using (var cn = new SqlConnection { ConnectionString = _connectionString })
            {
                using (var cmd = new SqlCommand { Connection = cn, CommandText = deleteStatement })
                {

                    cmd.Parameters.AddWithValue("id", memberIdentifier);
                    cn.Open();
                    success = cmd.ExecuteNonQuery() == 1;
                }
            }

            return success;

        }
        /// <summary>
        /// Provides a list which in the frontend a user can select
        /// a member such as a ComboBox or ListBox etc.
        ///
        /// When they select a user you have code to cast the selected item
        /// to a MemberDetails instance and pass the primary key to the RemoveSingleOrder
        /// method.
        ///
        /// Note in the frontend if there are many members consider providing a incremental
        /// search feature to the frontend control displaying member details.
        /// </summary>
        /// <returns></returns>
        public static List<MemberDetails> GetMemberDetails()
        {
            var memberDetails = new List<MemberDetails>();

            /*
             * - Create a connection and command object with
             *   SELECT column names including 'id' primary key
             *
             * - Use a DataReader to loop through records into
             *   memberDetails variable
             */

            return memberDetails;
        }
    }
    /// <summary>
    /// Place in a class file named  MemberDetails.cs
    /// </summary>
    public class MemberDetails
    {
        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        // more properties

        /// <summary>
        /// Optional, can assist with debugging and/or
        /// for display purposes
        /// </summary>
        /// <returns></returns>
        public override string ToString() => $"{FirstName} {LastName}";

    }
}

Helper method for asking a question with the default button is No, not Yes.

using System.Windows.Forms;

namespace YourNamespace
{
    public static class Dialogs
    {
        public static bool Question(string text)
        {
            return (MessageBox.Show(
                text,
                Application.ProductName,
                MessageBoxButtons.YesNo,
                MessageBoxIcon.Question,
                MessageBoxDefaultButton.Button2) == DialogResult.Yes);
        }
    }
}

Form code

using System;
using System.Windows.Forms;

namespace YourNamespace
{
    public partial class Form1 : Form
    {
        private readonly BindingSource _membersBindingSource = new BindingSource();
        public Form1()
        {
            InitializeComponent();
            Shown += Form1_Shown;
        }
        /// <summary>
        /// Setup columns in the IDE for MembersDataGridView that
        /// exclude the primary key
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Form1_Shown(object sender, EventArgs e)
        {
            MembersDataGridView.AutoGenerateColumns = false;
            _membersBindingSource.DataSource = MembershipOperations.GetMemberDetails();
            MembersDataGridView.DataSource = _membersBindingSource;
        }

        private void RemoveMemberButton_Click(object sender, EventArgs e)
        {
            if (_membersBindingSource.Current == null) return;

            var member = (MemberDetails)_membersBindingSource.Current;

            if (!Dialogs.Question($"Remove {member}")) return;

            if (!MembershipOperations.RemoveSingleOrder(member.Id))
            {
                MessageBox.Show($"Failed to remove {member}");
            }
        }
    }
}