请写出c#代码

好的，下面是一个简单的C#用户登录管理系统的示例代码：

1. 创建用户数据表的SQL语句：

CREATE TABLE [dbo].[users] (
    [id] INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
    [username] VARCHAR(50) NOT NULL,
    [password] VARCHAR(50) NOT NULL,
    [name] VARCHAR(50) NOT NULL,
    [phone] VARCHAR(50) NOT NULL,
    [email] VARCHAR(50) NOT NULL
);

2. 用户登录界面的代码：

private void btnLogin_Click(object sender, EventArgs e)
{
    string username = txtUsername.Text.Trim();
    string password = txtPassword.Text.Trim();

    // 查询数据库中是否存在该用户
    string sql = "SELECT * FROM users WHERE username=@username AND password=@password";
    SqlParameter[] parameters = {
        new SqlParameter("@username", SqlDbType.VarChar, 50),
        new SqlParameter("@password", SqlDbType.VarChar, 50)
    };
    parameters[0].Value = username;
    parameters[1].Value = password;

    DataTable dt = SqlHelper.ExecuteDataTable(sql, parameters);

    if (dt.Rows.Count > 0)
    {
        // 登录成功，保存用户信息并关闭当前窗口
        User user = new User();
        user.Id = Convert.ToInt32(dt.Rows[0]["id"]);
        user.Username = dt.Rows[0]["username"].ToString();
        user.Name = dt.Rows[0]["name"].ToString();
        user.Phone = dt.Rows[0]["phone"].ToString();
        user.Email = dt.Rows[0]["email"].ToString();

        this.DialogResult = DialogResult.OK;
        this.Close();
    }
    else
    {
        MessageBox.Show("用户名或密码错误！");
    }
}

3. 用户管理界面的代码：

private void btnAdd_Click(object sender, EventArgs e)
{
    UserForm form = new UserForm();
    if (form.ShowDialog() == DialogResult.OK)
    {
        // 添加新用户到数据库
        string sql = "INSERT INTO users(username, password, name, phone, email) VALUES(@username, @password, @name, @phone, @email)";
        SqlParameter[] parameters = {
            new SqlParameter("@username", SqlDbType.VarChar, 50),
            new SqlParameter("@password", SqlDbType.VarChar, 50),
            new SqlParameter("@name", SqlDbType.VarChar, 50),
            new SqlParameter("@phone", SqlDbType.VarChar, 50),
            new SqlParameter("@email", SqlDbType.VarChar, 50)
        };
        parameters[0].Value = form.User.Username;
        parameters[1].Value = form.User.Password;
        parameters[2].Value = form.User.Name;
        parameters[3].Value = form.User.Phone;
        parameters[4].Value = form.User.Email;

        int rows = SqlHelper.ExecuteNonQuery(sql, parameters);
        if (rows > 0)
        {
            MessageBox.Show("添加用户成功！");
            BindUserList();
        }
        else
        {
            MessageBox.Show("添加用户失败！");
        }
    }
}

private void btnEdit_Click(object sender, EventArgs e)
{
    if (dgvUserList.SelectedRows.Count > 0)
    {
        int id = Convert.ToInt32(dgvUserList.SelectedRows[0].Cells["Id"].Value);

        UserForm form = new UserForm();
        form.User = GetUserById(id);

        if (form.ShowDialog() == DialogResult.OK)
        {
            // 更新用户信息到数据库
            string sql = "UPDATE users SET username=@username, password=@password, name=@name, phone=@phone, email=@email WHERE id=@id";
            SqlParameter[] parameters = {
                new SqlParameter("@username", SqlDbType.VarChar, 50),
                new SqlParameter("@password", SqlDbType.VarChar, 50),
                new SqlParameter("@name", SqlDbType.VarChar, 50),
                new SqlParameter("@phone", SqlDbType.VarChar, 50),
                new SqlParameter("@email", SqlDbType.VarChar, 50),
                new SqlParameter("@id", SqlDbType.Int)
            };
            parameters[0].Value = form.User.Username;
            parameters[1].Value = form.User.Password;
            parameters[2].Value = form.User.Name;
            parameters[3].Value = form.User.Phone;
            parameters[4].Value = form.User.Email;
            parameters[5].Value = id;

            int rows = SqlHelper.ExecuteNonQuery(sql, parameters);
            if (rows > 0)
            {
                MessageBox.Show("修改用户成功！");
                BindUserList();
            }
            else
            {
                MessageBox.Show("修改用户失败！");
            }
        }
    }
}

private void btnDelete_Click(object sender, EventArgs e)
{
    if (dgvUserList.SelectedRows.Count > 0)
    {
        int id = Convert.ToInt32(dgvUserList.SelectedRows[0].Cells["Id"].Value);

        // 从数据库中删除该用户
        string sql = "DELETE FROM users WHERE id=@id";
        SqlParameter[] parameters = {
            new SqlParameter("@id", SqlDbType.Int)
        };
        parameters[0].Value = id;

        int rows = SqlHelper.ExecuteNonQuery(sql, parameters);
        if (rows > 0)
        {
            MessageBox.Show("删除用户成功！");
            BindUserList();
        }
        else
        {
            MessageBox.Show("删除用户失败！");
        }
    }
}

private void BindUserList()
{
    string sql = "SELECT * FROM users";
    DataTable dt = SqlHelper.ExecuteDataTable(sql);
    dgvUserList.DataSource = dt;
}

private User GetUserById(int id)
{
    string sql = "SELECT * FROM users WHERE id=@id";
    SqlParameter[] parameters = {
        new SqlParameter("@id", SqlDbType.Int)
    };
    parameters[0].Value = id;

    DataTable dt = SqlHelper.ExecuteDataTable(sql, parameters);

    User user = new User();
    user.Id = Convert.ToInt32(dt.Rows[0]["id"]);
    user.Username = dt.Rows[0]["username"].ToString();
    user.Password = dt.Rows[0]["password"].ToString();
    user.Name = dt.Rows[0]["name"].ToString();
    user.Phone = dt.Rows[0]["phone"].ToString();
    user.Email = dt.Rows[0]["email"].ToString();

    return user;
}

以上代码仅供参考，具体实现还需要根据实际需求进行调整和完善。