Hospital/hospital/admin.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace hospital
{
    public partial class admin : Form
    {
        SqlConnection sqlConnection = null;
        public admin()
        {
            InitializeComponent();
        }

        private void admin_Load(object sender, EventArgs e)
        {
            FormCreating.openConnection(ref sqlConnection);


            SqlDataAdapter dataAdapter = new SqlDataAdapter($"select * from employees", sqlConnection);
            DataSet dataSet = new DataSet();
            dataAdapter.Fill(dataSet);
            dataGridView1.DataSource = dataSet.Tables[0];

            SqlCommand a = new SqlCommand($"select name_post from posts", sqlConnection);
            SqlDataReader reader = a.ExecuteReader();
            while (reader.Read())
            {
                comboBox1.Items.Add(reader.GetString(0));
            }
            reader.Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (!comboBox1.Text.Equals(""))
            {
                string row = dataGridView1.Rows[dataGridView1.CurrentCell.RowIndex].Cells[0].Value.ToString();
                SqlCommand b = new SqlCommand($"select id_post from posts where name_post like N'{comboBox1.SelectedItem}'", sqlConnection);
                SqlCommand a = new SqlCommand($"update employees set id_post = @id_post where id_employee like {row}", sqlConnection);
                a.Parameters.AddWithValue("id_post", b.ExecuteScalar().ToString());
                a.ExecuteNonQuery();

                SqlDataAdapter dataAdapter = new SqlDataAdapter($"select * from employees", sqlConnection);
                DataSet dataSet = new DataSet();
                dataAdapter.Fill(dataSet);
                dataGridView1.DataSource = dataSet.Tables[0];
            }
            else
            {
                MessageBox.Show("Ошибка", "", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void admin_FormClosing(object sender, FormClosingEventArgs e)
        {
            Application.Exit();
        }
    }
}