namespace LoginFormForUsers
{
	public partial class Form1 : Form
	{
		public Form1()
		{
			InitializeComponent();
		}

		private void button1_Click(object sender, EventArgs e)
		{
			if(LoginExist(this.textBox1.SelectedText))
			{
				//
			}
		}

		private bool LoginExist(string userLogin)
		{
			System.Data.SqlClient.SqlConnection dbConnect = new System.Data.SqlClient.SqlConnection();

			dbConnect.ConnectionString = "вот тут как бы надо подключаться к базе...";
			dbConnect.Open();


			string sql2 = @"SELECT LOGIN FROM S_LOGINS WHERE (LOGIN=@LOGIN)";
			System.Data.SqlClient.SqlCommand loginCount = new System.Data.SqlClient.SqlCommand(sql2);
			
			loginCount.Parameters.Add("@LOGIN", SqlDbType.NVarChar);
			loginCount.Parameters["@LOGIN"].Value = userLogin;


			System.Data.SqlClient.SqlDataReader dbRows = loginCount.ExecuteReader();

			bool result = dbRows.HasRows;

			dbRows.Close();

			dbConnect.Close();



			return result;
		}
	}
}