create procedure ValidateUserLogin
  @UserName varchar(30)
  , @Password varchar(30)
as
begin
  if exists (select * from UsersTable as ut
    where ut.UserName = @UserName AND ut.Password = @Password)
    select 1;
  else
    select 0;
end

private bool IsValidatedUser( string username, string password ) {
  try {
    bool rv = false;
 
    using ( SqlConnection con = new SqlConnection( connectionString ) ) {
      using ( SqlCommand cmd = new SqlCommand() ) {
        con.Open();
 
        cmd.Connection = con;
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandText = "ValidateUserLogin";
 
        cmd.Parameters.Add( "@UserName", SqlDbType.VarChar, 30 ).Value = username;
        cmd.Parameters.Add( "@Password", SqlDbType.VarChar, 30 ).Value = password;
 
        rv = Convert.ToBoolean( cmd.ExecuteScalar() );
 
        con.Close();
      }
    }
 
    return rv;
  } catch ( Exception ex ) {
    // Log errors
    throw;
  }
}

void buttonOK_Click( object sender, EventArgs e ) {
  bool isValid = this.IsValidatedUser(
    this.textBoxUserName.Text
    , this.textBoxPassword.Text );
 
  if ( isValid ) {
    // Do whatever for a valid login
  } else {
    // Show an error for an invalid login
  }
}

using System.Data.SqlClient;