Create "Login Form" with authentication

using System.Security.Principal;
using System.Threading;
using System.IO;
using System;
using System.Data;
using System.Windows.Forms;

public class Users
{

   public bool IsLogin(string strName, string strPassword)
   {
        // Procedure checks that the login exists in the XML file

        DataSet dsUsers = new DataSet();
		DataRow[] drRows;
	   bool ret = false;

        try {

            // Read the XML into a Dataset and filter on name and password for a collection of DataRows.             

            dsUsers.ReadXml(@"..\..\Users.xml");

            drRows = dsUsers.Tables[0].Select("name = '" + 
                        strName + "' and password = '" + strPassword + "'");

            // Code must be implemented when adding users to the list to insure 
            // that there are no 2 users with the same name 
            // if there is a row in the collection then a record was found

			if (drRows.Length > 0) 
			{	ret = true; 	}
			else 
			{	ret = false;	}

       } catch(FileNotFoundException e)
		{
            MessageBox.Show("Users.Xml file not found.", "Unable to Authenticate user.", MessageBoxButtons.OK, MessageBoxIcon.Warning);
			Application.Exit();
        }
	   return ret;
    }

    public GenericPrincipal GetLogin(string strName, string strPassword)
		{
        // Procedure returns a Generic Principal representing the login account

        DataSet dsUsers = new DataSet();
		DataRow[] drRows = null;

        try {

            // Read the XML into a Dataset and filter for a collection of DataRows

            dsUsers.ReadXml(@"..\..\Users.xml");

            drRows = dsUsers.Tables[0].Select("name = '" + 
                    strName + "' and password = '" + strPassword + "'");

       } catch( FileNotFoundException e)
		{
            MessageBox.Show("Users.Xml file not found.","Shutting Down...", MessageBoxButtons.OK, MessageBoxIcon.Warning);
			Application.Exit();
        }

        // Create the Generic Identity representing the User

        GenericIdentity GenIdentity = new GenericIdentity(strName);

        // Define the role membership an array

        string[] Roles  = {Convert.ToString(drRows[0]["Role"]), ""};
        GenericPrincipal GenPrincipal = new GenericPrincipal(GenIdentity, Roles);
        return GenPrincipal;
    }

    public bool IsAdministrator()
	{
        // Procedure checks if the Windows Login is an Administrator
        // For single role-based validation
        // WinPrincipal new WindowsPrincipal(WindowsIdentity.GetCurrent())
        // For repeated role-based validation

        AppDomain.CurrentDomain.SetPrincipalPolicy(PrincipalPolicy.WindowsPrincipal);

        WindowsPrincipal WinPrincipal = (WindowsPrincipal) Thread.CurrentPrincipal;

        // Check if the user account is an Administrator

		if (WinPrincipal.IsInRole(WindowsBuiltInRole.Administrator)) 
		{
			return true;
		}
		else 
		{
			return false;
		}
    }
}

private void btnOK_Click(object sender, System.EventArgs e) 
{
        // Instantiate a custom Users class

        Users objUser = new Users();
        GenericPrincipal GenPrincipal;
        string strName = txtUserName.Text;
        string strPassword = txtPassword.Text;

        // Check for Windows Administrator.  Administrator can bypass
        // custom security system.

		if (chkAdministratorAccount.Checked) 
		{
			if (objUser.IsAdministrator()) 
			{
				// Display the Users Name (Windows or Generic)

				MessageBox.Show(Thread.CurrentPrincipal.Identity.Name + 
					" has logged in successfully!","Login Successful",
					MessageBoxButtons.OK, MessageBoxIcon.Information);				
			}
			else 
			{
				// Increment login attempts

				intLoginAttempts += 1;

				MessageBox.Show("User not an Administrator.  Please provide a User Name and Password.", this.Text,
					MessageBoxButtons.OK,MessageBoxIcon.Exclamation);
			}
		}
		else 
		{
			// Check that the login exists

			if (objUser.IsLogin(strName, strPassword)) 
			{
				GenPrincipal = objUser.GetLogin(strName, strPassword);
				Thread.CurrentPrincipal = GenPrincipal;

				// Display the Users Name (Windows or Generic)

				MessageBox.Show(Thread.CurrentPrincipal.Identity.Name + 
					" has logged in successfully!", "Login Successful",
					MessageBoxButtons.OK,MessageBoxIcon.Information);				
			}
			else 
			{
				// Increment login attempts
				intLoginAttempts += 1;
				// After the 3 attempts quit the application
				if (intLoginAttempts >= 3) 
				{
					MessageBox.Show("Too many failed login attempts",this.Text,
						MessageBoxButtons.OK,MessageBoxIcon.Exclamation);
					Application.Exit();
				}
				else 
				{
					MessageBox.Show("User Name not found.  Please try again", this.Text,MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
				}
			}
		}
}