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;

namespace Form_Library
{
    public partial class Login : Form
    {
        private int intAttemptCount = 0;
        private bool blnAllowClosing = false;
        private int intUserID;
        public delegate void _LoginFailed(Object sender, EventArgs e);
        public delegate void _LoginSucceeded(Object sender, LoginEventArgs e);
        public delegate void _LoginCancelled(Object sender, EventArgs e);
        public event _LoginFailed LoginFailed;
        public event _LoginSucceeded LoginSucceeded;
        public event _LoginCancelled LoginCancelled;

        public int UserID
        {
            get
            {
                return intUserID;
            }
        }




        public Login()
        {
            InitializeComponent();
        }


        private void Login_Activated(object sender, EventArgs e)
        {
            txtusername.Text = Environment.UserName;
            txtpassword.Focus();
        }

        private void Login_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (!blnAllowClosing)
            {
                e.Cancel = true;
            }
        }

        private void btnok_Click(object sender, EventArgs e)
        {
            // Was a user name entered?
if(txtusername.Text.Trim().Length > 0)
{
// Was the password correct?
if(txtpassword.Text == "secret")
{
// Successful login, set the User ID
intUserID = 27;
// Raise the LoginSucceeded event
if(this.LoginSucceeded != null)
this.LoginSucceeded(this,
new LoginEventArgs(intUserID));
// Turn on the allow closing flag
blnAllowClosing = true;
}
else
{
// Inform the user
// that the password was invalid
MessageBox.Show("The password you entered" +" was invalid.","Login");
// Increment the attempt count
intAttemptCount += 1;
// Check the attempt count
if (intAttemptCount == 3)
{
// Raise the LoginFailed event
if(this.LoginFailed != null)
this.LoginFailed(this,
new EventArgs());
// Set the Cancel dialog result
    
this.DialogResult =
DialogResult.Cancel;
// Turn on the allow closing flag
blnAllowClosing = true;
}
}
}
else
{
// Inform the user
// that they must supply a user name
MessageBox.Show("You must supply a User Name.","Login");



// Was a user name entered?
if (txtusername.Text.Trim().Length > 0)
{
    // Was the password correct?
    if (txtpassword.Text == "secret")
    {
        // Successful login, set the User ID
        intUserID = 27;
        // Raise the LoginSucceeded event
        if (this.LoginSucceeded != null)
            this.LoginSucceeded(this,
            new LoginEventArgs(intUserID));
        // Turn on the allow closing flag
        blnAllowClosing = true;
    }
}

else
{
    // Inform the user
    // that the password was invalid
    MessageBox.Show("The password you entered" +" was invalid.", "Login");
    // Increment the attempt count
    intAttemptCount += 1;
    // Check the attempt count
    if (intAttemptCount == 3)
    {
        // Raise the LoginFailed event
        if (this.LoginFailed != null)
            this.LoginFailed(this,
            new EventArgs());
        // Set the Cancel dialog result
        this.DialogResult =
        DialogResult.Cancel;
        // Turn on the allow closing flag
        blnAllowClosing = true;
    }


    else
    {
        // Inform the user
        // that they must supply a user name
        MessageBox.Show("You must supply a User Name.", "Login");
    }
}







}
}

        private void btncancel_Click(object sender, EventArgs e)
        {
            // Raise the LoginCancelled event
            if (this.LoginCancelled != null)
                this.LoginCancelled(this, new EventArgs());
            // Turn on the allow closing flag
            blnAllowClosing = true;


            // Raise the LoginCancelled event
            if (this.LoginCancelled != null)
                this.LoginCancelled(this, new EventArgs());
            // Turn on the allow closing flag
            blnAllowClosing = true;

        }
        }
    }
  

می خواستم بدونم که چرا کد زیر که  براتون گذاشتم هنگام اجرا کردن برنامه پیغام خطا میده ؟؟

کد نوشته شده صحیح است و من حتی Rebuild solution هم کردم پیغام successfulبه من داد ولی نمی دونم هنگام اجرا کردن برنامه پیغام خطا میده ؟؟

اینم متن خطا :

a project with on output type of class library can not be started directly 

   in order to  this project, add an executable project to this solution wich refrences the library project . set the executable project as the start up project