Delegate vs Method

Question

Is there any advantige using delegates instead of methods? For example:

public void LoginAuthenticate(object sender, AuthenticateEventArgs e)
{
 if (Membership.ValidateUser(lgUserLogin.UserName, lgUserLogin.Password))
     {
       Func<string, string> getDestinationPage = (userLogin) => { //definition };

     e.Authenticated = true;
     lgUserLogin.DestinationPageUrl = getDestinationPage(lgUserLogin.UserName);
  }

else
   {
    Action<string> invalidLogin = (msg) =>
     {
       lgUserLogin.FailureText = string.IsNullOrWhiteSpace(msg) ? "Invalid login" : msg;
       e.Authenticated = false;
     };

     MembershipUser user = Membership.GetUser(lgUserLogin.UserName);
      if (user != null)
       {
        if (user.IsLockedOut)
         {
           invalidLogin("User has been blocked");

         }
         else if (!user.IsApproved)
          {
           invalidLogin("User is not activated");
          }
          else
           {  
             invalidLogin(null);  
           }

       }

       else
         {
           invalidLogin(null);
         }
   }
}

If I getDestinationPage define as private string getDestinationPage(string login){...} and invalidLogin as private void invalidLogin(string msg){...}

does it gives any benefits?

Answer 1

In your case it makes the code cleaner and more readable to use methods. There is no benefit in using delegates like this.

Your code would look like this:

private string getDestinationPage(string userLogin)
{
    // definition
}

private void invalidLogin(string msg)
{
    lgUserLogin.FailureText = string.IsNullOrWhiteSpace(msg)
                              ? "Invalid login"
                              : msg;
}

public void LoginAuthenticate(object sender, AuthenticateEventArgs e)
{
    if (Membership.ValidateUser(lgUserLogin.UserName, lgUserLogin.Password))
    {
        e.Authenticated = true;
        lgUserLogin.DestinationPageUrl = getDestinationPage(lgUserLogin.
                                                              UserName);
    }

    else
    {
        e.Authenticated = false;
        MembershipUser user = Membership.GetUser(lgUserLogin.UserName);
        if (user != null)
        {
            if (user.IsLockedOut)
            {
                invalidLogin("User has been blocked");
            }
            else if (!user.IsApproved)
            {
                invalidLogin("User is not activated");
            }
            else
            {
                invalidLogin(null);
            }
        }
        else
        {
            invalidLogin(null);
        }
    }
}

Answer 2

Methods are basically "what the object can do". Delegates are the mechanism to decide "what to do" and "do it" in different places. It's like making premature decisions when comparing to if(a) then b(); else c();

Answer 3

Lambdas are anonymous methods. You use them mostly in cases where you want to pass a bit of code into a method as a parameter.

For example all LINQ methods work with lambdas because

var result = list.Where(x => x % 2 == 0);

is much more concise then

var result = list.Where(IsEven);

public bool IsEven(int i)
{
    return i%2 == 0;
}

An other usage are expression trees.

In your case a normal method is the better choice regarding readability/maintainability.

Answer 4

You can access (capture) the local variables of the calling function LoginAuthenticate from within getDestinationPage and invalidLogin. invalidLogin captures e. If you make this a method, you have to pass e as an additional parameter.

If you don't need this, it is indeed cleaner to just declare a private method.

Answer 5

If you are going to work with different scenarious where users may be redirected to different destination pages (programatically), and there are several different methods handling this depending on the situation with some conditionals etc., then yes delegates have the advantage as you may change the method to call just by assigning the new method to the delegate instance. Other than that, I don't see any point.