public string variable get throws null exception

[SomeRandomOnlineDev]

I am making a very simple database reader that can create a file with information (first name, last name, age, gender), read a file and decode that information and put it into a class, and delete said files. In a certain part of the program, I have set a class's variables to null to prevent some errors being thrown elsewhere. However, in this class, one of the variables depends on the information of another variable, the first name, adds "_Info.txt" to it and calls it the file name. I have added a try catch statement when this assignment occurs to prevent this exception, however it seems to be happening in the actual get; set; function, and I'm not sure how to fix it.
Here is the class in question (There's slightly more to it than this but it's not relevant):

class Person {
    public Person(string firstName, string lastName, int age, string gender) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.gender = gender;
        try {
            this.fileName = this.firstName.ToLower() + "_Info.txt"; // this is where I thought it was being troublesome
        }
        catch (System.NullReferenceException) {
            this.fileName = "_Info.txt";
        }
    }

    public string firstName { get; set; } // this is where it's throwing the error inside the class
    public string lastName { get; set; }
    public int age { get; set; }
    public string gender { get; set; }
    public string fileName { get; set; }
}

And here is where I have set an instance of the class to null to prevent errors:

Person person = new Person(null, null, 0, null);

And here is why I have it set to null:

if (choice != "exit") {
    for (int i = 0; i < database.Count; i++) {
        if (database[i].firstName == choice) {
            person = database[i];
        }
    }

    Console.Clear();

    Console.Write("ACTION LIST\n\t1. Create a file for this person\n\t2. Delete the file for this person\nEnter your choice with the corresponding number");
    choice = Console.ReadLine();

    if (choice == "1") {
        person.createPersonFile(); // this line here was being problematic, so I have the person set to null earlier
    }
    else if (choice == "2") {
        person.deletePersonFile();
    }
}

 

[jmcilhinney]

And here is where I have set an instance of the class to null to prevent errors:

Person person = new Person(null, null, 0, null);

There's no such thing as "set[ting] an instance of a class to null". An instance of a class is an object and null is no object. Something cannot be equal to nothing.

Let's look at what I actually said in your other thread:

initialise the variable to null before the loop

I didn't say to initialise the variable to an object that has its properties set to null. I specifically said to set the variable to null. You're trying to make a simple situation complicated.

Person person = null;

That is going to prevent any compilation errors related to using an unassigned variable. What it won't do is prevent any runtime exceptions if an object isn't assigned to the variable before it is used. In your code, if database[i].firstName == choice is never true then person is never going to be set, so it will continue to have its initial value. You should be using the debugger to find out if and why that's the case.

 

[SomeRandomOnlineDev]

There's no such thing as "set[ting] an instance of a class to null". An instance of a class is an object and null is no object. Something cannot be equal to nothing.

Let's look at what I actually said in your other thread:

I didn't say to initialise the variable to an object that has its properties set to null. I specifically said to set the variable to null. You're trying to make a simple situation complicated.

Person person = null;

That is going to prevent any compilation errors related to using an unassigned variable. What it won't do is prevent any runtime exceptions if an object isn't assigned to the variable before it is used. In your code, if database[i].firstName == choice is never true then person is never going to be set, so it will continue to have its initial value. You should be using the debugger to find out if and why that's the case.

Oh, thank you very much for the clarification! Sorry for being a bit dense.

 

[Skydiver]

There's no such thing as "set[ting] an instance of a class to null". An instance of a class is an object and null is no object. Something cannot be equal to nothing.

But there is the Null Object design pattern. It's usually not introduced to beginner programmers, though because it tends to make them lax about their validation discipline, but later in a developers career, it actually becomes quite useful.

Null object pattern - Wikipedia

en.wikipedia.org