C#•2mo ago

✅ User defined Exception Handling

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();
            if (!File.Exists(filepath))
            {
                throw new FileDoesNotExistException(filepath);
            }

            TextFileReader.StreamTextFile(filepath);




        }
    }
}

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The filepath {filepath} does not exist! Press Enter to try again. ")
        {
            Console.ReadLine();
            ReadTextFileMenu.ReadTextFile();
        }
    }
}

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();
            if (!File.Exists(filepath))
            {
                throw new FileDoesNotExistException(filepath);
            }

            TextFileReader.StreamTextFile(filepath);




        }
    }
}

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The filepath {filepath} does not exist! Press Enter to try again. ")
        {
            Console.ReadLine();
            ReadTextFileMenu.ReadTextFile();
        }
    }
}

Why do i get a "An object reference is required for the non-static field, method, or property 'ReadTextFileMenu.ReadTextFile()' error?

88 Replies

Pobiega•2mo ago

because TextFileReader.StreamTextFile(filepath); looks like a static invocation when you invoke a static method, you do so by ClassName.MethodName() for an instance method, you need a class instance example:

var x = new MyClass();
x.MethodName();

var x = new MyClass();
x.MethodName();

Merineth 🇸🇪•2mo ago

Hmm I'm not sure i'm following. The method inside ReadTextFileMenu.cs named ReadTextFile. If the if statement triggers, it will throw an exception i created. Type out the message and then restart the method ReadTextFile() Static method and instance method are new concepts

Pobiega•2mo ago

why would it restart? I see no such code. and this code is unrelated to your error

Merineth 🇸🇪•2mo ago

I was hoping ReadTextFileMenu.ReadTextFile(); would make it restart

Pobiega•2mo ago

oh dear god I see now oh my this is very cursed. Congratulations, I've never seen this before 😄 okay, I see what you were trying to do, but using exceptions like this is not the way to do it. but no worries, we can quickly sort up the confusion first, an Exception should only contain information about the error that occured, it should not try to fix it (thats what the catch block is for)

Merineth 🇸🇪•2mo ago

Pobiega•2mo ago

second, never do input/output code inside a constructor, but especially not in an exception constructor You knew some C, right?

Merineth 🇸🇪•2mo ago

Is that the text inside base () ? Yeah i know C

Pobiega•2mo ago

no, thats just calling the base exception handler so you've heard about while and for, right?

Merineth 🇸🇪•2mo ago

Yea

Pobiega•2mo ago

great I'd say while is a fairly appropriate tool here you want to ask the user for a valid path while the current path is not valid does that make sense?

Merineth 🇸🇪•2mo ago

Yeah i think i know what you mean. While the input from the user isn't valid, redo it

Pobiega•2mo ago

throwing exceptions is usually the answer to the problem "I know there was an error, but I don't have the ability to handle it in my current scope. I should let someone else above me know there was a problem, and let them decide what to do" so for example, if the menuitem code is responsible for asking for the path, but the path is only validated inside the ReadFromTextFile method you made before, thats where an exception could be used "Oh, this method might throw, so I need to handle the path being invalid. And if it was, I'd probably just wanna ask again, so lets put it all in a loop"

Merineth 🇸🇪•2mo ago

Hmmm

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();
            
            while (!File.Exists(filepath)) 
            {
                throw new FileDoesNotExistException(filepath);
                Console.WriteLine("Enter filepath..");
                filepath = Console.ReadLine();
            }

            TextFileReader.StreamTextFile(filepath);
        }
    }
}

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();
            
            while (!File.Exists(filepath)) 
            {
                throw new FileDoesNotExistException(filepath);
                Console.WriteLine("Enter filepath..");
                filepath = Console.ReadLine();
            }

            TextFileReader.StreamTextFile(filepath);
        }
    }
}

I guess this was not how you meant? Considering i'm getting the same problem :catlaugh:

Pobiega•2mo ago

you'd need to remove the throw new... line here

Merineth 🇸🇪•2mo ago

Yeah i realized the code below becomes unreachable Ok the while loop works now. But it's not triggering the exception

Pobiega•2mo ago

well yeah, we removed the exception

Merineth 🇸🇪•2mo ago

The goal is to make a user defined exception for practice

Pobiega•2mo ago

oh okay

Merineth 🇸🇪•2mo ago

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The user tried to input something that isn't valid. They tried {filepath} ")
        {
        }
    }
}

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The user tried to input something that isn't valid. They tried {filepath} ")
        {
        }
    }
}

This would be my constructor for my exception?

Pobiega•2mo ago

yes

Merineth 🇸🇪•2mo ago

Inside the {} what would go there?

Pobiega•2mo ago

that error message should not include instructions to the user, btw exceptions should not be shown to users, they are intended for the programmer or an error log file

Merineth 🇸🇪•2mo ago

Ohh i see

Pobiega•2mo ago

most of the time, nothing. If your exception contains detailed information about the error, like an error code, or stuff like that, you might put it there for example:

public class MyFileException : Exception
{
    public string FilePath { get; }

    public MyFileException(string message, string filePath) : base(message)
    {
        FilePath = filePath;
    }
}

public class MyFileException : Exception
{
    public string FilePath { get; }

    public MyFileException(string message, string filePath) : base(message)
    {
        FilePath = filePath;
    }
}

Merineth 🇸🇪•2mo ago

I updated it. So something like this is more appropriate?

Pobiega•2mo ago

this would not be out of place at all in an exception based around files Sure, that could work. Its a bit verbose and "friendly", which is unusual for exceptions but it works A more common message might be "Specified path '{path}' does not exist" but what you wrote is fine too 🙂

Merineth 🇸🇪•2mo ago

So now i include an if to check if the filepath does exist, and throw an exception?

Pobiega•2mo ago

from your previous thread, you used to have 2 methods right? the menu item, and your own stream-file-reader thing

Merineth 🇸🇪•2mo ago

Ah that was a different namespace

Pobiega•2mo ago

not working on that problem anymore? if you want to demonstrate proper usage of exceptions, I suggest you throw the exception in the file-reading method, and handle it in the menu item method exceptions are rarely (never?) used to handle errors within the same method that made them - because if you CAN do that, you can handle it without an exception

Merineth 🇸🇪•2mo ago

No i finished it. It works as intended. The next part is : "In the file FileDoesNotExistException.cs (in the folder Exceptions), there is a code skeleton for the class FileDoesNotExistException. Use this class to implement a user-defined exception that can be thrown if the text file in Chapter 2 doesn’t exist in the file system. The exception should contain an error message and the path to the non-existent file. Then modify the code in the class TextFileReader (Chapter 2), so that the exception is thrown if the text file doesn’t exist "

Pobiega•2mo ago

exceptions unique ability is to "bubble up" (move upwards) in the call stack okay so theres nothing there about catching the exception, might that be the next step?

Merineth 🇸🇪•2mo ago

Ah crap the exception should be thrown in the TextFileReader.cs and not in the menu

Pobiega•2mo ago

yeah that makes more sense 😄

Merineth 🇸🇪•2mo ago

Alright i see. So they want me to catch the exception here with my own user defined exception

Merineth 🇸🇪•2mo ago

Damn i can't seem to find a solution. My program stops completely but in my teachers code which is nearly identical they get to try again

Pobiega•2mo ago

can you show me your latest code, both files?

Merineth 🇸🇪•2mo ago

namespace Workshop3.IO
{
    public class TextFileReader
    {
        public static void StreamTextFile(string filepath)
        {
            // string str = File.ReadAllText(filepath);
            // Console.WriteLine(str);

            if (!File.Exists(filepath)) 
            {
                throw new FileDoesNotExistException(filepath);

            }
            // I create an object of StreamReader
            StreamReader sr = new StreamReader(filepath);

            // I read the first row of text. '?' indicates that the file might be empty.
            string? line = sr.ReadLine();

            // As long as the current line/row isn't empty.
            while (line != null)
            {
                // Write the row of text onto the terminal.
                Console.WriteLine(line);

                // Read the next row of text.
                line = sr.ReadLine();
            }




        }
    }
}

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The user tried to input something that isn't valid. They tried {filepath} ")
        {
        }
    }
}

namespace Workshop3.IO
{
    public class TextFileReader
    {
        public static void StreamTextFile(string filepath)
        {
            // string str = File.ReadAllText(filepath);
            // Console.WriteLine(str);

            if (!File.Exists(filepath)) 
            {
                throw new FileDoesNotExistException(filepath);

            }
            // I create an object of StreamReader
            StreamReader sr = new StreamReader(filepath);

            // I read the first row of text. '?' indicates that the file might be empty.
            string? line = sr.ReadLine();

            // As long as the current line/row isn't empty.
            while (line != null)
            {
                // Write the row of text onto the terminal.
                Console.WriteLine(line);

                // Read the next row of text.
                line = sr.ReadLine();
            }




        }
    }
}

namespace Workshop3.Exceptions
{
    public class FileDoesNotExistException : Exception
    {
        public FileDoesNotExistException(string filepath) : base($"The user tried to input something that isn't valid. They tried {filepath} ")
        {
        }
    }
}

Pobiega•2mo ago

That seems fine now show me your menu code

Merineth 🇸🇪•2mo ago

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();


            TextFileReader.StreamTextFile(filepath);
        }
    }
}

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();


            TextFileReader.StreamTextFile(filepath);
        }
    }
}

It stops the program at

            if (!File.Exists(filepath)) 
            {
                throw new FileDoesNotExistException(filepath);

            }

Pobiega•2mo ago

yes your menu code does not use try/catch so it just crashes

Merineth 🇸🇪•2mo ago

I'll try and add that okay i think i did it!

            try
            {
                TextFileReader.StreamTextFile(filepath);
            }
            catch (FileDoesNotExistException)
            {
                Console.WriteLine("Press Continue to try again..");
                Console.ReadLine();
                ReadTextFile();
            }

            try
            {
                TextFileReader.StreamTextFile(filepath);
            }
            catch (FileDoesNotExistException)
            {
                Console.WriteLine("Press Continue to try again..");
                Console.ReadLine();
                ReadTextFile();
            }

I added this try and catch

Pobiega•2mo ago

almost! see, that ReadTextFile(); down there will actually cause issues

Merineth 🇸🇪•2mo ago

Oh i see. What happens after a catch has been found? Does it restart automatically?

Pobiega•2mo ago

calling a method from itself is called recursion no, it runs the code in the catch then nothing if you want to restart, you must manually do that let me show you

var person = new Person("John");

while (true)
{
    try
    {
        Console.WriteLine("Enter an age for John: ");
        var age = int.Parse(Console.ReadLine());
        person.SetAge(age);
        break; // exit the loop
    }
    catch (TooYoungException ex)
    {
        Console.WriteLine($"Age {ex.Age} is too young, please try again.");
    }
}

Console.WriteLine("John is {person.age} years old.");

// program ends here

public class Person
{
    public string Name { get; }

    public Person(string name)
    {
        Name = name;
    }

    public void SetAge(int age)
    {
        if (age < 18)
        {
            throw new TooYoungException(age);
        }
    }
}

public class TooYoungException : Exception
{
    public int Age { get; }

    public TooYoungException(int age) : base($"Age {age} is too young.")
    {
        Age = age;
    }
}

var person = new Person("John");

while (true)
{
    try
    {
        Console.WriteLine("Enter an age for John: ");
        var age = int.Parse(Console.ReadLine());
        person.SetAge(age);
        break; // exit the loop
    }
    catch (TooYoungException ex)
    {
        Console.WriteLine($"Age {ex.Age} is too young, please try again.");
    }
}

Console.WriteLine("John is {person.age} years old.");

// program ends here

public class Person
{
    public string Name { get; }

    public Person(string name)
    {
        Name = name;
    }

    public void SetAge(int age)
    {
        if (age < 18)
        {
            throw new TooYoungException(age);
        }
    }
}

public class TooYoungException : Exception
{
    public int Age { get; }

    public TooYoungException(int age) : base($"Age {age} is too young.")
    {
        Age = age;
    }
}

Merineth 🇸🇪•2mo ago

Oh interesting I manually added the recursion there such that the program doesn't just end if the user inputs a filepath that doesn't exist. Is recursion ok in c#? I'm very used to it in C

Pobiega•2mo ago

its generally avoided definately do not use if a normal loop does the trick its very useful for stuff like traversing a tree, or pathfinding etc because otherwise, imagine if I am a silly user and I keep entering a bad path it doesnt "restart" the method, it runs the method again, in a new stack frame

Pobiega•2mo ago

meaning...

Merineth 🇸🇪•2mo ago

Oh yeah crap yeah i can def see an issue with that

Pobiega•2mo ago

every time you "restart" you actually open a new frame, further down the stack

Merineth 🇸🇪•2mo ago

Aahhh okay that makes sense now. I read through the code and with the Show() I can actually just view all the options again from the menu in the same Frame So i just simply got rid of the recursion

Pobiega•2mo ago

can you? show me

Merineth 🇸🇪•2mo ago

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();

            try
            {
                TextFileReader.StreamTextFile(filepath);
            }
            catch (FileDoesNotExistException)
            {
                Console.WriteLine("Press Continue to try again..");
                Console.ReadLine();
            }
            Show();
        }
    }
}

namespace Workshop3.Menus
{
    public class ReadTextFileMenu : Menu
    {
        public ReadTextFileMenu(string name)
            : base(name)
        {
            Add(new MenuItem("Read Text File", new Command<ReadTextFileMenu>(this, r => r.ReadTextFile())));
        }

        public void ReadTextFile()
        {
            // Clear the console from previous text
            Console.Clear();
            // Asking the user for filepath
            Console.WriteLine("Enter filepath..");
            // Saving the filepath as a string object
            string filepath = Console.ReadLine();

            try
            {
                TextFileReader.StreamTextFile(filepath);
            }
            catch (FileDoesNotExistException)
            {
                Console.WriteLine("Press Continue to try again..");
                Console.ReadLine();
            }
            Show();
        }
    }
}

Pobiega•2mo ago

Show comes from the base class, but I would guess it invokes the command action for the menu item somehow Im pretty sure you are creating another frame here too

Merineth 🇸🇪•2mo ago

RIp :catHeyHello:

Pobiega•2mo ago

read my age example with John again and you'll figure it out

Merineth 🇸🇪•2mo ago

That was my teachers solution xD Oki 1 sec

Pobiega•2mo ago

your teacher is a hack, we already knew this 😄

Merineth 🇸🇪•2mo ago

public class TooYoungException : Exception
{
    public int Age { get; }

    public TooYoungException(int age) : base($"Age {age} is too young.")
    {
        Age = age;
    }
}

public class TooYoungException : Exception
{
    public int Age { get; }

    public TooYoungException(int age) : base($"Age {age} is too young.")
    {
        Age = age;
    }
}

Quick question. What does the public int Age { get; } and Age = age; do?

Pobiega•2mo ago

the first declares a property. its an instance member variable that normally holds public information. in this case, its read only (no setter) Age = age sets the value of the Age property to the value given to the constructor I just wanted to show that the main benefit of making your own exceptions is that you can include actual data inside them not just a message

Merineth 🇸🇪•2mo ago

Ah yeah Hmm i'm not entirely sure i understand the first part. public TooYoungException(int age) I get that this is a method that accepts the argument int age public int Age { get; } this however hmm

Pobiega•2mo ago

well you know how you must declare a variable before you can use it?

Merineth 🇸🇪•2mo ago

It defines a read only property? Would this be a struct?

Pobiega•2mo ago

this is declaring a property, which is a form of instance member

Merineth 🇸🇪•2mo ago

Right yeah you got to declare a variable before you can use it In this case it's name is Age.

Pobiega•2mo ago

this is just like that, but instead of a local variable that only exists inside a method, this is an instance member meaning it will "keep" the value for as long as the instance lives so for this particular exception instance, it will have this Age value if I make another instance, it has its own Age

Merineth 🇸🇪•2mo ago

Ahhhh so the get that you use indicates that it's only a getter? i.e can't be changed only used?

Pobiega•2mo ago

correct!

Merineth 🇸🇪•2mo ago

Neat

Pobiega•2mo ago

you might have seen { get; set; } before

Merineth 🇸🇪•2mo ago

Yeah litearlly 5 minutes ago hahah, just read up about them

Pobiega•2mo ago

thats "both a getter, and a setter"

Merineth 🇸🇪•2mo ago

But yeah that makes a lot of sense

Pobiega•2mo ago

there is also { get; private set; } which means anyone can read, but only I can write

Merineth 🇸🇪•2mo ago

In my own mind i'd like to do public int age; But i'd assume this would be both a getter and setter?

Pobiega•2mo ago

that would be just a field a field doesnt have the concept of having getters or setters thats why we dont use them for public things

Merineth 🇸🇪•2mo ago

In C if i do int age; it would get a garbage value, deos this apply to c# also?

Pobiega•2mo ago

no you'd just get an uninitialized variable, which cant be used in C#

Merineth 🇸🇪•2mo ago

I see

Pobiega•2mo ago

C# has a runtime, so its not like C that just runs machine code so no "use after free" etc

Merineth 🇸🇪•2mo ago

I'm pretty sure C# uses machine code also?

Pobiega•2mo ago

not directly C# does not compile to native machine code

Merineth 🇸🇪•2mo ago

Ah it had something to do with .NET compiling iirc

Pobiega•2mo ago

yeah

Merineth 🇸🇪•2mo ago

public int Age { set; } Does this even have any use? Only being able to set a value without retrieving it sounds dumb

Pobiega•2mo ago

that has essentially no value, correct but regarding machine code, C# did quite recently get the ability to do "AOT compilation" this does result in machine code, but its... its complicated 😄

Merineth 🇸🇪•2mo ago

:catlaugh: I'm gonna continue with the program now. With writing this time... will open a poster later if something comes up thanks again (:

Gaming

Programming

✅ User defined Exception Handling