Erro with useing tostring in tostring override

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

private List<Freight> freights;

public Truck(string name,double weightCapacity)
{
    this.Name = name;
    this.WeightCapacity = weightCapacity;
    List<Freight> freights = new List<Freight>();
}

In this code you are defining a private class-level variable called "freights", however in the constructor you are creating a second variable called freights and assigning a List to that, however that variable only exists within the constructor, the class-level "freights" is still null.

sb.Append(string.Join(", ", freights)).Append(Freight.ToString());

You can only call "ToString" on an instance of a type;

Freight f = new Freight();
f.ToString(); // f is an instance of Freight so this is valid
Freight.ToString(); // Freight is a type, not an instance of a type so this is not valid

I suspect you can just remove this ToString and have only this

sb.Append(string.Join(", ", freights));

What this code is going to do is go through each item in freights and call ToString() on it to convert that item to a string, and then comma separate the list. As long as you have a ToString override on the Freight class as well then that code will probably get you what you want.