I give up on C#!

using System;

class Program { static double Price(int quantity) { double pricePerUnit; if (quantity >= 100) { pricePerUnit = 1.5; }

    It won't RUN

I couldn't see where that code came from to see context (whether it's intended as complete code expected to run, or snippet to demonstrate some facet) or the problem it's trying to solve. However, if it's helpful, I was able to run the following in an online csharp repl.

using System;

class Program { 
    static double Price(int quantity) {
        double pricePerUnit = 1; 
        if (quantity >= 100) 
        { 
            pricePerUnit = 1.5;
        }
        return pricePerUnit * quantity;
    }
    public static void Main(){        
        Console.WriteLine(Price(10)); //prints 10
        Console.WriteLine(Price(100)); //prints 150
    }
}

The code you posted seemed incomplete. I just added enough code to guarantee that Price returned a value and then made sure the code could execute by adding a main method. You'd presumably need to make sure you save the code and compiled it prior to running. Under most circumstances, if the code is faulty, compiling or running should produce a human readable error message.