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copied!<p><em>Note: in C# the term "function" is often replaced by the term "method". For the sake of this question there is no difference, so I'll just use the term "function".</em></p> <p>The other answers have already given you a quick way to <em>fix</em> your problem (just make <code>Add</code> a <code>static</code> function), but I'd like to explain why.</p> <p>C# has a fundamentally different design paradigm than C. That paradigm is called <strong>object-oriented programming</strong> (OOP). Explaining all the differences between OOP and functional programming is beyond the scope of this question, but here's the short version as it applies to you.</p> <p>Writing your program in C, you would have created a function that adds two numbers, and that function would exist independently and be callable from anywhere. In C# most functions don't exist independently; instead, they exist in the context of an object. In your example code, only an instance (an object) of the class <code>Program</code> knows how to perform <code>Add</code>. Said another way, you have to create an instance of <code>Program</code>, and then ask <code>Program</code> to perform an <code>Add</code> for you.</p> <p>The solutions that people gave you, using the <code>static</code> keyword, route around that design. Using the <code>static</code> keyword is kind of like saying, "Hey, this function I'm defining doesn't need any context/state, it can just be called." Since your <code>Add</code> function is very simple, this makes sense. As you start diving deeper into OOP, you're going to find that your functions get more complicated and rely on knowing their state/context.</p> <p>My advice: Pick up an OOP book and get ready to switch your brain from functional programming to OOP programming. You're in for a ride. </p>

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