Console.WriteLine("Hello, world!");

Run the project:

dotnet run

This prints a line to standard output. In a small C# app, Console.WriteLine is the usual starting point.

string name = "Dan";
int count = 1;
bool active = true;
var language = "C#";

This shows common C# types plus var, which lets the compiler infer the type from the value on the right.

static string Greet(string name)
{
    return $"Hello, {name}!";
}

This is a basic C# method: it accepts an argument, builds a string, and returns the result.