is_even.cs

using System.Text;

namespace IsEven;

internal class Program
{
	private const string PATH = ""; //Your path here
	public static void Main(string[] args) { GenerateIf(); }

	private static void GenerateIf()
	{
		const int init = Int32.MinValue;
		const int max = Int32.MaxValue;

		// Delete the file if it exists.
		if (File.Exists(PATH))
		{
			File.Delete(PATH);
		}

		using (var fs = new FileStream(File.OpenHandle(PATH, FileMode.OpenOrCreate, FileAccess.ReadWrite), FileAccess.ReadWrite))
		{
			// Includes
			AddText(fs, "#include <stdlib.h>\n" +
			            "#include <stdio.h>\n" +
			            "#include <stdbool.h>\n\n");
			
			// isEven
			AddText(fs, "bool isEven(int n) {\n");

			const double comp = (double) max;
			for (int i = init; i < max; i++)
			{
				double progress = (i / comp) * 100;

				if ((i % 500000) == 0)
				{
					Console.WriteLine($"{progress}%");
				}

				if (i == init)
				{
					AddText(fs, template_even(i, true) + "\n");
				}
				else
				{
					AddText(fs, (i % 2) == 0 ? template_even(i) + "\n" : template_odd(i) + "\n");
				}
			}
			// Close isEven
			AddText(fs, "}\n\n");
			
			// Main
			AddText(fs, "int main(int argc, char *argv[]) {\n\t" +
			            "int n = atoi(argv[1]);\n\t" +
			            "if(isEven(n)) {\n\t\t" +
			            "printf(\"%d is even!\\n\", n);\n\t}\n\t" +
			            "else {\n\t\t" +
			            "printf(\"%d is odd!\\n\", n);\n\t}\n}");
		}
	} 
	private static string template_even(int n, bool use_if = false)
	{
		string base_str = $"if (n == {n}) {{\n\t\treturn true;\n\t}} ";
		return use_if ? "\t" + base_str : $"\telse {base_str}";
	}

	private static string template_odd(int n) { return $"\telse if (n == {n}) {{\n\t\treturn false;\n\t}} "; }

	private static void AddText(FileStream fs, string value)
	{
		byte[] info = new UTF8Encoding(true).GetBytes(value);
		fs.Write(info, 0, info.Length);
	}
}

👍 LGTM!