Foobar.cs

using System.Text;

namespace FoobarPattern;

public class Foobar 
{
	private int _patternLength;
	private Dictionary<int, string> _patternDict = new();
	private StringBuilder _sb = new();

	public Foobar(int patternLength)
	{
		CheckNegativeLength(patternLength);
		_patternDict = new Dictionary<int, string>(){{3, "foo"}, {5, "bar"}};
		_patternLength = patternLength;
	}
	
	public Foobar(Dictionary<int, string> patternDict, int patternLength)
	{
		CheckNegativeKey(patternDict);
		CheckNegativeLength(patternLength);
		_patternDict = patternDict;
		_patternLength = patternLength;
	}
	
	public void SetPatternLength(int patternLength) 
	{
		CheckNegativeLength(patternLength);
		_patternLength = patternLength;
	}
	
	public void Add(int key, string value) 
	{
		CheckNegativeKey(key);
		_patternDict.Add(key, value);
	}
	
	public void Add(Dictionary<int, string> patternDict) 
	{
		CheckNegativeKey(patternDict);
		foreach (var kvp in patternDict) 
		{
			_patternDict.Add(kvp.Key, kvp.Value);
		}
	}
	
	public void Update(int key, string newValue) 
	{
		if (_patternDict.ContainsKey(key)) _patternDict[key] = newValue;
		else throw new KeyNotFoundException();
	}
	
	public void Delete(int key) 
	{
		if (_patternDict.ContainsKey(key)) _patternDict.Remove(key);
		else throw new KeyNotFoundException();
	}
	
	private void CheckNegativeKey(Dictionary<int, string> patternDict) 
	{
		foreach (var kvp in patternDict) 
		{
			if (kvp.Key < 0) throw new NegativeNumberException();
		}
	}
	
	private void CheckNegativeKey(int key) 
	{
		if (key < 0) throw new NegativeNumberException();
	}
	
	private void CheckNegativeLength(int patternLength) 
	{
		if (patternLength < 0) throw new NegativeNumberException();
	}
	
	public string GetPattern() 
	{
		_sb.Clear();
		for (int i = 0; i <= _patternLength; i++) 
		{
			string pattern = "";
			bool hasPattern = false;
			if (i == 0) _sb.Append("0, ");
			else 
			{
				foreach (var kvp in _patternDict) 
				{
					if (i % kvp.Key == 0) 
					{
						pattern += kvp.Value;
						hasPattern = true;
					}
				}
				if (!hasPattern) 
				{
					pattern = i.ToString();
				}
				pattern += ", ";
				_sb.Append(pattern);	
			}
		}
		return _sb.Replace(", ", ".", _sb.Length - 2, 2).ToString();
	}
}

NegativeNumberException.cs

namespace FoobarPattern;

public class NegativeNumberException : Exception
{
	public NegativeNumberException(string message = "Number can't be negative.") : base(message) 
	{
		
	}
}

Program.cs

using FoobarPattern;
public class Program 
{
	static void Main() 
	{
		Foobar foobar = new Foobar(new Dictionary<int, string>(){{3, "foo"}, {5, "bar"}}, 15);
		Console.WriteLine(foobar.GetPattern());
		
		foobar.Add(7, "fizz");
		Console.WriteLine(foobar.GetPattern());
		
		foobar.Update(5, "buzz");
		Console.WriteLine(foobar.GetPattern());
		
		foobar.Delete(3);
		Console.WriteLine(foobar.GetPattern());

		/*
		
		This code produces the following output:
		
		0, 1, 2, foo, 4, bar, foo, 7, 8, foo, bar, 11, foo, 13, 14, foobar.
		0, 1, 2, foo, 4, bar, foo, fizz, 8, foo, bar, 11, foo, 13, fizz, foobar.
		0, 1, 2, foo, 4, buzz, foo, fizz, 8, foo, buzz, 11, foo, 13, fizz, foobuzz.
		0, 1, 2, 3, 4, buzz, 6, fizz, 8, 9, buzz, 11, 12, 13, fizz, buzz.
		
		*/
	}
}

Lalu ada masukan, coba pikirkan ketika user akan menambahkan kondisi tambahan selain foo 3 dan 5 bar diatas, misal menambah 7 sebagai fizz, atau mengubah 5 menjadi buzz, apakah program kamu saat ini sudah support dengan penambahan kondisi tersebut

Sudah saya tambahkan mas, bisa dicek misal perlu saran lagi 😁