Get first missing number in an array. (LINQ independent)

Program.cs

using System;
using System.Collections.Generic;

public class Program
{
	public static int GetFirstMissingNumber(int[] positions)
	{
		int maxPositions;
		var posMax = 0;
		var posCount = 0;
		
		foreach(int pos in positions){
			
			if(posMax < pos)
				posMax = pos;
			
			posCount++;
		}
		
		if (posCount != 0)
		{	
			maxPositions = posMax > 0 && posMax > posCount ? posMax : posCount;
		}
		else
		{
			maxPositions = 1;
		}
		
		byte[] posOrder = new byte[maxPositions];
		foreach (int pos in positions)
		{
			if (pos > 0)
				posOrder[pos - 1] = 1;
		}
		
		for (var i = 0; i < maxPositions; i++)
		{
			if (posOrder[i] == 0)
				return i + 1;
		}
		
		return maxPositions + 1;
	}

	public static void Main()
	{
		List<int[]> testList = new List<int[]>{
			new int[4] {5, 1, 3, 4},
			new int[4] {0, 0, 0, 4},
			new int[4] {0, 4, 3, 2},
			new int[4] {1, 0, 3, 2}
		};
		
		foreach(var arr in testList)
		{
			Console.WriteLine("Missing Number {0}", GetFirstMissingNumber(arr));
		}
	}
}