Как добавить в строку символы?

Random rnd = new Random();
int a=0;
string strNumber = "123456789qwerty;
string strSymbols = "_|'|*|~|-";
string[] arrSymbols = strSymbols.Split('|').ToArray();
List<string> lstSymbols = new List<string>();

	foreach (string arrSymbol in arrSymbols)
	{
		lstSymbols.Add(arrSymbol);
	}

int intCountNumber = strNumber.Length;

for(a=0; a<intCountNumber; a++)
{
	string strRandomSymbols = Convert.ToString(lstSymbols[rnd.Next(0, lstSymbols.Count)]);
		
	int intRandom = rnd.Next(0, 10);
		
	if (intRandom>3)
	{
		
		try
		{
			strNumber = strNumber.Insert(a+a, strRandomSymbols);	
			
		}catch{
			
			break;	
			
		}
		
	}
}