Ejemplo LinQ, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] n1 = new int [10];
            
            //Relleno los numeros rapido, tu haz a tu manera
            Random ran = new Random();
            for(int i = 0; i < n1.Length; i++)
                n1[i] = ran.Next(100);
            
            
            //Que queremos buscar
            string strPattern = "3";
            
            //Aqui hacemos la cuenta
            int nCount = n1.Where(x=>x.ToString().Substring(0,1) == strPattern).Count();
            
            
            //Comprobacion
            n1.ToList().ForEach(x=>Console.WriteLine(x));
            Console.WriteLine(string.Format("Se han encontrado {0} repeticiones",nCount));
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 14 Mb, average nr of threads: 3