http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_4_B&lang=jp

BinarySearch.cs

<Query Kind="Program">
  <Reference>&lt;RuntimeDirectory&gt;\System.dll</Reference>
  <Reference>&lt;RuntimeDirectory&gt;\System.Net.dll</Reference>
  <Reference>&lt;RuntimeDirectory&gt;\System.Net.Http.dll</Reference>
  <Reference>&lt;RuntimeDirectory&gt;\System.Web.dll</Reference>
  <NuGetReference>Microsoft.Net.Http</NuGetReference>
  <NuGetReference>Newtonsoft.Json</NuGetReference>
  <NuGetReference>Sendgrid</NuGetReference>
  <Namespace>Newtonsoft.Json</Namespace>
  <Namespace>Newtonsoft.Json.Bson</Namespace>
  <Namespace>Newtonsoft.Json.Converters</Namespace>
  <Namespace>Newtonsoft.Json.Linq</Namespace>
  <Namespace>Newtonsoft.Json.Schema</Namespace>
  <Namespace>Newtonsoft.Json.Serialization</Namespace>
  <Namespace>SendGrid.SmtpApi</Namespace>
  <Namespace>System.IO</Namespace>
  <Namespace>System.Net</Namespace>
  <Namespace>System.Web.Mail</Namespace>
</Query>
//using System;
//using System.Linq;
//using System.Collections.Generic;
public class BinarySearch
{
    public static void Main()
    {
        Console.ReadLine();
        var s = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        Console.ReadLine();
        var t = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
    
        SearchWithHashSet(s, t);
        //SearchWithBinarySearch(s, t);
    }
    
    private static void SearchWithHashSet(int[] s, int[] t)
    {
        // Contains用にHashset生成
        var hashSet = new HashSet<int>(s);
        var c = t.Count(x => hashSet.Contains(x));
        
        // AIZU ONLINEがmonoなのでParallel使えない。。。使えたらこっちもあり
        //ThreadPool.SetMinThreads(32, 32);
        //var c = t.AsParallel().Count(x => hashSet.Contains(x));
        
        Console.WriteLine(c);
    }
    
    private static void SearchWithBinarySearch(int[] s, int[] t)
    {
        var c = t.Count(x => BinarySearchCore(s, x) > 0);
        Console.WriteLine(c);
    }
    
    // 擬似コードをそのまま写してみました。
    private static int BinarySearchCore(int[] s, int key)
    {
        var left = 0;
        var right = s.Count();
        
        while (left < right)
        {
            var mid = (left + right) / 2;
            if (s[mid] == key)
                return mid;
            if (key < s[mid])
                right = mid;
            else
                left = mid + 1;
        }
        return -1;
    }
}