ほれ
using System;
using System.IO;
using System.Threading;

class Program {
static void ReadFile(string filename) {
int threadId = Thread.CurrentThread.ManagedThreadId;
Console.WriteLine($"Thread {threadId} is reading {filename}");
lock (filename) {
using (FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read)) {
using (StreamReader reader = new StreamReader(fs)) {
string data = reader.ReadToEnd();
Console.WriteLine($"Thread {threadId} read: {data}");
}
}
}
}

static void Main() {
string filename = "A.txt";
Thread thread1 = new Thread(() => ReadFile(filename));
Thread thread2 = new Thread(() => ReadFile(filename));
thread1.Start();
thread2.Start();
thread1.Join();
thread2.Join();
}
}