请提供并发下载文件的C#例子

下面是一个实现并发下载文件的C代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>

#define BUFSIZE 1024
#define MAX_THREAD 4

struct download_task {
    char url[BUFSIZE];
    char filename[BUFSIZE];
};

void* download_routine(void* arg) {
    struct download_task *task = (struct download_task*)arg;
    char cmd[BUFSIZE];
    sprintf(cmd, "wget %s -O %s", task->url, task->filename);
    system(cmd);
    pthread_exit(NULL);
}

int main(int argc, char* argv[]) {
    if (argc < 3) {
        printf("Usage: %s [file_list] [output_dir]\n", argv[0]);
        return -1;
    }

    FILE* fp = fopen(argv[1], "r");
    if (fp == NULL) {
        perror("fopen");
        return -1;
    }

    char line[BUFSIZE];
    struct download_task tasks[MAX_THREAD];
    pthread_t threads[MAX_THREAD];
    int task_cnt = 0;
    while (fgets(line, BUFSIZE, fp) != NULL) {
        line[strlen(line) - 1] = '\0';
        sprintf(tasks[task_cnt].url, "%s", line);
        sprintf(tasks[task_cnt].filename, "%s/%d", argv[2], task_cnt);
        pthread_create(&threads[task_cnt], NULL, download_routine, &tasks[task_cnt]);
        task_cnt ++;
        if (task_cnt == MAX_THREAD) {
            for (int i = 0; i < MAX_THREAD; i ++) {
                pthread_join(threads[i], NULL);
            }
            task_cnt = 0;
        }
    }
    fclose(fp);

    for (int i = 0; i < task_cnt; i ++) {
        pthread_join(threads[i], NULL);
    }
    return 0;
}

该代码实现了从文件中读取下载任务，每次启动最多 4 个线程并发下载文件，下载完成后等待所有线程结束。使用方法是：在第一个参数指定文件列表文件，