String path = @"X:\xxx\xxx";

//第一种方法

string[] files = Directory.GetFiles(path, "*.txt");

foreach (string file in files)

{

    Console.WriteLine(file);

}

//第二种方法

DirectoryInfo folder = new DirectoryInfo(path);

foreach (FileInfo file in folder.GetFiles("*.txt"))

{

    Console.WriteLine(file.FullName);

}