String path = @"X:\xxx\xxx";
 
//第一种方法
var files = Directory.GetFiles(path, "*.txt");
             
foreach (var 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);
}