how to read all files inside particular folder

using System.IO;
...
foreach (string file in Directory.EnumerateFiles(folderPath, "*.xml"))
{
    string contents = File.ReadAllText(file);
}

Note the above uses a .NET 4.0 feature; in previous versions replace EnumerateFiles with GetFiles). Also, replace File.ReadAllText with your preferred way of reading xml files - perhaps XDocument, XmlDocument or an XmlReader.

using System.IO;

DirectoryInfo di = new DirectoryInfo(folder);
FileInfo[] files = di.GetFiles("*.xml");

using System.IO;

//...

  string[] files;

  if (Directory.Exists(Path)) {
    files = Directory.GetFiles(Path, @"*.xml", SearchOption.TopDirectoryOnly);
    //...

You can use the DirectoryInfo.GetFiles method:

FileInfo[] files = DirectoryInfo.GetFiles("*.xml");

If you are looking to copy all the text files in one folder to merge and copy to another folder, you can do this to achieve that:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace HowToCopyTextFiles
{
  class Program
  {
    static void Main(string[] args)
    {
      string mydocpath=Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);     
      StringBuilder sb = new StringBuilder();
      foreach (string txtName in Directory.GetFiles(@"D:\Links","*.txt"))
      {
        using (StreamReader sr = new StreamReader(txtName))
        {
          sb.AppendLine(txtName.ToString());
          sb.AppendLine("= = = = = =");
          sb.Append(sr.ReadToEnd());
          sb.AppendLine();
          sb.AppendLine();   
        }
      }
      using (StreamWriter outfile=new StreamWriter(mydocpath + @"\AllTxtFiles.txt"))
      {    
        outfile.Write(sb.ToString());
      }   
    }
  }
}