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;

//...

  string[] files;

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

using System.IO;

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