using System;
using System.Text;
using System.Runtime.InteropServices;
using System.CodeDom.Compiler;
 
public class FindExecutableUtils{
 
	[DllImport("shell32.dll", EntryPoint="FindExecutable")] 
	private static extern long FindExecutableA(string file, 
											  string directory, 
											  StringBuilder res);
 
	public static string FindExecutable(string fileName){
		StringBuilder sb = new StringBuilder(1024);
		long res = FindExecutableA(fileName, string.Empty, sb);
		if(res >= 32){
			return sb.ToString();
		}
		else{
			return "";
		}
	}
 
	public static string FindExecutableByExtension(string extension){
		String tempFileName = null;
		String res = null;
		try{
			tempFileName = new TempFileCollection(Environment.GetEnvironmentVariable( _
  "TEMP"))
															 .AddExtension(extension);  
			System.IO.File.Create(tempFileName).Close();
			res = FindExecutable(tempFileName);
		}
		finally{
			System.IO.File.Delete(tempFileName);
		}
		return res;
	}
}

MessageBox.Show (FindExecutableUtils.FindExecutableByExtension("html"));