System.Data.SQLite

In case you missed the subject line, this only applies to NTFS volumes.  It also doesn't necessarily have to be a database file.  NTFS file compression is also transparent.  Once the compression bit is set, you continue to access the file normally.  It does not need to be decompressed.  The decompression function is shown below only for demonstration purposes.

using System;
using System.Data;
using System.Data.Common;
using System.Data.SQLite;
 
namespace test
{
  class Program
  {
    static void Main(string[] args)
    {
      SQLiteConnection.CompressFile("c:\\mydatabasefile.db3");
      SQLiteConnection.DecompressFile("c:\\mydatabasefile.db3");
    }
  }
}