TagBackupExample.cs

// The NBT tags to copy from the player file to the backup.
static string[] tagsToCopy = { "XpLevel", "XpP", "Inventory", "EnderItems" };

===

var player = TagBase.Load("<player>.dat"); // Load the file
var backup = new TagCompound(); // Create an empty NBT tag

// Copy over the wanted tags from the file to the empty NBT tag
for (var tag in tagsToCopy)
  // .Copy() or .Clone() would be a good idea here, but that's not in
  // the library at the moment. Though it doesn't matter in this case.
  backup[tag] = player[tag];

int experienceLevel = (int)player["XpLevel"];
float experienceProgress = (float)player["XpP"];

// Go over all the items in the inventory.
for (var item in (TagList)player["Inventory"]) {
  var slot = (byte)item["Slot"];
  var id = (string)item["id"];
  var count = (byte)item["Count"];
  var damage = (short)item["Damage"];
  
  // Do something with the item..?
  
  if (id == "minecraft:written_book") {
    var author = (string)item["tag"]["author"];
    var title = (string)item["tag"]["title"];
    var pages = ((TagList)item["tag"]["pages"]).Select(x => (string)x).ToList(); // Requires using System.Linq
    
    // Do something with books..?
  }
}

===

// Turn the backup tag into a byte array to store in the database
using (var ms = new MemoryStream()) {
  // Compressing might not be worth it, but you could use GZip compression of you wanted.
  backup.Save(backup, null, NbtCompression.None);
  var bytes = ms.ToArray();
  
  // Do something with raw NBT data..?
}

===

// Grab the backup data from the database and restore everything
var bytes = ... // Get this from the database somehow
using (var ms = new MemoryStream(bytes)) {
  var backup = TagBase.Load(ms, NbtCompression.None); // Wouldn't need to specify compression, will auto-detect it.
  var player = TagBase.Load("<player>.dat");          // Just like it does with the player file.
  
  // Copy backup data to player tag, overwriting them
  for (var tag in tagsToCopy)
    player[tag] = backup[tag];
  
  // Save player data again
  player.Save("<player>.dat", NbtCompression.GZip);
}