Serialize an in Memory Sqlite DB C#

Question

I am trying to serialize my in memory Sqlite db, so I can convert it back into a file later on. I see how you would serialize the db in node.js How does `db.serialize` work in `node-sqlite3` and in C Serialize a database in C. I just have not been able to find a way to accomplish this in C#. If anyone has any insight it would be appreciated!
What I have so far

using System.Data.SQLite;
using Newtonsoft.Json;
using System.Text;

public byte[] SerializeDB(string sql)
{
   using(var con = new SQLiteConnection($"Data Source=:memory:;Version=3;New=True;")) {
     con.Open();
     using(SQLiteCommand cmd = new SQLiteCommand(sql, con)) {
       cmd.ExecuteNonQuery();

       return Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(**WHAT SHOULD GO HERE**));
     }
   }
}

Cincidial · Accepted Answer · 2024-07-25 20:45:56Z

I've been looking for a few days on a way to do this in C# as well.

SQLite defines as part of it's C-Interface a method to do just the thing: https://www.sqlite.org/c3ref/serialize.html

unsigned char *sqlite3_serialize(
  sqlite3 *db,           /* The database connection */
  const char *zSchema,   /* Which DB to serialize. ex: "main", "temp", ... */
  sqlite3_int64 *piSize, /* Write size of the DB here, if not NULL */
  unsigned int mFlags    /* Zero or more SQLITE_SERIALIZE_* flags */
);

The sqlite3_serialize(D,S,P,F) interface returns a pointer to memory that is a serialization of the S database on database connection D

For an ordinary on-disk database file, the serialization is just a copy of the disk file. For an in-memory database or a "TEMP" database, the serialization is the same sequence of bytes which would be written to disk if that database where backed up to disk.

However, the C# library I'm using (sqlite-net-base with SQLitePCLRaw.provider.winsqlite3 as the provider) does not DllImport this method as part of it's provider implementation. The method is still available though as the SQLite specification includes it.

So, what I've done is import the method myself along side the library. This has worked in my case (using winsqlite3). You should be able to do the same by changing the DLL name to the one you are using.

[DllImport("winsqlite3", CallingConvention = CallingConvention.StdCall, ExactSpelling = true)]
public static extern IntPtr sqlite3_serialize(sqlite3 db, [MarshalAs(UnmanagedType.LPStr)] string a, IntPtr piSize, uint mFlags);

In the docs there is an important note that

The caller is responsible for freeing the returned value to avoid a memory leak

As this is calling C code it's important to remember that C# won't be garbage collecting this for us until we get any memory it gives us back into C# managed memory.

So, a method to wrap this call and make it more C# friendly can be used:

private byte[] SeralizeDb()
{
    var lengthInput = Marshal.AllocHGlobal(sizeof(long));
    var unmanagedResult = sqlite3_serialize(this.sqliteDbConnection.Handle, "main", lengthInput, 0); // https://www.sqlite.org/c3ref/serialize.html main is mentioned as an example and is what works

    // Handle null result or a bad length.

    var lengthResult = (int)Marshal.ReadInt64(lengthInput); // Cast as int for the Marshel.Copy. Db is only a cache of a few Mb ever in our case
    var managedResult = new byte[lengthResult];
    Marshal.Copy(unmanagedResult, managedResult, 0, lengthResult);

    // Free unmanaged memory allocations
    Marshal.FreeHGlobal(lengthInput);
    Marshal.FreeHGlobal(unmanagedResult);

    return managedResult;
}

bmurf17 · Accepted Answer · 2022-08-04 19:43:47Z

0

After looking into further, I think that it is impossible. SQLite does not let you access the in memory file. This could work great if I was willing to back the file up and then access the backup. But then I would not have any need for the in memory file.

answered Aug 4, 2022 at 19:43

bmurf17

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