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A Portable Class Library (PCL) for low-level (raw) access to SQLite
C C++ C# Other
branch: master

update the readme for 0.8.0

latest commit 786afe4c2f
Eric Sink authored

README.md

SQLitePCL.raw

SQLitePCL.raw is a Portable Class Library (PCL) for low-level (raw) access to SQLite.

Is this open source?

Yes. Apache License v2.

What is a Portable Class Library (PCL)?

A Portable Class Library is not merely a class library which happens to avoid using things that are not portable. The PCL concept also includes tooling support. You can tell the compiler which .NET targets you want to support (a profile) and then it will complain at you if you try to use something that wouldn't work.

Which flavor of PCL is this?

See my blog entry about the two different ways of doing PCLs.

SQLitePCL.raw uses the Bait and Switch approach. It provides a portable assembly (the Bait) which can be referenced by projects that are libraries. When building an executable or app, reference the appropriate platform-specific alternative.

Which platform assemblies are provided in this library?

In the pcl subdirectory, each csproj with a name of the form SQLitePCL.platform.* contains an implementation of a platform assembly.

The following platforms are currently supported:

  • .NET 4.5 on Windows or Mac OS X

  • .NET 4.0 on Windows or Mac OS X

  • Windows Phone 8

  • Windows Phone 8.1, both SL and RT flavors

  • WinRT

  • Xamarin.iOS, both Classic and Unified

  • Xamarin.Android

Some of these platforms have more than one implementation of the platform assembly, for reasons explained later in this README.

Is this available in NuGet?

Yes. The package ID is SQLitePCL.raw

There's another NuGet package called SQLitePCL.raw_basic. What is it??

That's the old package ID. With the 0.8.0 release, I transitioned to just 'SQLitePCL.raw' as the package ID (without the _basic suffix). Eventually I may stop publishing updates to the old ID. For now, the package gets published under both IDs and they are identical.

Is this available in the Xamarin Component Store?

No.

What is the state of this code? Is it completely done?

(I'm not sure it'll ever be completely done, but...)

The code itself is in pretty good shape. There are some more tests I want to write. There are some platform configurations I still want to support.

Look in todo.txt for an informal working list of tasks pending. This file is not always kept terribly current. :-)

How do I build this?

For all practical purposes, it's impossible. :-)

But if you're dying to try it, look at gen_build.cs. It's a script which generates the build system. I build everything on Windows. The script will expect you to have Visual Studio 2012 and Visual Studio 2013, the latter with Update 2 installed for the Phone 8.1 stuff. You also need the Xamarin stuff installed. And the relevant Android SDK(s).

Which PCL profile is supported?

The build system contains projects to build the PCL with profile 78, profile 136, profile 158, and profile 259. Others might work as well, but those are the ones I have tried.

The test suites are configured to use the profile 78 version in most places, but profile 259 for Windows Phone 8.1.

The main reason to keep profile 158 is because MvvmCross is using it. If they end up switching to the new reflection APIs and profile 259, then 158 won't be very important here either. Folks are saying that profile 259 is the new 78.

Can this library be used to write a mobile app?

Technically, yes, but that's not what you want to do. This is not the sort of SQLite library you would use to write an app. It is a very thin C# wrapper around the C API for SQLite. It's "raw".

Consequently, as much as possible, this library follows the stylistic conventions of SQLite, not those of the .NET/C# world.

For example, the C function for opening a SQLite file is sqlite3_open(), so this API provides a method called sqlite3_open(), not Sqlite3Open().

Similarly, the functions in this API return integer error codes rather than throwing .NET exceptions, because that's how the SQLite C API works.

As a library for app developers, this library is downright hostile. It feels like using C. Intentionally.

So if this library is so unfriendly, why does it exist at all?

This library is designed to be the common portable layer upon which friendlier wrappers can be built. Right now, every C# SQLite library writes their own P/Invoke and COM and marshaling and stuff. Build on this library instead and focus more on the upper layer and its goal of providing a pleasant, easy-to-use API for app developers.

How does this compare to sqlite-net?

sqlite-net is a very popular SQLite wrapper by Frank Krueger (@praeclarum). Unlike SQLitePCL.raw, it is designed to make writing apps easier. It even includes a lightweight ORM, and some basic support for LINQ.

SQLitePCL.raw wants to replace the bottom half of sqlite-net so that it can become a PCL.

In fact, that has happened. Frank Krueger has released a NuGet package (sqlite-net-pcl) which is SQLite-net with SQLitePCL.raw underneath:

https://www.nuget.org/packages/sqlite-net-pcl/

When people ask me to recommend a friendlier SQLite wrapper, sqlite-net is the one that I usually recommend.

How does this compare to SQLitePCL.pretty?

SQLitePCL.pretty is another friendly SQLite API wrapper. It is built on top of SQLitePCL.raw, and its name resembles my SQLitePCL.Ugly wrapper (described below), but I am not the developer, and I'm afraid I have no actual experience using it. SQLitePCL.pretty is developed by @bordoley, who has also been a contributor of several fine pull requests to SQLitePCL.raw itself.

@bordoley's own description of SQLitePCL.pretty:

"It is designed to make interacting with the SQLite API easier, exposing the full feature set of SQLite in an idiomatic and pretty C# API.

Interesting features include the ability to iterate through query result sets using LINQ, support for binary streaming of data in and out of SQLite using .NET streams, and a powerful async API built on the RX framework."

How does this compare to the SQLite stuff in MvvmCross?

Same story. MvvmCross is using a PCL-ified fork of sqlite-net. I would welcome MvvmCross to use my version of sqlite-net built on SQLitePCL.raw.

How does this compare to SQLite.Net-PCL?

SQLite.Net-PCL appears to be a PCL fork of sqlite-net, apparently with a bit more proactive stance toward eliminating ifdefs.
I don't know much else about it.

How does this compare to System.Data.SQLite?

System.Data.SQLite is an ADO.NET-style SQLite wrapper developed by the core SQLite team. It is very full-featured, supporting LINQ and Entity Framework. And for obvious reasons, it does a fantastic job of the SQLite side of things. But it is not at all mobile-friendly.

How does this compare to Mono.Data.Sqlite?

Mono.Data.Sqlite is an ADO.NET-style SQLite wrapper which is built into Mono and the Xamarin platform. It shares a common ancestry with System.Data.SQLite, as both began as forks from the same code.

How does this compare to SQLitePCL?

SQLitePCL is a SQLite Portable Class Library released on Codeplex by MS Open Tech.

This library is a fork of that code. Sort of.

It is a fork in the 2007 sense of the word. I made significant use of the code. I preserved copyright notices.

However, this is not the the sort of fork which is created for the purpose of producing a pull request. The changes I've made are so extensive that I do not plan to submit a pull request unless one is requested. I plan to maintain this code going forward.

So how is SQLitePCL.raw different from what you started with?

Since beginning with the SQLitePCL code, I've made a bunch of improvements to the build and its supported platforms:

  • Added support for more PCL profiles

  • Added build support for Xamarin.iOS

  • Added build support for Xamarin.Android

  • Added support for Windows Phone 8.1, RT and SL

  • Updated WinRT to support both P/Invoke and C++ Interop

  • Updated Net45 to support both P/Invoke and C++ Interop

  • Added a new set of automated test projects, sharing the same test code for MS Test and NUnit

  • Simplified the platform injection interfaces from 3 down to 1

  • Reorganized the tree to eliminate code duplication

  • Lots of bug fixes

I've also implemented nearly the entire SQLite API, including the following highlights which were not present in the original code:

  • Have SQLite call a .NET delegate when a transaction ends (sqlite3_commit_hook, sqlite3_rollback_hook)

  • Give SQLite a .NET delegate to be called for diagnostic purposes (sqlite3_trace, sqlite3_profile)

  • Write custom functions and collations in managed code (sqlite3_create_function, sqlite3_create_collation)

  • Access SQLite blobs using the streaming API, to avoid the requirement of reading the entire blob into memory (sqlite3_blob_etc)

  • Obtain the SQLite extended result codes and error messages

  • Use the SQLite backup API (sqlite3_backup_etc)

  • Ask the SQLite library for information about itself (sqlite3_compileoption_*, sqlite3_libversion, etc)

Bottom line, this library is a robust low-level wrapper which stays very true to the SQLite library itself, while presenting a portable API to the layer above.

So what is the architecture of this library?

Even within this thin library, there are three layers to be explained:

At the very bottom is the SQLite library itself. Written in C. This is unmanaged code, compiled for x86 or ARM or whatever.

Building on that, SQLitePCL.raw has three layers.

(1) We need something that can call the C code. For the Xamarin implementations this is DllImport (aka P/Invoke). For Windows Phone 8, this is a C++ wrapper. For the others, it is both. In either case, this presents an extremely low-level API. C pointers become System.IntPtr. Strings are not actually .NET strings, but rather, are in utf-8 encoding, represented in C# as either an IntPtr or a byte[].

(2) Next layer up, we have a C# layer which makes things just a tiny bit friendlier. Not much. It maps strings to/from utf8. And it manages delegates. The implementation of this layer varies depending on which assembly it is. For the PCL assembly itself (the Bait assembly), this is implemented as a set of stubs that throw errors.

(3) Finally, we have one more layer called "raw".
This layer is the top layer of the PCL, the one that is presented publicly. It is identical in all of the assemblies, portable or not. It adds one more C# nicety, which is that all IntPtrs are packaged up inside typed wrapper classes. For example, at the level of the C API, a database connection is represented by a sqlite3*. One layer up, inside the C# code, this becomes an IntPtr, and it remains an IntPtr at each layer until the top one (raw) which instead uses an instance of the sqlite3 class, which does nothing much except contain an IntPtr. In other words, it adds nothing except type checking.

For example, consider the C function sqlite3_prepare_v2(). In C, this function looks like this:

int sqlite3_prepare_v2(
  sqlite3* db,
  const char* pzSql,
  int nByte,
  sqlite3_stmt** ppStmt,
  const char** pzTail
  );

One layer up, it becomes:

[DllImport(SQLITE_DLL, CallingConvention = CallingConvention.Cdecl)]
public static extern int sqlite3_prepare_v2(
  IntPtr db, 
  byte[] pSql, 
  int nBytes, 
  out IntPtr stmt, 
  out IntPtr ptrRemain
  );

Both the sqlite3 and the sqlite3_stmt pointers became IntPtr, thus losing their type info and our ability to distinguish them by type. The string argument became a byte[], because it's utf8. The pzTail argument for returning a string becomes an IntPtr, but is also a utf8 C string.

One layer up, in the interface, the function looks like this:

int sqlite3_prepare_v2(
  IntPtr db, 
  string sql, 
  out IntPtr stmt, 
  out string remain
  );

The utf8 stuff is gone, and we've got strings. But IntPtr is still there.

Finally, in the raw API, this function is:

static public int sqlite3_prepare_v2(
  sqlite3 db, 
  string sql, 
  out sqlite3_stmt stmt, 
  out string tail
  );

The sqlite3 and sqlite3_stmt classes are those typed wrappers for IntPtrs that I mentioned. They have the exact same names as their counterparts in the SQLite C code.

Is there anything else in those IntPtr classes?

Actually, yes, glad you asked. Several of them support IDisposable as well. And there is also a little bit of plumbing to make sure that each pointer from the C layer lives inside only one instance of its corresponding IntPtr class.

So those IntPtr classes don't have any methods on them?

Nope.

What is SQLitePCL.Ugly?

Well, it's a bunch of extension methods for the IntPtr classes. It's like a fourth layer which provides method call syntax. It also switches the error handling model from integer return codes to exception throwing.

For example, the sqlite3_stmt class represents a statement handle, but you still have to do things like this:

int rc;

sqlite3 db;
rc = raw.sqlite3_open(":memory:", out db);
if (rc != raw.SQLITE_OK)
{
    error
}
sqlite3_stmt stmt;
rc = raw.sqlite3_prepare(db, "CREATE TABLE foo (x int)", out stmt);
if (rc != raw.SQLITE_OK)
{
    error
}
rc = raw.sqlite3_step(stmt);
if (rc == raw.SQLITE_DONE)
{
    whatever
}
else
{
    error
}
raw.sqlite3_finalize(stmt);

The Ugly layer allows me to do things like this:

using (sqlite3 db = ugly.open(":memory:"))
{
    sqlite3_stmt stmt = db.prepare("CREATE TABLE foo (x int)");
    stmt.step();
}

This exception-throwing wrapper exists so that I can have something easier against which to write tests. It retains all the "lower-case and underscores" ugliness of the layer(s) below.
It does not do things "The C# Way". As such, this is not a wrapper intended for public consumption.

But why did you have to make the Ugly layer, er, ugly?

I am very familiar with the underlying SQLite C API. I just wanted to write tests against something similar to it.

Wait a minute -- you're not a true camelCase believer, are you?

Guilty. I actually kinda like the old "lower-case and underscores" convention from my Unix days.

Also, sometimes when I am driving alone in my truck, I listen to country music.

What are the SQLitePCL.Test.* projects?

The test suite is a shared code file located in src/test/test_cases.cs. Each platform has a test project. For .NET, Windows Phone, and WinRT, I use the unit testing features of Visual Studio (MS Test). For iOS and Android, I use the unit testing features built into Xamarin Studio (NUnit).

Why do some tests fail on iOS and/or Android?

Because the version of SQLite preinstalled on the device or emulator is too old.

For example, the function sqlite3_close_v2() was added in SQLite version 3.7.14. As of Android KitKat, all versions of Android have shipped with SQLite 3.7.11 or older.

In practice, these issues are commonly handled by avoiding the use of new-ish SQLite functions. Alternatively, you can bundle a recent version of SQLite into your mobile app rather than using the build that is preinstalled on the platform. (See below for how to get SQLitePCL.raw to do this for you.)

Why do some of the platforms have multiple platform assemblies?

Because there are multiple options available in how the native SQLite code gets included in an app.

First of all, there are two ways to call native code from C#:

  1. P/Invoke

  2. C++ Interop

Xamarin platforms only support P/Invoke. The Silverlight flavors of Windows Phone 8 only support C++ Interop. The other platforms support both.

P/Invoke requires a platform-native shared library (such as a .dll or .so).

The C++ Interop builds include the SQLite bits directly, resulting in a mixed mode assembly.

On iOS and Android, a sqlite3 library is included as part of the OS. Most people just use it.

Some people want/need to include a SQLite version bundled as part of their app. Reasons to do this include:

  1. They want something more recent than the version provided with iOS/Android.

  2. They're on WinRT or something else that doesn't bundle SQLite at all.

  3. They need a custom SQLite, built with compile options customized for their situation. A common case here is wanting to use the FTS (full text search) feature of SQLite.

  4. They are substituting SQLCipher as their implementation of SQLite.

And then, consider the fact that for a single processor (Intel, ARM, etc) the native SQLite code can be either 32 bit or 64 bit, and the distinction is important.

This library wants to support all of these use cases and hide all the details behind a PCL.

But why? Why are you making this so complicated?

Hey, don't blame me. I'm not making this complicated. I'm just trying to support all the valid use cases.

And more importantly, it is critical for the app to get this right. Here's why:

If you have two instances of the SQLite library linked into your app, you can corrupt a SQLite database file.

That's bad. And it's kind of an easy mistake to make, especially on mobile platforms where the OS provides SQLite preinstalled.

Whoa. I don't want my SQLite files corrupted. How do I make sure that won't happen?

On iOS/Android, you have two choices:

  1. Only use the SQLite provided by the OS. Make sure your that no part of your app bundles another copy of the SQLite library.

  2. Understand all the linkage issues and be very careful.

On other platforms, make sure you are including exactly one instance of the SQLite library.

How can I use SQLitePCL.raw with a recent version of the sqlite3 lib on iOS/Android?

Don't even ask this question until you read the previous three questions above and understand the risks.

OK, I understand the risks. How do I do this?

As of SQLitePCL.raw 0.8.0, you can use a bundled/recent version of sqlite3 by setting an MSBuild property in your project file. The name of the property is "UseSQLiteFrom", and if you set this property value to "packaged_sqlite3" then SQLitePCL.raw will choose an assembly with a bundled copy of the native sqlite3 library instead of referencing the one built-in to iOS/Android.

<UseSQLiteFrom>packaged\_sqlite3</UseSQLiteFrom>

This feature does not work for iOS Classic (but iOS Unified, the new 64-bit stuff you should be using anyway, works fine).

For those native sqlite3 libraries built-in to SQLitePCL.raw: How were they compiled?

SQLITE_DEFAULT_FOREIGN_KEYS=1
SQLITE_ENABLE_FTS4
SQLITE_ENABLE_FTS3_PARENTHESIS
SQLITE_ENABLE_COLUMN_METADATA

Is SQLitePCL.raw compatible with SQLCipher?

<UseSQLiteFrom>packaged\_sqlcipher</UseSQLiteFrom>

This only works on iOS Unified and Android.

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