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A generic implementation of the repository pattern in C#.
C# PowerShell
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README.md

Repository

Repository is a generic implementation of the Repository pattern in C#. It provides a repository base class Repository<T> that exposes functions to store/retrieve data, and an object context base class ObjectContext<T> that enables manipulation of data once it's retrieved. It also exposes an Items property that returns an IQueryable<T> which can be used to perform LINQ queries on the repository.

Implementations

This (code) repository comes with a few Repository implementations. The first and simplest is InMemoryRepository, which acts as a temporary in-memory store useful mainly for testing. The second is an Entity Framework repository named EFRepository, which uses Entity Framework as its storage interface. The thid is AzureRepository, an implementation of Repository for Azure Blob Storage. And the fourth is FileSystemRepository, an implementation that serializes objects directly to the filesystem.

Pull requests for additional implementations (and improvements to existing ones) are welcome and encouraged.

Change Tracking

As you'll see in the examples below, changes to objects accessed via a Repository instance are tracked automatically and committed back to the underlying data store whenever a call to SaveChanges is made. This functionality is enabled automatically for any class which derives from Repository; if you're writing a new implementation, you'll get change tracking for free. See the 'Modify an object' example below to get an idea for how this works.

Examples

Store an object:

var repository = new MyConcreteRepository<int>();
repository.Insert(1); // Store a single value
repository.SaveChanges();


repository.Insert(new[] { 2, 3, 4, 5 }); // Store a bunch of values
repository.SaveChanges();

Retrieve an object:

class MyClass 
{
    public String Key { get; set; }
}

/* ... */

using (var repository = new MyConcreteRepository<MyClass>())
{
    var objectContext = repository.Find("myKey");
    var obj = objectContext.Object;

    // Do cool things with this value
}

Query stored objects:

class MyClass
{
    public String Key { get; set; }
    public int Value { get; set; }
}

/* ... */

using (var repository = new MyConcreteRepository<MyClass>())
{
    // Query objects based on their Value field
    var filteredItems = repository.Items.Where(x => x.Value > 5);

    // Do stuff with query result
}

Remove an object:

class MyClass 
{
    public String Key { get; set; }
}

/* ... */

using (var repository = new MyConcreteRepository<MyClass>())
{
    repository.RemoveByKey("myKey");
    repository.SaveChanges();

    // Or...

    var obj = new MyClass { Key = "myKey" };
    repository.Remove(obj);
    repository.SaveChanges();

    // Or...

    repository.RemoveByKey("myKey");
    repository.SaveChanges();

    // Or...

    repository.RemoveAllByKey("myKey");
    repository.SaveChanges();

    // Or...
    repository.RemoveAll();
    repository.SaveChanges();
}

Modify a stored object:

class MyClass 
{
    public String Key { get; set; }
    public int Value { get; set; }
}

/* ... */

// Pull the object from the repository
using (var repository = new MyConcreteRepository<MyClass>())
{
    // Pull the object from the repository
    var valueContext = repository.Find("myKey");

    // Modify it
    valueContext.Object.Value = 5;

    // Save the changes
    repository.SaveChanges();
}

Typed Key Repositories

Stored objects are accessed by their keys, as in the examples above. Normally, keys are passed into Repository methods as untyped params arguments; as a result, the compiler will not perform any checks for either the number or types of keys passed in. This allows for flexibility in the types of keys you can use, but also negates some of the benefits of having a strongly-typed generic repository in the first place.

In order to fix this potential problem, this library contains a series of Repository base classes that take type parameters for keys. These classes simply wrap the "untyped" repositories with strongly-typed versions of certain methods, like Find and RemoveByKey. In order to make typed versions of your repositories available, simply derive from the strongly-typed versions of Repository and pass in instances of your untyped repositories to the base constructor.

For example, EFRepository exposes strongly-typed versions (for classes with up to two key values) like so:

public class EFRepository<TContext, TValue, TKey> : Repository<TValue, TKey> where TValue : class where TContext : DbContext
{
    //===============================================================
    public EFRepository(Func<TContext, DbSet<TValue>> setSelector, TContext context = null)
        : base(new EFRepository<TContext, TValue>(setSelector, context))
    {}
    //===============================================================
}

public class EFRepository<TContext, TValue, TKey1, TKey2> : Repository<TValue, TKey1, TKey2>
    where TValue : class
    where TContext : DbContext
{
    //===============================================================
    public EFRepository(Func<TContext, DbSet<TValue>> setSelector, TContext context = null)
        : base(new EFRepository<TContext, TValue>(setSelector, context))
    { }
    //===============================================================
}

All these classes do is instantiate untyped instances of EFRepository and pass them into the base Repository constructor. Once instantiated, these strongly-typed versions of EFRepository will enforce type safety on all key lookups. You can see this by looking at methods that make use of keys. For example, the signature of Find in the strongly-typed version of Repository looks like this:

public ObjectContext<TValue> Find(TKey key)
{
    return InnerRepository.Find(key);
}

Notice how, instead of params Object[], the sole argument here is of type TKey. This will allow the compiler to check for invalid lookups at compile-time, which can be very handy.

EFRepository

In order to create an instance of EFRepository, you need to first declare a derived class of DbContext with DbSet instances for all of the types for which you want to have repositories. An example DbContext might look like this:

class TestClass
{
    [Key]
    public String Key { get; set; }
}

/* ... */

class MyContext : DbContext
{
    public DbSet<TestClass> TestClasses { get; set; }
}

Once a subclass of DbContext is defined, you can instantiate an EFRepository for type TestClass like so:

var repository = new EFRepository<MyContext, TestClass>(x => x.TestClasses);

The sole argument to the constructor is a Func<MyContext, TestClass> that tells the repository how to find the appropriate DbSet in the DbContext.

Note that the procedures normally followed when using Entity Framework must still be followed here; that is, you should still specify connection strings in your Web.config and decorate your key properties with [Key] attributes if necessary.

AzureRepository

All you need for AzureRepository is a proper connection string for Azure blob storage. You can either pass this connection string directly to an instance of AzureRepository, or you can store it in a App.config/Web.config file and reference it by name. There's also a static ForStorageEmulator method that will create an instance of AzureRepository pointed at the local Azure storage emulator.

Re-using the TestClass class from above, instantiating an AzureRepository looks like this:

// From explicit connection string
var repository = AzureRepository<TestClass>.FromExplicitConnectionString(x => x.Key, myConnectionString);

// From named connection string
var repository = AzureRepository<TestClass>.FromNamedConnectionString(x => x.Key, "myConnectionStringName");

// For storage emulator
var repository = AzureRepository<TestClass>.ForStorageEmulator(x => x.Key);

There is also an AzureOptions class that allows you to specify how e.g. objects are serialized to blob storage, access rules for stored objects, content type, etc.

FileSystemRepository

FileSystemRepository is an implementation that (surprise) saves objects to the local filesystem. Instantiating one is very simple:

var repository = new FileSystemRepository<TestClass>(x => x.Key);

You can also optionally specify a FileSystemOptions parameter that configures how objects are serialized to disk, where they're stored, the file extension, etc.

Implementing Your Own Repositories

To understand how to go about doing implementing your own repositories, we need to briefly go over how repositories work under the hood.

All operations on Repository implementations are deferred; that is, they are not committed until a call to SaveChanges is made. This is achieved by storing a list of pending operations and applying them sequentially when SaveChanges is called. These pending operations are objects which implement the interface Operation, which has a single method called Apply:

public interface Operation
{
    //===============================================================
    void Apply();
    //===============================================================
} 

There are three main types of operations: Insert, Remove, and Modify. These are abstract base classes. Each Repository implementation will provide its own concrete versions of these classes which perform the necessary operations. As an example, the abstract Insert class looks like this:

public abstract class Insert<TValue> : Operation
{
    //===============================================================
    public Insert(IEnumerable<Object> keys, TValue value)
    {
        Keys = keys;
        Value = value;
    }
    //===============================================================
    public IEnumerable<Object> Keys { get; private set; }
    //===============================================================
    public TValue Value { get; private set; }
    //===============================================================
    public abstract void Apply();
    //===============================================================
}

Notice that this base class stores all of the information necessary to perform an insert into some underlying data store, namely the object to be inserted and the keys under which it should be stored.

Now take a look at an implementation of Insert, this one for FileSystemRepository:

internal class FileSystemInsert<T> : Insert<T>
{
    //===============================================================
    public FileSystemInsert(IEnumerable<object> keys, T value, FileSystemInterface<T> fsInterface)
        : base(keys, value)
    {
        FileSystemInterface = fsInterface;
    }
    //===============================================================
    private FileSystemInterface<T> FileSystemInterface { get; set; }
    //===============================================================
    public override void Apply()
    {
        FileSystemInterface.StoreObject(Value, Keys);
    }
    //===============================================================
}

All this class does in its Apply method is call a supplied FileSystemInterface object to perform the actual insert. The FileSystemInterface class is a utility class that handles filesystem manipulation, but you could just as easily implement the filesystem serialization logic directly in Apply. The point is that the implementation of Insert is responsible for initiating the actual insertion operation into the underlying data store.

In order to implement your own repositories, you have to implement three methods from the base Repository class which correspond to the three types of operations mentioned above:

//===============================================================
protected abstract Insert<T> CreateInsert(IEnumerable<object> keys, T value);
//===============================================================
protected abstract Remove CreateRemove(IEnumerable<object> keys);
//===============================================================
protected abstract Modify<T> CreateModify(IEnumerable<object> keys, T value, Action<T> modifier);
//===============================================================

These methods are responsible for returning implementations of the three types of operations, as shown above.

In addition to these methods, you must also implement three others:

//===============================================================
protected abstract ObjectContext<T> FindImpl(object[] keys);
//===============================================================
public abstract bool ExistsByKey(params Object[] keys);
//===============================================================
public abstract EnumerableObjectContext<T> Items { get; } 
//===============================================================

FindImpl is reponsible for looking up an object from the underlying data store, ExistsByKey is responsible for checking for an object's existence by its key values, and Items is responsible for enumerating the objects in the repository. These will likely call methods directly on the underlying data store or make use of some sort of interface such as FileSystemInterface (mentioned above). Again, the point is that these methods are responsible for interfacing with the underlying data store. As an example, if you were implementing a repository for Amazon S3 storage, the FindImpl method would likely call some sort of lookup method in the S3 API.

License

Repository is licensed under the New BSD License:

Copyright (c) 2013, Matthew Schrager
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
    * Neither the name of Matthew Schrager nor the
      names of its contributors may be used to endorse or promote products
      derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL MATTHEW SCHRAGER BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
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