Introducing LazyLinq: Internals

August 17, 2009 — Keith Dahlby

This is the second in a series of posts on LazyLinq, a wrapper to support lazy initialization and deferred disposal of a LINQ query context:

  1. Introducing LazyLinq: Overview
  2. Introducing LazyLinq: Internals
  3. Introducing LazyLinq: Queryability
  4. Simplifying LazyLinq
  5. Introducing LazyLinq: Lazy DataContext

My first post introduced the three interfaces that LazyLinq provides. Next, we get to implement them.

Implementing ILazyQueryable

First, the interface:

    public interface ILazyQueryable<TContext, TSource, TQuery>
        : IQueryable<TSource>
        where TQuery : IQueryable<TSource>
    {
        ILazyContext<TContext> Context { get; }
        Func<TContext, TQuery> QueryBuilder { get; }
    }

We’ll start simple with an implicit implementation of the interface and a trivial constructor:

    class LazyQueryableImpl<TContext, TSource, TQuery>
            : ILazyQueryable<TContext, TSource, TQuery>
            where TQuery : IQueryable<TSource>
    {
            public ILazyContext<TContext> Context { get; private set; }
            public Func<TContext, TQuery> QueryBuilder { get; private set; }

            internal LazyQueryableImpl(ILazyContext<TContext> deferredContext, Func<TContext, TQuery> queryBuilder)
            {
                if (deferredContext == null) throw new ArgumentNullException("deferredContext");
                if (queryBuilder == null) throw new ArgumentNullException("queryBuilder");

                Context = deferredContext;
                QueryBuilder = queryBuilder;
            }

Next, a lazy-loaded query built from our lazy context:

            protected TQuery Query
            {
                get
                {
                    if (query == null)
                    {
                        query = QueryBuilder(Context.Context);
                        if (query == null)
                            throw new InvalidOperationException("Query built as null.");
                    }
                    return query;
                }
            }

And the internals of managing Context, which implements IDisposable:

            private void Dispose()
            {
                Context.Dispose();
                query = default(TQuery);
            }

            private IEnumerator<TSource> GetEnumerator()
            {
                try
                {
                    foreach (var i in Query)
                        yield return i;
                }
                finally
                {
                    Dispose();
                }
            }

Since Query depends on Context, once Context is disposed we need to reset Query so a new one can be built (if possible). Note that we use an iterator here to return an IEnumerator<TSource>, rather than the usual IEnumerable<>.

Finally, we’ll close out by explicitly implementing IQueryable:

            IEnumerator<TSource> IEnumerable<TSource>.GetEnumerator()
            {
                return GetEnumerator();
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }

            Type IQueryable.ElementType
            {
                get { return Query.ElementType; }
            }

            Expression IQueryable.Expression
            {
                get { return Query.Expression; }
            }

            IQueryProvider IQueryable.Provider
            {
                get { return Query.Provider; }
            }
        }

If this seemed relatively simple, you’re right. We’re just building a lazy-loaded Query proxy, with a bit of plumbing to clean up our Context.

Implementing ILazyOrderedQueryable

Not very exciting, but for completeness:

        public interface ILazyOrderedQueryable<TContext, TSource, TQuery>
            : ILazyQueryable<TContext, TSource, TQuery>, IOrderedQueryable<TSource>
            where TQuery : IOrderedQueryable<TSource>
        { }

        class LazyOrderedQueryableImpl<TContext, TSource, TQuery>
                : LazyQueryableImpl<TContext, TSource, TQuery>, ILazyOrderedQueryable<TContext, TSource, TQuery>
                where TQuery : IOrderedQueryable<TSource>
        {
            internal LazyOrderedQueryableImpl(ILazyContext<TContext> lazyContext, Func<TContext, TQuery> queryBuilder)
                : base(lazyContext, queryBuilder)
            {
            }
        }

LazyQueryable Factory

Consumers of this API should never need to know about these implementation details, so we can hide them behind a factory class:

        public static class LazyQueryable
        {
            public static ILazyQueryable<TContext, TResult, TQuery> CreateQuery<TContext, TResult, TQuery>(
                ILazyContext<TContext> context, Func<TContext, TQuery> queryBuilder)
                where TQuery : IQueryable<TResult>
            {
                return new LazyQueryableImpl<TContext, TResult, TQuery>(context, queryBuilder);
            }
            public static ILazyOrderedQueryable<TContext, TResult, TQuery> CreateOrderedQuery<TContext, TResult, TQuery>(
                ILazyContext<TContext> context, Func<TContext, TQuery> queryBuilder)
                where TQuery : IOrderedQueryable<TResult>
            {
                return new LazyOrderedQueryableImpl<TContext, TResult, TQuery>(context, queryBuilder);
            }
        }

Implementing ILazyContext

Again, we’ll start with the interface:

        public interface ILazyContext<TContext> : IDisposable
        {
            TContext Context { get; }

            ILazyQueryable<TContext, TResult, TQuery>
                CreateQuery<TResult, TQuery>(Func<TContext, TQuery> queryBuilder)
                where TQuery : IQueryable<TResult>;

            ILazyOrderedQueryable<TContext, TResult, TQuery>
                CreateOrderedQuery<TResult, TQuery>(Func<TContext, TQuery> queryBuilder)
                where TQuery : IOrderedQueryable<TResult>;

            TResult Execute<TResult>(Func<TContext, TResult> action);
        }

Now we can start fulfilling our requirements:

1. Lazily expose the Context.

        class LazyContextImpl<TContext> : ILazyContext<TContext>, IDisposable
        {
            public Func<TContext> ContextBuilder { get; private set; }

            private TContext context;
            public TContext Context
            {
                get
                {
                    if (context == null)
                    {
                        context = ContextBuilder();
                        if (context == null)
                            throw new InvalidOperationException("Context built as null.");
                    }
                    return context;
                }
            }

2. Produce lazy wrappers to represent queries retrieved from a context by a delegate.

            public ILazyQueryable<TContext, TResult, TQuery> CreateQuery<TResult, TQuery>(
                Func<TContext, TQuery> queryBuilder)
                where TQuery : IQueryable<TResult>
            {
                return LazyQueryable.CreateQuery<TContext, TResult, TQuery>(this, queryBuilder);
            }

            public ILazyOrderedQueryable<TContext, TResult, TQuery> CreateOrderedQuery<TResult, TQuery>(
                Func<TContext, TQuery> queryBuilder)
                where TQuery : IOrderedQueryable<TResult>
            {
                return LazyQueryable.CreateOrderedQuery<TContext, TResult, TQuery>(this, queryBuilder);
            }

3. Execute an action on the context.

There are two ways to “complete” a query, and we need to clean up context after each. The first was after enumeration, implemented above. The second is on execute, implemented here:

            public TResult Execute<TResult>(Func<TContext, TResult> expression)
            {
                try
                {
                    return expression(Context);
                }
                finally
                {
                    Dispose();
                }
            }

4. Ensure the context is disposed as necessary.

We don’t require that TContext is IDisposable, but we need to handle if it is. We also clear context to support reuse.

            public void Dispose()
            {
                var disposable = context as IDisposable;
                if (disposable != null)
                    disposable.Dispose();

                context = default(TContext);
            }

Constructors

With our requirements met, we just need a way to create our context. We provide two options:

            internal LazyContextImpl(TContext context) : this(() => context) { }

            internal LazyContextImpl(Func<TContext> contextBuilder)
            {
                if (contextBuilder == null) throw new ArgumentNullException("contextBuilder");

                ContextBuilder = contextBuilder;
            }

The former wraps an existing TContext instance in a closure, meaning every time ContextBuilder is called it returns the same instance. The latter accepts any delegate that returns a TContext. The most common such delegate would be a simple instantiation: () => new MyDataContext().

It should be clear now why we would want to clear our context on dispose. If ContextBuilder returns a new context instance each time, it’s perfectly safe to discard of the old (disposed) context to trigger the creation of a new one. Conversely, if the builder returns a single instance, using the context after disposal would trigger an ObjectDisposedException or something similar.

LazyContext Factory

For consistency, we should also provide factory methods to hide this specific implementation:

            public static class LazyContext
            {
                public static ILazyContext<T> Create<T>(T context)
                {
                    return new LazyContextImpl<T>(context);
                }
                public static ILazyContext<T> Create<T>(Func<T> contextBuilder)
                {
                    return new LazyContextImpl<T>(contextBuilder);
                }
            }

Lazy Extensions

And last, but certainly not least, we’re ready to reimplement our Use() extension methods:

    public static class Lazy
    {
        public static ILazyContext<TContext> Use<TContext>(this TContext @this)
        {
            return LazyContext.Create<TContext>(@this);
        }

        public static ILazyContext<TContext> Use<TContext>(this Func<TContext> @this)
        {
            return LazyContext.Create<TContext>(@this);
        }
    }

With several usage possibilities:

    var r1 = from x in new MyDataContext().Use() ...;

    Func<MyDataContext> f1 = () => new MyDataContext();
    var r2 = from x in f1.Use() ...;

    var r3 = from x in new Func<MyDataContext>(() => new MyDataContext()).Use() ...;

    var r4 = from x in Lazy.Use(() => new MyDataContext()) ...;

Or maybe we can make it even easier. Maybe…

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