How to wrap Entity Framework to intercept the LINQ expression just before execution?

Question

I want to rewrite certain parts of the LINQ expression just before execution. And I\'m having problems injecting my rewriter in the correct place (at all actually).

Answer 1

I have exactly the sourcecode you'll need - but no idea how to attach a File.

Here are some snippets (snippets! I had to adapt this code, so it may not compile):

IQueryable:

public class QueryTranslator : IOrderedQueryable
{
    private Expression _expression = null;
    private QueryTranslatorProvider _provider = null;

    public QueryTranslator(IQueryable source)
    {
        _expression = Expression.Constant(this);
        _provider = new QueryTranslatorProvider(source);
    }

    public QueryTranslator(IQueryable source, Expression e)
    {
        if (e == null) throw new ArgumentNullException("e");
        _expression = e;
        _provider = new QueryTranslatorProvider(source);
    }

    public IEnumerator GetEnumerator()
    {
        return ((IEnumerable)_provider.ExecuteEnumerable(this._expression)).GetEnumerator();
    }

    IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return _provider.ExecuteEnumerable(this._expression).GetEnumerator();
    }

    public Type ElementType
    {
        get { return typeof(T); }
    }

    public Expression Expression
    {
        get { return _expression; }
    }

    public IQueryProvider Provider
    {
        get { return _provider; }
    }
}

IQueryProvider:

public class QueryTranslatorProvider : ExpressionTreeTranslator, IQueryProvider
{
    IQueryable _source;

    public QueryTranslatorProvider(IQueryable source)
    {
        if (source == null) throw new ArgumentNullException("source");
        _source = source;
    }

    #region IQueryProvider Members

    public IQueryable CreateQuery(Expression expression)
    {
        if (expression == null) throw new ArgumentNullException("expression");
        return new QueryTranslator(_source, expression) as IQueryable;
    }

    public IQueryable CreateQuery(Expression expression)
    {
        if (expression == null) throw new ArgumentNullException("expression");

        Type elementType = expression.Type.FindElementTypes().First();
        IQueryable result = (IQueryable)Activator.CreateInstance(typeof(QueryTranslator<>).MakeGenericType(elementType),
            new object[] { _source, expression });
        return result;
    }

    public TResult Execute(Expression expression)
    {
        if (expression == null) throw new ArgumentNullException("expression");
        object result = (this as IQueryProvider).Execute(expression);
        return (TResult)result;
    }

    public object Execute(Expression expression)
    {
        if (expression == null) throw new ArgumentNullException("expression");

        Expression translated = this.Visit(expression);

        return _source.Provider.Execute(translated);            
    }

    internal IEnumerable ExecuteEnumerable(Expression expression)
    {
        if (expression == null) throw new ArgumentNullException("expression");

        Expression translated = this.Visit(expression);

        return _source.Provider.CreateQuery(translated);
    }

    #endregion        

    #region Visits
    protected override MethodCallExpression VisitMethodCall(MethodCallExpression m)
    {
        return m;
    }

    protected override Expression VisitUnary(UnaryExpression u)
    {
         return Expression.MakeUnary(u.NodeType, base.Visit(u.Operand), u.Type.ToImplementationType(), u.Method);
    }
    #endregion
}

Usage (warning: adapted code! May not compile):

private Dictionary _table = new Dictionary();
public override IQueryable GetObjectQuery()
{
    if (!_table.ContainsKey(type))
    {
        _table[type] = new QueryTranslator(
            _ctx.CreateQuery("[" + typeof(T).Name + "]"));
    }

    return (IQueryable)_table[type];
}

Expression Visitors/Translator:

http://blogs.msdn.com/mattwar/archive/2007/07/31/linq-building-an-iqueryable-provider-part-ii.aspx

http://msdn.microsoft.com/en-us/library/bb882521.aspx

EDIT: Added FindElementTypes(). Hopefully all Methods are present now.

    /// 

    /// Finds all implemented IEnumerables of the given Type
    /// 
    public static IQueryable FindIEnumerables(this Type seqType)
    {
        if (seqType == null || seqType == typeof(object) || seqType == typeof(string))
            return new Type[] { }.AsQueryable();

        if (seqType.IsArray || seqType == typeof(IEnumerable))
            return new Type[] { typeof(IEnumerable) }.AsQueryable();

        if (seqType.IsGenericType && seqType.GetGenericArguments().Length == 1 && seqType.GetGenericTypeDefinition() == typeof(IEnumerable<>))
        {
            return new Type[] { seqType, typeof(IEnumerable) }.AsQueryable();
        }

        var result = new List();

        foreach (var iface in (seqType.GetInterfaces() ?? new Type[] { }))
        {
            result.AddRange(FindIEnumerables(iface));
        }

        return FindIEnumerables(seqType.BaseType).Union(result);
    }

    /// 

    /// Finds all element types provided by a specified sequence type.
    /// "Element types" are T for IEnumerable<T> and object for IEnumerable.
    /// 
    public static IQueryable FindElementTypes(this Type seqType)
    {
        return seqType.FindIEnumerables().Select(t => t.IsGenericType ? t.GetGenericArguments().Single() : typeof(object));
    }