Custom Linq Extension Methods

class ExpandableMethodAttribute :
        Attribute
{
    public ExpandableMethodAttribute()
    {
    }
}

static class Extensions
{
    // Example extension method
    //[ExpandableMethod]
    //public static IQueryable<YOUR_ENTITY> ExampleMethod(this IQueryable<YOUR_ENTITY> source, other args... )
    //{
    //  return source.Where( ... ) or something similar;
    //}

    public static IQueryable<T> AsExtendable<T>(this IQueryable<T> source)
    {
        if (source is ExtendableQuery<T>)
        {
            return (ExtendableQuery<T>)source;
        }

        return new ExtendableQueryProvider(source.Provider).CreateQuery<T>(source.Expression);
    }
}

class ExtendableVisitor : System.Linq.Expressions.ExpressionVisitor
{
    IQueryProvider _provider;

    internal ExtendableVisitor(IQueryProvider provider)
    {
        _provider = provider;
    }

    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
        bool expandNode = node.Method.GetCustomAttributes(typeof(ExpandableMethodAttribute), false).Any();
        if (expandNode && node.Method.IsStatic)
        {
            object[] args = new object[node.Arguments.Count];
            args[0] = _provider.CreateQuery(node.Arguments[0]);

            for (int i = 1; i < node.Arguments.Count; i++)
            {
                Expression arg = node.Arguments[i];
                args[i] = (arg.NodeType == ExpressionType.Constant) ? ((ConstantExpression)arg).Value : arg;
            }

            Expression exp = ((IQueryable)node.Method.Invoke(null, args)).Expression;

            return exp;
        }

        return base.VisitMethodCall(node);
    }
}

class ExtendableQuery<T> : IQueryable<T>, IOrderedQueryable<T>
{
    ExtendableQueryProvider _provider;
    Expression _expression;

    public ExtendableQuery(ExtendableQueryProvider provider, Expression expression)
    {
        _provider = provider;
        _expression = expression;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return _provider.ExecuteQuery<T>(_expression).GetEnumerator();
    }

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

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

    public Expression Expression
    {
        get {
            return _expression;
        }
    }

    public IQueryProvider Provider
    {
        get {
            return _provider;
        }
    }


}

class ExtendableQueryProvider : IQueryProvider
{
    IQueryProvider _underlyingQueryProvider;

    private ExtendableQueryProvider()
    {
    }

    internal ExtendableQueryProvider(IQueryProvider underlyingQueryProvider)
    {
        _underlyingQueryProvider = underlyingQueryProvider;
    }

    public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
    {
        return new ExtendableQuery<TElement>(this, expression);
    }

    public IQueryable CreateQuery(Expression expression)
    {
        Type elementType = expression.Type.GetElementType();
        try
        {
            return (IQueryable)Activator.CreateInstance(typeof(ExtendableQuery<>).MakeGenericType(elementType), new object[] { this, expression });
        }
        catch (System.Reflection.TargetInvocationException tie)
        {
            throw tie.InnerException;
        }
    }

    internal IEnumerable<T> ExecuteQuery<T>(Expression expression)
    {
        return _underlyingQueryProvider.CreateQuery<T>(Visit(expression)).AsEnumerable();
    }

    public TResult Execute<TResult>(Expression expression)
    {
        return _underlyingQueryProvider.Execute<TResult>(Visit(expression));
    }

    public object Execute(Expression expression)
    {
        return _underlyingQueryProvider.Execute(Visit(expression));
    }

    private Expression Visit(Expression exp)
    {
        ExtendableVisitor vstr = new ExtendableVisitor(_underlyingQueryProvider);
        Expression visitedExp = vstr.Visit(exp);

        return visitedExp;
    }
}

// Usage
// YOUR_OBJECT_SET.AsExtendable().ExampleMethod( ... );