If you use ASP.NET MVC with F# CTP (Monday brings Beta 1 and probably many changes), you might run into an issue of how to scope certain things. For example, an IDisposable is used to create blocks, for things like an HTML form. Example in C#:

This is needed because VB and C# didn't have any easy function/block syntax (VB 10 should fix this), and many C# developers are still wary of higher order functions. How does this play out in F#? First off, whitespace is important. This can get really messy with the current F# ASP.NET integration. Basically, always open the script blocks on a separate line, and indent from the first column. Example:

<% let i = 0

   this.Response.Write(sprintf "i = %d" i) %>

(Note that the 'this' variable is bound to the current page.) This fails:

Compiler Error Message: FS0010: Unexpected keyword 'let' or 'use' in expression. Expected incomplete structured construct at or before this point or other token

Source Error:

Line 111: let mutable parameterContainer = parameterContainer Line 112: __w.Write("\r\n") |> ignore Line 113: let i = 0 Line 114: this.Response.Write(sprintf "i = %d" i) Line 115: __w.Write("\r\n <h2>Create</h2>\r\n\r\n ") |> ignore

Note how the <% is counted as space, so the let starts off indented 3 spaces. Instead, we need to write it so:

This works fine. You can also put the %> on the next line if you like. Now, on to ASP.NET MVC's IDisposable usage. A straightforward use of the F# using function won't work:

< b >Demo</b>

 

<%

using (Mvc.Html.FormExtensions.BeginForm this.Html) (fun _ -> %>

 

    <p>Inside a form</p>

 

<% ) %>

Compiler Error Message: FS0191: The mutable variable '__w' is used in an invalid way. Mutable variables may not be captured by closures. Consider eliminating this use of mutation or using a heap-allocated mutable reference cell via 'ref' and '!'.

Source Error:

Line 114: __w.Write("\r\n\r\n<b>Demo</b>\r\n\r\n") |> ignore Line 115: Line 116: using (Mvc.Html.FormExtensions.BeginForm this.Html) (fun _ -> Line 117: __w.Write("\r\n\r\n <p>Inside a form</p>\r\n\r\n") |> ignore Line 118: )

As the error says, this is because the __w variable is mutable, so we can't play with it inside a lambda. I'm not sure if this will be worked around -- they'd have to change the codegen quite a bit, I'd think. As a side note, the F# CTP does not support C# extension methods (hence the verbose calling of BeginForm), but F# will eventually -- maybe in the Beta.

The way we must scope is with a use binding. There's no way I see to accomplish this with whitespace alone. Due to the ASPX translation process, this would probably be very error prone. Instead, we can simply put the use binding inside a do expression:

< b >Demo</b>

 

<%

do (use form = Mvc.Html.FormExtensions.BeginForm this.Html %>

 

< b >Inside the form</b>

 

<% ) %>

 

< b >Outside of the form</b>

The parentheses setup the scope exactly how we want it.

Last time, I hacked up some rudimentary support for F# and Fluent NHibernate. It really looked ugly. I looked into what I could do to make it not look as sucky.

First off, having to write a full lambda for each mapping was annoying. F# quotations don't have to be lambdas, unlike C#'s expressions. So, instead of having to write, say, "fun x -> x.Foo", we can write "x.Foo", assuming there's a local variable x with the right type. The ClassMap subclass now expects these types of quotations than full lambdas.

Next, I experimented with using type extensions to overload functions like Id and Map, however I found out that F#, at least currently, does not add in type extensions for overload resolution. So I ended up having a new subclass of ClassMap<T>, 'T ClassMapQ. I used the Q suffix consistently to denote "quotations". I added type extensions for most of the other mapping types so that quotations could be used.

As to the question of having to tag on " |> ignore " to each mapping, I decided to write an extension propery for IMappingPart called Done, which is simply unit. Finally, the problem of lazy loading I took care of by setting "use_proxy_validator" to false, as mentioned here. The end result is that this mapping code:

            type
StoreMap() = inherit
ClassMap<Store>() dobase.Not.LazyLoad() base.Id
~@@ <@ fun x -> x.Id @> |> ignore base.Map
~@@ <@ fun x -> x.Name @> |> ignore (base.HasManyX
<@ fun x -> upcast x.Staff @>) .LazyLoad() .Inverse().Cascade.All() |>
ignore (base.HasManyToManyX
<@ fun x -> upcast x.Products @>) .Cascade.All() .WithTableName("StoreProduct")
|> ignore 
          

Is now:

type StoreMap() as m = inherit ClassMapQ<Store>() do
    let x = m.DefaultX
    (m.IdQ <@ x.Id @>).Done
    (m.MapQ <@ x.Name @>).Done
    (m.HasManyQ <@ seq x.Staff @>)
        .LazyLoad()
        .Inverse().Cascade.All()
        .Done
    (m.HasManyToManyQ <@ seq x.Products @>)
        .LazyLoad()
        .Cascade.All()
        .WithTableName("StoreProduct")
        .Done

This is pretty enough that I'm satisfied with how well F# interops. Most of the things I figured out here will apply to other .NET OO APIs, not just this one.

I am hoping that VS2010 Beta 1 will ship soon, as F# is getting some interesting upgrades then. With that, it should be easier to extend the support to other parts of NHibernate, such as querying, and perhaps integrate in NHibernate.Linq.

Fluent NHibernate is a nice way to be able to use NHibernate without having to deal with all that unchecked XML. This morning I decided to find out how well it works with F#. Things went relatively smooth. I've converted some code samples from the Fluent NHibernate First Project. I suggest having that open to fill in any gaps. I've also included the full project code and DB script at the end of this article.

If you're not familiar at all with Fluent NHibernate, basically it takes advantage of lambda expressions as Expression<T> to provide a somewhat strongly typechecked reflection system. Thus, instead of having attributes with hard-coded strings, or XML files, you have expressions that target the properties of objects you wish to map. The Fluent NHibernate library then takes care of hooking it up to NHibernate, and away you go. Something like that anyways.

Classes

So, first, we define the "entities" like the C# project does. Here's the first little pain. F# doesn't really support C#'s idea of "automatic properties". You can have vals on a class, which act like fields (although, they are implemented as properties). Or, you can manually specify them, like you would in earlier versions of C# which didn't have the auto-gen-a-field-for-me.

F# doesn't encourage uninitialized values. If you have uninitialized fields (vals), you need to mark them with an attribute to say you know what you're doing. So, that adds a bit more code overhead. What I do here is to alias the DefaultValueAttribute to "DV". So the first bit of our classes looks like this:

Now, NHibernate relies on having virtual properties so that it can dynamically create code to do nifty things like lazy loading. In F#, creating a virtual member means defining an abstract member and providing an implementation, in the same class. Since we don't have automatic properties, this means we'll have to define the backing field ourselfs. In all, the full code for the virtual property "Store" (virtual so NHibernate can lazy-load it) is:

Not the pinnacle of short code, but not horrible all things considered. The rest of the entity mappings are rather straightforward so I'll skip them here. I stuck with vals for anything that didn't have to be virtual, to keep it more concise.

Mappings: Easy Start

OK, so now to the "real" work. Fluent NHiberate looks for classes that subclass ClassMap<T>. It then creates an instance of them, which allows you to call the mapping methods in the object's constructor. From the First Project:

          
            public
            class EmployeeMap
: ClassMap<Employee> { public EmployeeMap()
{ Id(x => x.Id); } } 
        

OK, so how do we convert this to F#? It's easy... except for that lambda. The lambda in this case compiles to an Expression<Func<Employee, object>>. F#'s compiler does not support Expression<T>. So, we turn to experimental support. Referencing the FSharp.PowerPack.Linq assembly gives us the Microsoft.FSharp.Linq.QuotationEvaluation module. This extends the F# quotation type, Expr, with "ToLinqExpression", which returns an untyped LINQ Expression object.

To get this untyped Expression (LINQ) out of an Expr (F#) and into an Expression<T> (F#) suitable for Fluent NHibernate's consumption, I started off writing this tiny helper module:

When an F# quotation of a lambda is turned into a LINQ expression, the root node is a useless MethodCallExpression. So, we unwrap that by taking it's argument and using it to generate a typed lambda Expression<T>.  

This is a good start. But many of the Expressions that Fluent NHibernate looks for have a return type of object. Instead of having to write "box" or "upcast" all over, I added another function called "ToLinqObj". This takes an Expr<'a -> 'b>, but returns an Expression<Func<'a, obj>>.

Finally, writing "ToLinq" and "ToLinqObj" seemed too verbose, so I added some operators to the LinqHelper module:

Now we can start mapping:

We start off by disabling LazyLoad because most of the properties are not virtual, and NHibernate will fail to validate the mapping. Instead, we explicitly LazyLoad things, like the Store reference.

Mappings: Modifying the Expressions

Unfortunately, things didn't stay so simple. The Fluent NHibernate methods "HasMany" and "HasManyToMany", for instance, don't work with F#'s type inference. This is because they have several overloads, a couple of which take expressions. If we try this in the StoreMap:

We get an error because F# doesn't know what x is. (error FS0055: This lookup uses a deprecated feature, where a class type is inferred from the use of a class field label. Consider using a type annotation to make it clear which class the field comes from.) Using ~@ to keep it strongly typed fails as well; it can't figure out the overload. This is nothing surprising -- overloading is the enemy of type inference.

So, what do we do? Type annotations are not what I consider fun. So instead, we'll add non-overloaded ClassMap<'t> type extensions into the LinqHelper module:

By providing these extensions that are explicit once, we enable type inference for the rest of the time. We can now finish the StoreMap:

The only type "annotation" we need here is the upcast for the HasManyX. This is because F# forces you to be explicit. Accessing "Staff" in the first quotation means type IList<Employee>, not seq<Employee>. The upcast will sort this out for us.

This compiles fine. But remember how I said using expressions was a "somewhat strongly typed" way of doing things? The expression trees are interpreted at runtime, which allows failures a more complex type system might prevent. This is one of the times. If we try to execute it as-is, we get this exception:

 ---> FluentNHibernate.Cfg.FluentConfigurationException: An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.

 ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: Not a member access Parameter name: member

Ouch. Fluent NHibernate does not appreciate our expression trees. Why? Using FSI, we can inspect what we're actually converting the F# quotations to:

type SomeClass() = member x.Stuff = [|1;2;3|];;
let myExpr : Expr<SomeClass -> seq<int>> = <@ fun x -> upcast x.Stuff @>;;
let myLinq = LinqHelper.ToLinq myExpr;;

> myLinq;;
val it : Linq.Expressions.Expression<Func<SomeClass,seq<int>>>
= x => (x.Stuff As IEnumerable`1)
    {Body = (x.Stuff As IEnumerable`1);
     NodeType = Lambda;
     Parameters = seq [x];
     Type = System.Func`2[FSI_0022+SomeClass,System.Collections.Generic.IEnumerable`1[System.Int32]];}

> myLinq.Body;;
val it : Linq.Expressions.Expression
= (x.Stuff As IEnumerable`1)
    {IsLifted = false;
     IsLiftedToNull = false;
     Method = null;
     NodeType = TypeAs;
     Operand = x.Stuff;
     Type = System.Collections.Generic.IEnumerable`1[System.Int32];}

So, what's going on? The upcast is modifying the F# quotation (as it should), and the ToLinqExpression is sticking this in as a "TypeAs" node. Fluent NHibernate does not seem to like this. Not. One. Bit.

But, it IS ok if we have a Convert node in the Expression tree. I imagine this is because C# generates such nodes in its expression trees (say, accessing an int member in a Func<T, object> expression). So, our last hack in making F# work right here is adding a fixup function to our LinqHelper, and using it from ToLinq:

Now it's happy with our expressions.

Finally

The rest of the First Project is pretty straightforward in F#. For instance, creating the Session Factory:

Everything seems to execute as it should. Fluent interfaces in F# generate a bit more noise, because of all the ignores that have to be added. I'm thinking of creating a type extension to obj to add Ignore as a method, to make it look a bit more uniform.

Future

It's my hope that F# has now addressed most of these issues -- 1.9.6 is 7 months old. Expression<T> is growing in importance, outside of LINQ querying, so I cannot see F# not being able to handle them easily and generating similar output to C#. It'd be really neat if F# would auto-convert quotations to Expression<T> when the expr is a syntactic argument like it does with delegates now. I'd also be surprised if abstract/virtual members still lack the ability to define accessibility.

Code: fhib.zip (4.76 KB)
I did not include Fluent NHibernate or any of its dependencies. The project expects them to be in the project's lib folder, so get them here if you don't have them and extract them to "lib".

I welcome comments on this and suggestions for making the code more concise.

This entry was triggered by this question. Someone asked how to use AES, and we got 2 sample classes that do it wrong. The flaw in both was that they shared the IV which means your ciphertext will can leak information. One answerer didn't believe me at first, but then got it and deleted his code. The other person got offended and said IVs, performing authentication, etc. are all "corner cases" and any problem is "contrived". So, I'm going to provide a bit of code and show two problems that arise from not generating a unique IV for each message and not authenticating the decrypted data.

First, I wrote this a while ago: What is an IV?. That describes what an IV is, and why you need a unique one for each message. Wikipedia also has good information on this. Now for the demo. I used the code at the bottom, but removed the hashing and random IV from the code. So it's just encrypting the with the same key and IV for each message -- very straightforward. Here are the messages and their ciphertext:

"Alice; Bob; Eve;: PerformAct1"
"Alice; Bob; Eve;: PerformAct2"

tZfHJSFTXYX8V38AqEfYVXU5Dl/meUVAond70yIKGHY=
tZfHJSFTXYX8V38AqEfYVcf9a3U8vIEk1LuqGEyRZXM=

Notice how the first block of ciphertext is the same? All messages starting with "Alice; Bob; Eve;" will have that same first block. That means an attacker, after getting this ciphertext once, now knows if any message is addressed the same way. Very, very straightforward, basic attack. Now, maybe for a specific implementation you have in mind, this is not an issue. But it's still improperly implemented cryptography.

For the next attack, we're going to show that even with a random IV, you need to authenticate your decrypted messages. This code generates a 64-bit integer and encrypts it with AES and a random key/IV. Then, it starts changing bytes until the decrypt succeeds. Presto: the attacker was able to present a completely different value, and the decryption was successful.

public static void Main() {
    var buff =newbyte[8];
    new Random().NextBytes(buff);
    var v = BitConverter.ToUInt64(buff, 0);
    Console.WriteLine("Value: "+ v.ToString());
    Console.WriteLine("Value (bytes): "+ BitConverter.ToString(BitConverter.GetBytes(v)));
    var aes = Aes.Create();
    aes.GenerateIV();
    aes.GenerateKey();
    var encBytes = aes.CreateEncryptor().TransformFinalBlock(BitConverter.GetBytes(v), 0, 8);
    Console.WriteLine("Encrypted: "+ BitConverter.ToString(encBytes));
    var dec = aes.CreateDecryptor();
    Console.WriteLine("Decrypted: "+ BitConverter.ToUInt64(dec.TransformFinalBlock(encBytes, 0, encBytes.Length), 0));
    for (int i = 0; i < 8; i++) {
        for (int x = 0; x < 250; x++) {
            encBytes[i]++;
            try {
                Console.WriteLine("Attacked: "+ BitConverter.ToUInt64(dec.TransformFinalBlock(encBytes, 0, encBytes.Length), 0));
                 return;
            } catch { }
        }
    }
}

          
            Here's
an example run:
          
        

          Value:
5686260040031435365

Value (bytes): 65-7A-92-1A-61-A7-E9-4E

Encrypted: F4-62-AC-02-2D-7D-43-6A-4D-97-68-4D-95-9F-8A-DF

Decrypted: 5686260040031435365

Attacked: 1603329786558177755
        

Since there's no authentication of the decrypted data, an attacker can just play with the ciphertext until it generates an acceptable value. Perhaps you have other mitigations in your implementation/application for this, but why rely on that?

Here's some demo code (I haven't tested it much, so it might have some major issues -- but not by design AFAIK). Note this just shows performing an encryption operation, including the IV in the message, and verifying the decrypted bytes. Other things like replay attacks are not considered. If you're trying to learn how to use crypto so you can drop it into an application, STOP, then go read enough to understand what you're doing and the implications for your particular application.

aesdemo.cs.txt (4.38 KB)

 EndpointAddress
address =new EndpointAddress("http://127.0.0.1:52620/Service.svc");
BasicHttpMessageInspectorBinding binding =new BasicHttpMessageInspectorBinding(new SilverlightFaultMessageInspector());
ServiceClient proxy =new ServiceClient(binding,
address); proxy.DoWorkCompleted += new System.EventHandler<System.ComponentModel.AsyncCompletedEventArgs>(proxy_DoWorkCompleted);
proxy.DoWorkAsync(); 

protectedvoid Application_EndRequest(object sender,
EventArgs e) {
if (HttpContext.Current.Request.PhysicalPath.EndsWith(".svc",
StringComparison.OrdinalIgnoreCase) &&

HttpContext.Current.Response.StatusCode == 500 &&

!HttpContext.Current.Request.Browser.Crawler &&

HttpContext.Current.Request.Browser.EcmaScriptVersion.Major > 0) {
//
Set 200 if its a faulted service request

HttpContext.Current.Response.StatusCode = 200;

}

}

publicstatic Func<int, int>
Add(int x)
{ return y
=> x + y;
} publicstatic Func<int, int>
Inc = Add(1); //
and "full application" of add looks like this: Add(1)(2)