Marius Bancila’s Blog

.NET allows you to expose components as COM and consume them from unmanaged code. There are many references on how to this (and you can only start with MSDN), and I will not talk about that part. What I want to explain here is something different. Suppose you have this interface:

[Guid("2F8433FE-4771-4037-B6B2-ED5F6585ED04")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IAccounts
{
      [DispId(1)]
      string[] GetUsers();
}

Method GetUsers() returns an array on string representing the user names. But what if you also wanted the user passwords or addresses? Since this is exposed as COM, you cannot return an array of User. But you can return multiple arrays of string. So, how would you deal with out string[]? This is what I want to show you in this tutorial.

This is a .NET interface exposed to COM. It has two methods, GetUsers() that returns an array of string representing user names, and GetUsers2() that returns an array of strings as an output parameters and a bool as return type, indicating whether any user was found.

namespace SampleLibrary
{
   [Guid("2F8433FE-4771-4037-B6B2-ED5F6585ED04")]
   [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
   public interface IAccounts
   {
      [DispId(1)]
      string[] GetUsers();

      [DispId(2)]
      bool GetUsers2(out string [] users);
   }
}

And this is the implementation:

namespace SampleLibrary
{
   [Guid("C4713144-5D29-4c65-BF9C-188B1B7CD2B6")]
   [ClassInterface(ClassInterfaceType.None)]
   [ProgId("SampleLibrary.DataQuery")]
   public class Accounts : IAccounts
   {
      List< string > m_users;

      public Accounts()
      {
         m_users = new List< string > {
            "marius.bancila",
            "john.doe",
            "anna.kepler"
         };
      }

      #region IDataQuery Members

      public string[] GetUsers()
      {
         return m_users.ToArray();
      }

      public bool GetUsers2(out string[] users)
      {
         users = m_users.ToArray();

         return users.Length > 0;
      }

      #endregion
   }
}

Note: If you are trying this example make sure you set the ComVisible attribute to true, either for each type or per assembly (in AssemblyInfo.cs)

[assembly: ComVisible(true)]

Second, you have to check the “Register for COM interop” setting in the Build page of the project properties.

The first thing to do in C++ is importing the .TLB file that was generated by regasm.exe.

#import "SampleLibrary.tlb"
using namespace SampleLibrary;

If we look in the .TLB file, we can see how the IAccounts interface looks like:

struct __declspec(uuid("2f8433fe-4771-4037-b6b2-ed5f6585ed04"))
IAccounts : IDispatch
{
    //
    // Wrapper methods for error-handling
    //

    // Methods:
    SAFEARRAY * GetUsers ( );
    VARIANT_BOOL GetUsers2 (
        SAFEARRAY * * users );
};

The following C++ functions, GetUsers1() retrieves the users users list using method GetUsers() from IAccounts. It puts the users in a CStringArray (notice that this container does not have an assignment operator, so the only way to return such an array is with a reference in the parameters list).

void GetUsers1(CStringArray& arrUsers)
{
   IAccountsPtr pAccounts(__uuidof(Accounts));

   SAFEARRAY* sarrUsers = pAccounts->GetUsers();

   _variant_t varUsers;
   varUsers.parray = sarrUsers;
   varUsers.vt = VT_ARRAY | VT_BSTR;

   UnpackBstrArray(varUsers, arrUsers);
   SafeArrayDestroy(sarrUsers);

   pAccounts->Release();
}

UnpackBstrArray() is a function (see below) that extracts the elements of a SAFEARRAY and adds them to a CStringArray.

Function GetUsers2() uses the second method, GetUsers2() from IAccounts. This needs the address of a pointer to a SAFEARRAY (i.e. SAFEARRAY**) that will hold the values returned by the COM method. This time we have to create an empty SAFEARRAY and then pass its address to the COM method. The rest is similar to the previous case.

void GetUsers2(CStringArray& arrUsers)
{
   IAccountsPtr pAccounts(__uuidof(Accounts));

   SAFEARRAYBOUND aDim[1];
   aDim[0].lLbound = 0;
   aDim[0].cElements = 0;

   SAFEARRAY* sarrUsers = SafeArrayCreate(VT_BSTR, 1, aDim);

   VARIANT_BOOL ret = pAccounts->GetUsers2(&sarrUsers);
   if(ret != VARIANT_FALSE)
   {
      _variant_t varUsers;
      varUsers.parray = sarrUsers;
      varUsers.vt = VT_ARRAY | VT_BSTR;
      UnpackBstrArray(varUsers, arrUsers);
   }

   SafeArrayDestroy(sarrUsers);

   pAccounts->Release();
}

The helper method UnpackBstrArray() used previous looks like this:

void UnpackBstrArrayHelper(VARIANT* pvarArrayIn, CStringArray* pstrarrValues)
{
   if (!pstrarrValues || !pvarArrayIn || pvarArrayIn->vt == VT_EMPTY)
      return;

   pstrarrValues->RemoveAll();

   VARIANT* pvarArray = pvarArrayIn;
   SAFEARRAY* parrValues = NULL;

   SAFEARRAYBOUND arrayBounds[1];
   arrayBounds[0].lLbound = 0;
   arrayBounds[0].cElements = 0;

   if((pvarArray->vt & (VT_VARIANT|VT_BYREF|VT_ARRAY)) == (VT_VARIANT|VT_BYREF) &&
      NULL != pvarArray->pvarVal &&
      (pvarArray->pvarVal->vt & VT_ARRAY))
   {
      pvarArray = pvarArray->pvarVal;
   }

   if (pvarArray->vt & VT_ARRAY)
   {
      if (VT_BYREF & pvarArray->vt)
         parrValues = *pvarArray->pparray;
      else
         parrValues = pvarArray->parray;
   }
   else
      return;

   if (parrValues != NULL)
   {
      HRESULT hr = SafeArrayGetLBound(parrValues, 1, &arrayBounds[0].lLbound);
      hr = SafeArrayGetUBound(parrValues, 1, (long*)&arrayBounds[0].cElements);
      arrayBounds[0].cElements -= arrayBounds[0].lLbound;
      arrayBounds[0].cElements += 1;
   }

   if (arrayBounds[0].cElements > 0)
   {
      for (ULONG i = 0; i < arrayBounds[0].cElements; i++)
      {
         LONG lIndex = (LONG)i;
         CString strValue = _T("");

         VARTYPE vType;
         BSTR bstrItem;

         ::SafeArrayGetVartype(parrValues, &vType);
         HRESULT hr = ::SafeArrayGetElement(parrValues, &lIndex, &bstrItem);

         if(SUCCEEDED(hr))
         {
            switch(vType)
            {
            case VT_BSTR:
               strValue = (LPCTSTR)bstrItem;
               break;
            }

            ::SysFreeString(bstrItem);
         }

         pstrarrValues->Add(strValue);
      }
   }
}

void UnpackBstrArray( const _variant_t &var, CStringArray &strarrValues  )
{
   UnpackBstrArrayHelper( &(VARIANT)const_cast< _variant_t & >(var), &strarrValues );
}

Attached you can find a demo project (C# and C++) with the complete example show in this tutorial.

[download id="4"]

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Project Tuva is an enhanced video player created by Microsoft Research to freely host the lectures give my Richard Feynman at the Cornell University in the ’60s. Bill Gates saw the lectures two decades ago, was impressed with them and wanted to make them freely available. Now, it finally happened. You can watch them at Microsoft Research.

The seven lectures given by professor Feynman are:

• Law of Gravity
• The Relation of Mathematics and Physics
• The Great Conservation Principles
• Symmetry in Physical Law
• The Distinction of Past and Future
• Probability and Uncertainty – The Quantum Mechanical View of Nature
• Seeking New Laws

These are great lectures given by one of the greatest physicists of the 20th century. They really worth watching.

fyenman, lectures, physics, tuva Hits for this post: 2577 .

Yesterday I wrote about list in F#. Today I’ll write about arrays, which unlike lists are a mutable flat storage and cannot be resized. That means you have to create a new array if you want to remove or add elements. Advantages include constant look-up time and the fact that they can store a large amount of data.

You can create a literal array in a similar way with the lists, placing the elements between [| |]:

let data1 = [|1;2;3;4|]
printfn "data1: %a" output_any data1

data1: [|1; 2; 3; 4|]

The empty literal array is [||].

To create an array you can either use Array.create or Array.init. They both create and initialize an array, but the second makes a lambda expression, which allows advance initialization possibilities. The following creates an array with 10 elements initialized to 1:

let data2 = Array.create 10 1
printfn "data2: %a" output_any data2

Here is the output:

data2: [|1; 1; 1; 1; 1; 1; 1; 1; 1; 1|]

The same can be achieved using Array.init:

let data3 = Array.init 10 (fun x -> 1)
printfn "data3: %a" output_any data3

data3: [|1; 1; 1; 1; 1; 1; 1; 1; 1; 1|]

But we can use Array.init to initialize the elements from 1 to N for instance:

let data4 = Array.init 10 (fun x -> x+1)
printfn "data4: %a" output_any data4

data4: [|1; 2; 3; 4; 5; 6; 7; 8; 9; 10|]

The arrays are mutable data structures. Elements are accessed with .[] or .(). The following code shows how to set the elements of an array:

let data5 = Array.create 10 0
for i = 0 to (Array.length data5)-1 do
   data5.[i] <- i+1

printfn "data5: %a" output_any data5

data5: [|1; 2; 3; 4; 5; 6; 7; 8; 9; 10|]

You can iterate over the elements of an array with Array.iter and Array.iteri, the second also providing access to the index of the elements.

data4 |> Array.iter (fun x -> printf "%d " x)
printfn ""

data4 |> Array.iteri (fun i x -> printfn "data4(%d) = %d" i x)

1 2 3 4 5 6 7 8 9 10
data4(0) = 1
data4(1) = 2
data4(2) = 3
data4(3) = 4
data4(4) = 5
data4(5) = 6
data4(6) = 7
data4(7) = 8
data4(8) = 9
data4(9) = 10

Retrieving the length of the array can either be done with Array.length arr or with arr.Length.

for i = 0 to data4.Length-1 do
   printfn "data4(%d) = %d" i data4.(i)

data4(0) = 1
data4(1) = 2
data4(2) = 3
data4(3) = 4
data4(4) = 5
data4(5) = 6
data4(6) = 7
data4(7) = 8
data4(8) = 9
data4(9) = 10

Like the lists, arrays provide mapping that creates a new array by applying a function on all the elements of an array (with Array.map) or two arrays (with Array.map2).

let data6 = data4 |> Array.map (fun x -> x*2)
printfn "data6: %a" output_any data6

let data7 = Array.map2 (fun x y -> x+y) data4 data6
printfn "data7: %a" output_any data7

data6: [|2; 4; 6; 8; 10; 12; 14; 16; 18; 20|]
data7: [|3; 6; 9; 12; 15; 18; 21; 24; 27; 30|]

A copy of an array can be done with Array.copy.

let data7 = Array.copy data6
printfn "data7: %a" output_any data7

data7: [|2; 4; 6; 8; 10; 12; 14; 16; 18; 20|]

Appending elements to an array is also possible with Array.append, but the result is a new array, created by concatenating two arrays.

let data8 = Array.append data7 [|100|]
printfn "data8: %a" output_any data8

data8: [|2; 4; 6; 8; 10; 12; 14; 16; 18; 20; 100|]

The last operation of arrays I'm going to mention here is the folding, which allows applying a function to all the elements of an array, threading an accumulator argument in the process. The following example shows how to compute the sum of the elements of an array.

let data9 = [|1;2;3;4|]
let sum1 = (Array.fold_left (fun acc x-> x + acc) 0 data9)
let sum2 = (Array.fold_right (fun acc x-> x + acc) data9 0)
printfn "sum1 = %d" sum1
printfn "sum2 = %d" sum2

sum1 = 10
sum2 = 10

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In this post I will talk about the lists in F#, one of the fundamental concepts of the language. What should be said from the very beginning is that list are imutable single linked list. That means whenever you change a list, a new list is created.

You can declare a list in the following ways:

let list1 = [1;2;3;4]
let list2 = 5::6::7::8::[]

To print the content of the list you can do this:

printfn “list1: %a” output_any list1
printfn “list2: %a” output_any list2

list1: [1; 2; 3; 4]
list2: [5; 6; 7; 8]

You can concatenate two lists with operator @:

let list3 = list1 @ list2
printfn "list3: %a" output_any list3

list3: [1; 2; 3; 4; 5; 6; 7; 8]

and you can append elements to the beginning of the list with operator ::

let list4 = -1::0::list3
printfn "list4: %a" output_any list4

list4: [-1; 0; 1; 2; 3; 4; 5; 6; 7; 8]

You can also use the List (defined in Microsoft.FSharp.Code) functionality to print a list by iterating over its elements:

list3: [1; 2; 3; 4; 5; 6; 7; 8]

1 2 3 4

The same can be achieved using the pipe operator:

list1 |> List.iter (fun x -> printf "%d " x)

You can also iterate and get the index of the list elements, with List.iteri:

list1 |> List.iteri (fun i x -> printfn "list1[%d] : %d " i x)

list1[0] : 1
list1[1] : 2
list1[2] : 3
list1[3] : 4

List have a special representation, a head followed by a tail, that is in turn another list (including empty list []). Let's consider the list [1;2;3]. It has the head 1, and the tail [2;3]. The tail, in turn, has the head 2 and the tail [3]. This tail has the head 3 and the tail [], which is the empty list.
You can see the head and tail of a list with List.hd and List.td:

printfn "head list1: %a" output_any (List.hd list1)
printfn "tail list1: %a" output_any (List.tl list1)

The ouput for list1 [1;2;3] is:

head list1: 1
tail list1: [2;3]

Enough with basic things. Let's try working with lists.

1. Minimum and maximum from a list

We can compute the maximum (or minimum) of a list using the following algorithm:

• if the list is empty, indicate error
• if the list has only one element, that is the maximum (or minimum)
• if the list has at least to elements, compute the maximum between that element and the maximum from the rest of the list

That sounds like a recursive operation, which can be simply put in F# like this:

let rec greatest_element l =
    match l with
    | [] -> failwith "empty list"
    | [x] -> x
    | x::rest -> max x (greatest_element rest)

let rec smallest_element l =
    match l with
    | [] -> failwith "empty list"
    | [x] -> x
    | x::rest -> min x (smallest_element rest)

We can use that like this:

let list1 = [1;2;3;4;-4;-3;-2;-1]
let list2 = []   

try
   printfn "maximum from list1: %d" (greatest_element list1)
   printfn "minimum from list1: %d" (smallest_element list1)

   printfn "maximum from list2: %d" (greatest_element list2)
   printfn "minimum from list2: %d" (smallest_element list2)
with
   Failure msg ->
      printfn "Error: %s" msg

and the output would be:

maximum from list1: 4
minimum from list1: -4
Error: empty list

2. Reversing a list

How would we reverse a list? We should take the last element and append to it the one before the last. To the new list we append the one before the one before the end, etc. That again sounds recursive.

let rec revert_list l =
   match l with
   | [] -> []
   | x::rest -> (revert_list rest) @ [x]

let list1 = [1;2;3;4;-4;-3;-2;-1]

printfn "list1: %a" output_any list1
printfn "list2: %a" output_any (revert_list list1)

And here is the output:

list1: [1; 2; 3; 4; -4; -3; -2; -1]
list2: [-1; -2; -3; -4; 4; 3; 2; 1]

3. Inserting in a list

So how could we insert an element in a list, before or after a specified element? We can use the following algorithm:

• if the list is empty, the new list has one element (the one to insert)
• else, if the head is the element we are looking for, create a list, with the new element either before the head, or between the head and the tail
• else, if the head is not the element we are looking for, append the head to a list created by inserting the new element in the tail.

You got that right, recursion again.

let rec insert_after elem newelem l =
    match l with
    | [] -> [newelem]
    | x::rest -> if x = elem then
                    (x::newelem::rest)
                 else
                     x::(insert_after elem newelem rest)

let rec insert_before elem newelem l =
    match l with
    | [] -> [newelem]
    | x::rest -> if x = elem then
                    (newelem::x::rest)
                 else
                    x::(insert_before elem newelem rest)        

let list1 = [1;2;3;4;-4;-3;-2;-1]
let list2 = insert_after 4 6 list1
let list3 = insert_before 6 5 list2

printfn "list1: %a" output_any list1
printfn "list2: %a" output_any list2
printfn "list3: %a" output_any list3

And the output is:

list1: [1; 2; 3; 4; -4; -3; -2; -1]
list2: [1; 2; 3; 4; 6; -4; -3; -2; -1]
list3: [1; 2; 3; 4; 5; 6; -4; -3; -2; -1]

4. Removing elements from a list

As a last exercise, let's consider the removing of elements from a list. The following steps can be used to remove elements:

• if the list is empty, return an empty list
• if the list is not empty and the head meets the removing criteria, return a list obtained by reiterating the algorithm on the tail of the list
• if the list is not empty and the head does not meet the removing criteria, return a list obtained by appending the head to a list optained by reiterating the algorithm on the tail of the list

let rec remove_if l predicate =
    match l with
    | [] -> []
    | x::rest -> if predicate(x) then
                    (remove_if rest predicate)
                 else
                     x::(remove_if rest predicate)

The great thing about this implementation is that we can pass a lambda expression as a predicate, and use it to specify the criteria for removing elements. We can remove like that, for instance, the odd elements, or the even elements, or the negative elements. Here is some sample code:

let list1 = [1;2;3;4;-4;-3;-2;-1]

let list2 = remove_if list1 (fun x -> (abs x &&&1) = 1)
let list3 = remove_if list1 (fun x -> (abs x &&&1) = 0)
let list4 = remove_if list1 (fun x -> x < 0)

printfn "%a" output_any list1
printfn "%a" output_any list2
printfn "%a" output_any list3
printfn "%a" output_any list4

The output for this sample is:

[1; 2; 3; 4; -4; -3; -2; -1]
[2; 4; -4; -2]
[1; 3; -3; -1]
[1; 2; 3; 4]

I hope this will help you to get a grip on how you can work on lists in F#.

F# tutorial article list recursive Hits for this post: 11035 .