IT / Administration / Development / other stuff

Wednesday, May 27, 2015

dynamic dispatch using "as dynamic" in c#

I found a new entry for our internal coding guidelines: "don't use dynamic dispatching if not absolutely necessary!".

wikipedia:

...

In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. Dynamic dispatch contrasts with static dispatch in which the implementation of a polymorphic operation is selected at compile-time. The purpose of dynamic dispatch is to support cases where the appropriate implementation of a polymorphic operation can't be determined at compile time because it depends on the runtime type of one or more actual parameters to the operation.

...

Dynamic dispatch is often used in object-oriented languages when different classes contain different implementations of the same method due to common inheritance.

...

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Dynamic
{
    public interface IInt
    {
    }

    public class A : IInt
    {
    }

    public class B : A
    {

    }

    class Program
    {
        static void Main(string[] args)
        {
            Program p = new Program();

            IInt x = new A();
            p.Print(x);
            p.Print((A)x);
            p.Print(x as B);
            p.Print(123);
            p.Print(x as dynamic);

            Debugger.Break();
        }

        public void Print(IInt p)
        {
            Console.WriteLine("IInt");
            //if (p is B)
            //{
            //    Print(p as B);
            //} 
            //else if(p is A)
            //{
            //    Print(p as A);
            //}
        }

        public void Print(A p)
        {
            Console.WriteLine("A");
        }

        public void Print(B p)
        {
            Console.WriteLine("B");
        }
        /*
        public void Print(object p)
        {
            Console.WriteLine("obj");
        }*/

        public void Print(dynamic p)
        {
            Console.WriteLine("dynamic");
        }
    }
}

Output:
IInt
A
B
dynamic
A

The reason why line 61 to 65 is commented out is that it is technically not possible to keep both functions in code ... the one with the object parameter and the one with the dynamic (will be translated to object later). At first sight it seems to be elegant to call line 30 and get the interface function and then call line 34 and get the "right" function, but don't forget the difference... you put compiler decisions to run-time, so you trick the compiler in some ways. Don't do that!

kind regards,
Daniel

sql recursive functions (linked lists in sql)

In modern programming languages it makes sense to work with lists, stacks, dictionaries and other collections. This can be crucial when you want to persist the data into the database, because some considerations have to be made about sorting and stuff like that.

Now implementing a linked list in a table is easy:

table: mylist

ID: int (not null)
data: nvarchar(50)
nextID: int (null)

... more tricky is to walk through the list and find e.g.: the leaf of a corresponding root (seeing a list as a special sort of tree).

the solution is described at:

http://stackoverflow.com/questions/16749095/sql-recursive-function-that-gets-all-ancestors-of-an-item

...but the other way round using parent_id instead of a next_id

data_table:

ID parent_id name

---------------------

1 2 first

2 4 second

3 3 third

4 5 fourth

5 - fifth

the solution looks as follows:

with parents as 
(
  select ID, parent_ID
  from data_table t
  where parent_ID is not null
  union all 
  select p.ID, t.parent_ID
  from parents p
    inner join data_table t on p.parent_ID = t.ID
      and t.parent_ID is not null
      and t.ID <> t.parent_ID
)
select *
  , parents = '(' + stuff
    (
      (
        select ', ' + cast(p.parent_ID as varchar(100))
        from parents p 
        where t.ID = p.ID
        for xml path('')
      ), 1, 2, ''
    ) + ')'
from data_table t
order by ID

line 1 gives the inner statement a name to call it e.g.: more often like in
with y as (select * from x where id > 12) select * from y, y
... here you have a smaller set of data to cross join
line 3 to 5 makes a select as a starting point for the recursion (the original records)
union all in line 6 adds to the starting point a data set defined below
line 7 to 11 was at first sight simply magical to me

line 7 selects the columns corresponding to line 2 (required by the union and makes sense with the recursive algorithm) ... the own id and the parent of parent
line 8 requests data from the data table itself (the recursion using the name), but
line 9 joins parent data to the original data

line 13 to 24 uses the "expanded" data evaluated and shows it nicely in an own column using "for xml path".

the expanded result is:

| ID | parent_ID |
|----|-----------|
|  1 |         2 |
|  2 |         4 |
|  3 |         3 |
|  4 |         5 |
|  2 |         5 |
|  1 |         4 |
|  1 |         5 |

this will be transformed to

| ID | parent_id |   name |   parents |
|----|-----------|--------|-----------|
|  1 |         2 |  first | (2, 4, 5) |
|  2 |         4 | second |    (4, 5) |
|  3 |         3 |  third |       (3) |
|  4 |         5 | fourth |       (5) |
|  5 |    (null) |  fifth |    (null) |

...

There is an addition for select statements to overwrite the max recursion depth (in sql server it is default set to 100) to work with "deep" lists or trees.

OPTION (MAXRECURSION 0)

kind regards,

Daniel

Monday, May 11, 2015

Tool for load tests and performance tests

I needed to make some load and performance tests ... I thought about using the visual studio components, but I realized that I have no studio installed on the machine where I wanted to make my testing ... I needed another tool! Test Studio of Telerik costs 80$ a month ... #dislike

Finally I thought about using an open source tool. After some google search I found some opportunities ( http://blazemeter.com/blog/open-source-load-testing-tools-which-one-should-you-use ), but JMeter seems to be outstanding...

... and it worked for me! The following tutorial with 20 minutes was enough to setup my first easy use case...

kind regards,
Daniel

Wednesday, May 6, 2015

C# Debug-Info

Hi,

a few days ago I found a quite cool feature:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestProject
{
    class Program
    {
        [DebuggerDisplay("Person: '{Name}'")]
        public class Person
        {
            public string Name { get; set; }
            public int Age { get; set; }
        }
        static void Main(string[] args)
        {
            var persons = new List<Person>(new Person[] { 
                new Person() { Name = "Johnny Galecki", Age = 40},
                new Person() { Name = "Kaley Cuoco-Sweeting", Age = 30},
            });

            Debugger.Break();

            if (Debugger.IsLogging())
            {
                persons.ForEach(x => Debugger.Log(0, "output", x.Name + Environment.NewLine));
            }
        }
    }
}

In line 12 DebuggerDisplay tells the Debugger to show the information "person" and then the person's name. E.g.: if you are running this example application the debugger will break (like setting a break-point) on line 25. When you check the variable using a "watch"-window or "local" you will see that the output is now something helpful instead of the full qualified class name.

Other important features related to Debug-Info:

DebuggerStepThrough-Attribute
DebuggerBrowsable-Attribute

kind regards,

Daniel

EDIT: check also DebuggerTypeProxy: https://msdn.microsoft.com/en-us/library/d8eyd8zc.aspx

Tuesday, April 21, 2015

DOS error levels

Today I was in the mood to quit my job and become a potato farmer...

I tried to work with batch scripts to create an "easy" install script (which is currently growing to a quite sophisticated all-in-one solution). (The post is related to http://itados.blogspot.co.at/2014/12/windows-start-script-for-applications.html )

The "easy" concept was to...

* delete and re-create a hard coded folder on the c: - drive (making always a clean "check-out" from the source path)

* connect to a UNC path on a server

* xcopy the files locally

My first problem was that connecting to the UNC path which is shared for the group "everyone", but which was not reachable by "everyone", because "everyone" seems to be "everyone" inside of the domain scope and not "everyone" as I would expect it. There are some related problems which can be solved by tweaking the windows security policy which can be found in secpol.msc at "Local Policies > Security Settings > Network Access: *", but I think it is a bad idea to change these (the infrastructure experts will definitely have an eye on these settings (= problems for me) and this means to open a new gate into your system and be open for new threats).

So I created a user on the system locally and gave him a secure password what leads me to my second problem, which was that if the "net use" command has credentials inserted and the credentials contain special characters then an error about "password or user incorrect" shows up. Attention: I copy/paste-d the command to my normal cmd-box and it worked properly (and created completely different output in windows event log on the server)! I used the same statement in a batch script... it failed. Now some magic: using "abc" as a password worked in both cases properly... (I really hate that...).

The net use command stores the credentials of this local account. In case these have changed or just to work proper I deleted them before setting clean new ones... in case it is the first time to connect, nothing can be deleted so we get an error message saying "hey, this can't be executed, nothing found to delete". So I printed the user (as an information only) that "something happened here, but don't worry everything is still fine"... using "if %errorlevel% NEQ 0" ( ... ATTENTION: there is an undocumented feature on /DELETE ... the /YES forces the delete of the connections even if the connection is currently open... it do is undocumented in /? of net use!

Next problem: the parenthesis of the if has to be on the same line as the if statement in the batch script to identify the following statement as a block which should be executed when the if condition is true. In case this would have made serious troubles a nice goto would make you jump out of the critical section probably to the end of the script (what equals to a "return" in common programming languages).

Back to step 1: To delete the folder is an easy call using "rd" or "rmdir" (i believe these are synonyms). Here I have troubles getting an error code and in fact the BUILT-IN COMMANDS have troubles setting the error code right!

http://stackoverflow.com/questions/11137702/batch-exit-code-for-rd-is-0-on-error-as-well

http://stackoverflow.com/questions/6500314/do-ms-dos-built-in-commands-return-an-error-exit-code

the answer of stackoverflow user "dbenham" brought me to solution that works: "rd testdir || rem" ... again... mind blowing! you have to pipe the result to a comment to get the errorcode of the original statement set up. W00t?

And now to my favorite part... setting and resetting the error level. To copy the application binaries I use "taskkill" to close any running applications (=processes). Resetting the errorcode (because some users really close their application before installing a new version leading to the error "process not found" = code 128) is a common problem and could be accomplished in many different ways like

* verify >nul

* ver >nul

* cmd /c "exit /b 0"

* (call )

* (call)

attention: Don't use "set %errorlevel% = 0", because this creates a local variable with the same name, what disables further error handling possibilities, because then the local variable and not the system variable will be evaluated for further checks ( http://stackoverflow.com/questions/1113727/what-is-the-easiest-way-to-reset-errorlevel-to-zero )

but none of them work inside of an if statement...

taskkill /f /im doesnotexist.exe

echo after taskkill %errorlevel%
if 0==0 (
 cmd /c "exit /b 0"
 echo cmd %errorlevel%
)

echo after if %errorlevel%

output:

FEHLER: Der Prozess "doesnotexist.exe" wurde nicht gefunden. (german for process not found)
after taskkill 128
cmd 128
after if 0

meaning ... if you have something like:

kill my process => errorlevel become 128

if folder exists (

  delete folder => errorlevel should become 0

  check error code (HERE THE ERRORCODE STAYS 128 AND THE ERRORCODE OF DELETE FOLDER WILL BE IGNORED)

)

the solution here is to reset the errorcode after killing the process and to make an "if folder exists" after deleting the folder and to get the information here... another valid solution is to jump to code parts outside of any ifs using goto.

The effect is explained in http://blogs.msdn.com/b/oldnewthing/archive/2006/08/23/714650.aspx who says that an if is executed as a single command what means that something like a preprocessor pushes the values as textreplace into the placeholders. So it does not count what happens inside the if because the text-replace was already done. 

Following example from the link shows it quite good:

C:\>set VAR=before
C:\>set VAR=after & echo %VAR%
before

There seems to be a solution for it with "SETLOCAL ENABLEDELAYEDEXPANSION" (please check the link for further information).

In the end i am not even sure if i am using the errorlevel right, because i am not sure if the %-signs are necessary (both ways seem valid)

cmd /c "exit /b 123"
if errorlevel == 123 (
 echo errorlevel 123 %errorlevel%
) else (
 echo else %errorlevel%
)

cmd /c "exit /b 123"
if %errorlevel% == 123 (
 echo errorlevel 123 %errorlevel%
) else (
 echo else %errorlevel%
)

for this example the next trap came across ... closing parenthesis in echo also close blocks!

cmd /c "exit /b 123"
if %errorlevel% == 123 (
 echo errorlevel 123 (%errorlevel%)
) else (
 echo else (%errorlevel%)
)

output:

errorlevel 123 (123
else (123)

now I know why everyone loves powershell (and also for small scripts) and also why install projects or ClickOnce-Deployment have their right to be... (nevertheless why it is cool to build websites without installing stuff)

kind regards,

Daniel

Null propagation in C# 6.0

Today I read ( http://www.volatileread.com/Wiki?id=2104 ) about the null propagation Operator ?. ... a C# 6.0 feature what enables you the possibility to make things like

string invalidResult = string.Empty;
 
if(this.CurrentUser != null)
{
    if(this.CurrentUser.Name != null)
    {
        return this.CurrentUser.Name;
    }
    else
    {
        return invalidResult;
    }
}
else
{
    return invalidResult;
}

in only one simple statement:

1	return this.CurrentUser?.Name ?? string.Empty;

... semantically "?." means "if not null evaluate property, else return null". 

cool stuff...

kind regards,

Daniel

Tuesday, April 7, 2015

performant vs clean code

Today I read a really good article about development performance vs simple-clean development.

http://arne-mertz.de/2015/03/simple-and-clean-code-vs-performance/

The article (and its comments) highlights some very good points I definitely agree with.

In short:

simple is better than performant:

before: performance (how fast to do something) != efficiency (how long to do it)
if you run 2 miles you can be more performant than a guy walking a single mile,
but if you reach the same goal by walking only one mile you are much more effective
try to write efficient code before you write performant code because your intention will still be understandable and the code keeps being readable
check whether the code to optimize is needed to be optimized (or called rarely, so it doesn't contribute to total run-time)
optimization needs tooling to find painful parts in the code
optimize the data store first
check for other bottlenecks (e.g.: hardware, user input,...) 
check for best practices, libraries and patterns
use caching instead of recalculating the hard stuff

kind regards,

Daniel