Category: .NET

Task and generic list

Could you spot the problem in transforming one code that is sequential into a Task ( threaded ) one?

First , this is the sequential one:

 

            var ret = new List<Indicator>();
            var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            ret.AddRange(array);
            var currentPage = 1;
            while (currentPage < page.pages)
            {
                currentPage++;
                jsonData = await data.JsonData(currentPage);
                jo = JArray.Parse(jsonData);
                array = jo[1].ToObject<Indicator[]>();
                ret.AddRange(array);

            }
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();


 

This is the modified with task:

 

            var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            var ret = new List<Indicator>(array);

            var currentPage = 1;
            var downloads = new List<Task>();
            while (currentPage < page.pages)
            {
                currentPage++;
                var itemPage = currentPage;
                var task = data.JsonData(itemPage)
                    .ContinueWith(it =>
                    {
                        var data = JArray.Parse(it.Result);
                        var pageNr = data[0].ToObject<Pagination>();
                        var indicators = data[1].ToObject<Indicator[]>();
                       ret.AddRange(indicators);

                    }
                );

                downloads.Add(task);
            }
            await Task.WhenAll(downloads);
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();

Ok. If you do not know , here is a helper:
Sometimes, at line

 ret.AddRange(indicators)

it gives the error:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. —> System.ArgumentException: Source array was not long enough. Check srcIndex and length, and the array’s lower bounds.
at System.Array.Copy(Array sourceArray, Int32 sourceIndex, Array destinationArray, Int32 destinationIndex, Int32 length, Boolean reliable)
at System.Collections.Generic.List`1.set_Capacity(Int32 value)
at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)

Ok. I let you think.
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The answer is simple : List < T > is not Thread safe. If you have small quantities of data, does not matter. I was having 16174 records – and sometimes , when multiple threads have come to .AddRange , the internal re-dimensioning of array it does not fulfill the task ( imagine one thread redim to 100, then other 3 threads coming with request of 50+ data)

Solution 1.

Use ConcurrentBag< T > instead of List < T > . This is thread safe and does the trick

Solution 2.
Add to the array in the final of data ( not use ContinueWith) . The code is slightly low performant

           var jsonData = await data.JsonData();
            var jo = JArray.Parse(jsonData);
            var page = jo[0].ToObject<Pagination>();
            var array = jo[1].ToObject<Indicator[]>();
            var ret = new List<Indicator>(array);

            var currentPage = 1;
            var downloads = new List<Task<string>>();
            while (currentPage < page.pages)
            {
                currentPage++;
                var itemPage = currentPage;
                var task = data.JsonData(itemPage);
                downloads.Add(task);
            }
            await Task.WhenAll(downloads);

            foreach (var t in downloads)
            {
                var data = JArray.Parse(t.Result);
                //var pageNr = data[0].ToObject<Pagination>();
                var indicators = data[1].ToObject<Indicator[]>();
                //Console.WriteLine($"reading {pageNr.page} with {item.Length}");
                ret.AddRange(indicators);

            }
            //Console.WriteLine($"total records {ret.Count}");
            Debug.Assert(ret.Count == page.total, $"{nameof(ret.Count)} : {ret.Count} should be equal {nameof(page.total)} : {page.total}");
            return ret.ToArray();

You will find the code at IndicatorRepository in https://github.com/ignatandrei/WorldBankAPi

Maintaining history of data

 

What is this about?

I tend to quote Fowler: “Usually when we see properties on a class, they represent questions we can ask of an object now. However there are times when we don’t want to just ask questions about a property of an object now, we also want to ask these questions about some point in the past when things may have changed. “

For example , we want to track every change that occurs at an Employee ( changing name, salary) and see the previous versions( what was the salary 2 years ago ?)

So what are the software solutions to do an audit log ? First, we should have a history table for each table that we want to maintain an audit log. E.g. Employee => EmployeeHstory

Solution 1 : Coding / Event Sourcing

That means code should maintain history himself – i.e. adding to the history table every modifications.

For Entity Framework this can be done easy, by overiding SaveChanges.

Demo at https://youtu.be/YZ9GDjFApns 

Solution 2 : Table Triggers

This can be done from database . There are triggers for insert , update, delete – and those triggers insert the data in the history table

Demo at https://youtu.be/9xsvCJ-1f7Q

Solution 3, Temporal Tables, Microsot  Sql Server 2016 :

Documentation at https://docs.microsoft.com/en-us/sql/relational-databases/tables/temporal-tables 

Limitations at https://docs.microsoft.com/en-us/sql/relational-databases/tables/temporal-table-considerations-and-limitations 

Demo at https://youtu.be/1Q3DmXJI5h8

Five common mistakes for ASP.NET MVC accesing resources : css, js, images, ajax

This is a Razor /MVC5 variant of older post
http://msprogrammer.serviciipeweb.ro/2010/10/09/five-common-mistakes-for-asp-net-mvc-accesing-resources-css-js-images-ajax/

To have once for all the link to show to people, because too much makes the same error again and again. (From here – you can use ResolveUrl or Url.Content – it’s the same for me. I use ResolveUrl because I used first …)

Case 1 The image does not display

Please check you have the following :

 <img src="@Url.Content("~/Content/images/YOURIMAGE.jpg" )" alt="image" style="border:0" /> or 
 
<img src="~/Content/images/YOURIMAGE.jpg" alt="image" style="border:0" /> 

Case 2 The css does not show

Please check you have the following :

 @string.Format("<link href="{0}" type="text/css" rel="stylesheet" />", Url.Content("~/Content/your.css"))  

or

 <style type="text/css">
@import '@Url.Content("~/Content/your.css")';

</style>  
  <style type="text/css">
<link href='~/Content/your.css' type='text/css' rel='stylesheet' />

</style>  

Case 3 The js does not execute (undefined error)

3.1 Please check you have the following :

 <script type="text/javascript" src="@Url.Content("~/Scripts/yourjs.js")"></script> 

or

 <script type="text/javascript" src="~/Scripts/yourjs.js"></script> 

This should be either in _Layout.cshtml, or in the scripts section

 @section scripts {   } 

in your .cshtml file.
3.2 please check in browser console ( press F12 ) for any javascript error that code may have

Case 4 The js does execute in .cshtml page, but did not execute under js file

Please check you DO NOT have the following @something in the js file. The js file is not interpreted by the same engine as cshtml, so can not interpret asp.net tags. Instead , add a parameter to your function for each @ variable that you have. Simple example : Let’s say in cshtml you have :

 <script type=”text/javascript”>

function Generate(){

window.open('@Url.Action(“About”)’);
}

</script> 

and you call Generate() from .cshtml file. Now you want the same in a .js file. The function Generate will have a parameter instead of Razor variable

 function Generate(url){  window.open(url); } 

and call like this from .cshtml file:

 Generate('@Url.Action("About")'); 

Case 5 The ajax request gives you a 404 error.

Please ensure that you have

 @Url.Content("~/path”) 

and not

 /path 

when you call the Ajax URL

Bonus 1: T4MVC , http://mvccontrib.codeplex.com/releases

Bonus 2: Edit the project file and put <MvcBuildViews>true</MvcBuildViews>

( short url: http://bit.ly/asp5Mistakes)

Async + sync completion + lock

Let’s suppose that you have 2 functions = one true sync and one true async . Let’s suppose more that the sync one can must finish before calling next time ( think about some weird COM component or , in our days, SqlConnection )

If we look just at the async function , we could wait for all. However, the sync function must be called sync

One solution is to make different calls:


var list = new List<Task<int>>(nr);
//add async to list

await Task.WhenAll(list)

//call each sync



But I want to make really async – and one solution is lock

We could wrote the sync function in async manner like this:


//SemaphoreSlim sem = new SemaphoreSlim(1);
static object myObject = new object();
async Task<int> syncTask(int i)         {             
lock (myObject)             
//try{                 
//await sem.WaitAsync();                                 
CallSyncFunction();                             
//}             
//finally             
//{                 
//sem.Release();             
//}                                   
}

The trick that I  used is lock, because the syntax is easier than SemaphoreSlim (BTW: if you want to learn about threading, jump directly to http://www.albahari.com/threading/)
In this manner, the sync function is modified in async -and, more, it waits for completion before entering the second time.

The code is on github on

More details in the Wiki https://github.com/ignatandrei/AsyncSyncLock/wiki or in the code https://github.com/ignatandrei/AsyncSyncLock/

spot errors in VB

Please spot the errors in the following code in VB.NET:

Dim Diff As String = con.ExecuteCommand(“SELECT DATEDIFF(Day, (select [date] from table where Username = ‘” + txtUserName.Text + “‘), ‘” & DateTime.Now.ToString() & “‘)”)
If (Diff > 30) Then
Response.Redirect(“PleaseChangeMyPassword.aspx”)
End If

 

( find at least 3…)

Side note:

No wonder Visual Basic has died ( more or less). Guess future of server javascript ….

Generating Visual Studio solution( or project) references

A Visual Studio pacakge is a plugin of Visual Studio that can do (almost) anything inside Visual Studio

I have made a small project that generates a list of all solution ( or project) references from VS:

 

            foreach (var project in dte.Solution.Projects)
            {
                var p = project as Project;
                if (p == null)
                {
                    continue;
                }

                var vsproject = p.Object as VSLangProj.VSProject;
                if (vsproject == null)
                    continue;

                string nameProject = p.Name;
                foreach (var reference in vsproject.References)
                {
                    var r = reference as Reference;
                    if (r == null)
                        continue;

                    var pathRef = r.Path;

                    //this is a package                    
                    var pa = new PackageAdd();
                  
                    pa.ProjectName = p.Name;
                    pa.IdentifierPackage = r.Identity;
                    pa.VersionPackage = r.Version;
                    pa.NamePackage = r.Name;

                  



                    if (r.SourceProject != null)
                    {
                        
                        pa.NamePackage = r.SourceProject.Name;
                    }
                    sb.AppendLine(pa.ToString());
                }
            }

You can find the solution at https://github.com/ignatandrei/ToolsAndUtilities/tree/master/VS2015/FindReferencesVS

Also the video is at https://youtu.be/a3YHVjJ9fm4

Enjoy!

Thank you for all the API

In our days it is simpler to make an application by putting together some API . I wanted to make a list of all my 5 Minutes .NET playlist videos . First, I have tried to find an application(nothing). Then I have read  You Tube API from https://console.developers.google.com/apis/library . With 2 http requests( one for retrieving all videos from a playlist, the other to find video details) and a NuGet package to export to Excel/Word/PDF/HTML it was simple. You can find the whole project at https://github.com/ignatandrei/YouTube . A video demo at https://www.youtube.com/watch?v=wNHDsHtLBq4 . And the whole list at https://github.com/ignatandrei/ToolsAndUtilities/tree/gh-pages

IDisposable and Marshal.ReleaseCOMObject and dynamic in C#

 

If you have worked with COM, you know that for every COM object that you access is not only necessary to null the reference, but also call Marshall.ReleaseCOMObject .

This is a very error prone task, because you have all try / finally blocks

 
// declare a, ws, w 
try{ 
a=new Application(); 
ws = a.Workbooks; 
w = ws.Add(); 
//code 
}
 catch(...) {
 if( w != null){ 
Marshal.ReleaseComObject(w); 
w = null; 
}
 if(ws !=null){ 
Marshal.ReleaseComObject(ws); 
ws = null; 
} 
if( a!= null) { 
Marshal.ReleaseComObject(a); a= null; 
} 
} 

All of this could be done easy with IDisposable, if

1. IDisposable calls the Marshal.ReleaseCOMObject

2. Find a way to call methods / properties on the COM object from the class that implements IDisposable

 

I have done this and the result is like this:

 
using (dynamic a = new ComDisposable(new Application())) { 
using (dynamic ws = a.Workbooks) { 
using (dynamic w = ws.Add()) { 
using (var shs = w.Sheets) { 
using (var s = shs[1]) { 
//Worksheet a; 
using (var r = a.Range("A1")) { 
r.Value2 = "http://ignatandrei.github.io/ToolsAndUtilities/";; 
using (var f = r.Font) { 
f.Bold = true; 
}
 }
 }
 }
 w.SaveAs(fileName); 
w.Close(); 
} 
}
 a.Quit(); } 

 

Video at https://youtu.be/2qbAcSjL1gU

NuGet package at https://www.nuget.org/packages/ReleaseComObjectDisposable/

AppContext

AppContext is like Application in ASP.NET (MVC). You can see the value all over the application

AppContext.SetSwitch("OCR", true);

And we can have the result in all dll’s by

 public bool IsOCREnabled()
        {
            bool test;
            if (!AppContext.TryGetSwitch("OCR", out test))
                return false;

            return test;
        }

Video demo at https://youtu.be/GMgn4o3yPqw

Instrumentation in C# / .NET

For many programmers a try/catch with stack trace it is enough

try
{
var p = new Person();
p.DateOfBirth = DateTime.Now.AddYears(-10);
p.Drink(10, 2);
}
catch (Exception ex)
{
Console.WriteLine("ERROR OCCURED " + ex.StackTrace);
}

Others want to see also functions arguments, like in


p.Drink(10, 2);

will produce the result

ERROR An exception occured in method InstrumentationPostSharp.Person.Drink with (beersNumber = 10 , vodkaNumber = 2)

For this you can install the package
https://www.nuget.org/packages/LoggerAspect.Nlog

that have full source code at
https://github.com/vnvizitiu/AOP/tree/master/PostSharpTutorial

and watch my video for more details
https://youtu.be/qe2kpuuWXkw