Category: HowTo

short tutorials

Circular references on .NET , Entity Framework and WebAPI

Imagine having a class Department( ID, Name) and Employee ( ID , Name, IDDepartment) . You want to return in the WebAPI the Departments with the Employee . It is simple to wrote this code:


[HttpGet]
public IEnumerable<Department> Get([FromServices] MyTestDatabaseContext context)
{
var departments = context.Department.Include(iterator=>iterator.Employee).ToArray();
return departments;
}

But the problem is with circular references when serializing  : The Department has a list of  Employees  that have a Department that have a list of Employees that …

 

 

Solution 1 :  Delete/Comment from the EF generated code what you do not need ( in this case , the Department reference that the Employee has)


public partial class Employee
{
public int Idemployee { get; set; }
public string NameEmployee { get; set; }
public int Iddepartment { get; set; }

//public Department IddepartmentNavigation { get; set; }
}

Solution 2 : Nullify the reference


[HttpGet]
public IEnumerable<Department> Get([FromServices] MyTestDatabaseContext context)
{
var departments = context.Department.Include(iterator=>iterator.Employee).ToArray();
foreach (var dep in departments)
{
foreach (var emp in dep.Employee)
{
emp.IddepartmentNavigation = null;
}
}
return departments;
}

( maybe this should be add to the partial class of the employee ?)

 

 

Solution 3: Handle circular references in serialization


services
.AddMvc()
.AddJsonOptions(opt =>
{
opt.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;
opt.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
});

Solution 4: Make a read DDD design. Read https://siderite.blogspot.com/2018/08/client-models-vs-data-transfer-objects.html

InternalsVisibleTo usage

I do not know if you know  InternalsVisibleToAttribute

https://msdn.microsoft.com/en-us/library/system.runtime.compilerservices.internalsvisibletoattribute%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396  .

I have used on a project https://github.com/ignatandrei/WorldBankAPi , that deals with World Bank API to get information

Let’s take an example: Countries: http://api.worldbank.org/countries?format=json

I have had a class CountriesRepository, that deals with transforming countries Json from WorldBank to Country classes. How can I test it WITHOUT relying on reading data from the internet ?

Simple:

1. Make an interface to grab JSON , IJSonData

    public interface IJsonData
    {
        Task<string> JsonData(int page = 1);
    }

2. Make a class JSONCountry that reads JSON from HTTP and implements IJSonData

3.  Make the CountryRepository class with 2 constructors  – one with IJSonData and the default calling JSONCountry

[assembly: InternalsVisibleTo("WorldBank.Test")]
namespace WorldBank.Repository
{
    public class CountriesRepository
    {
        private IJsonData data;
        public CountriesRepository():this(new JsonCountries())
        {

        }
//this is internal - could not see by outside projects with exception of WorldBank.Test
//see InternalsVisibleTo above
        internal CountriesRepository(IJsonData data)
        {
            this.data = data;
        }
    }
}

Optional: when testing manually, save the json into same files on the hard disk – you will need for point 5

4. The code that will use CountryRepository in official way will use the default – will not see the one with IJSonData

var c = new CountriesRepository();
var data = c.GetCountries().Result;

5. The testing code will define another class JSONFromHard that implements IJSonData and reads data from Hard

class JsonFromHard : IJsonData
{
//code to read files from disk 
// you have saved those when testing manually 
//see point 2 the optional part 
}

6. Because I have defined InternalsVisibleToAttribute to the WorldBank.Test, the Test dll can see the constructor with IJSonData  and then it class with JSONFromHard   – and I have no dependecy on the Http

 

public void GetAndInterpretData()
        {
            //uses [assembly: InternalsVisibleTo("WorldBank.Test")]
            var c = new CountriesRepository(new JsonFromHard("Countries"));
            var data = c.GetCountries().Result;
            Assert.Equal(218, data.Length);
        }

You can download the project from https://github.com/ignatandrei/WorldBankAPi  to see working.

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)

7 rules of logging ( and 7 notes)

Those are my 7 rules for logging (you can read also the side note for every rule)

So here they are:

  1. Logging is good for developers, not for the user .
  2. You should not re-invent logging framework – just use one that exists in your programming language.
  3. The logging should not affect the usual behavior of the program.
  4. You should log everywhere there is a layer made by you (including GUI).
  5. If the software returns/throws a generic code for error,then he should log the concrete details of what happened and the time * although this should be automatically done by the logging framework).
  6. The logging should be easy to configure ( where to log, what to log ).
  7. The format in which are you logging should be easy to understand by an automatic text parser

Note 1: Corollary : Every software needs logging and this should be performed as good as possible , with all specific details to understand the problem.

Note 2 : Choose one that works well in multi-threaded application, has multiple outputs ( database, log file, console , others) and provides various levels of logging( at least debug, info , error). For   .NET : log4net, nlog .  Do re-invent the wheel – if you believe that you can make something better .

Note 3 :  For example logging a function that modifies some parameters or logging variable.ToString() and this throws the infamous “null reference”

Note 4 : See http://liddellj.com/2014/09/09/log-all-the-things 

Note 5 : For example, if you throw a generic exception “ Something bad happened “ in order to not bother the user with mundane details, you should log the primary error and the reason that you throw the generic exception

Note 6:  This means it is easy for DevOps to configure the software to not log into production the debug levels .

Note 7: See http://dev.splunk.com/view/logging-best-practices/SP-CAAADP6

5 Minutes .NET–Memory Cache

 

At https://youtu.be/BL5yo_p7x-E you can find the new video about caching in .NET with Memory Cache.

The code is:

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Caching;
using System.Text;
using System.Threading.Tasks;

namespace MemoryCacheNet
{
    public static class GlobalData
    {
        static object lockMe = new object();

        public static List<string> CountryList()
        {
            string key = "countries";
            var data = MemoryCache.Default.Get(key) as List<string>;
            if(data == null)
            {
                lock (lockMe)
                {
                    data = MemoryCache.Default.Get(key) as List<string>;
                    if(data != null)
                    {
                        return data ;
                    }
                    data = CountryListFromDatabase();
                    var duration = DateTimeOffset.UtcNow.AddSeconds(5);
                    MemoryCache.Default.AddOrGetExisting(key,data,duration);
                   

                }
            }
            return data;
        }
        static List<string> CountryListFromDatabase()
        {
            Console.WriteLine("obtaining data from database");
            return new List<string>()
            {
                "Romania",
                "India",
                "USA"
                // add your country 😉
            };
        }
    }
}

and using from Console:

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

namespace MemoryCacheDOS
{
    class Program
    {
        static void Main(string[] args)
        {
            var data = GlobalData.CountryList();
            Console.WriteLine(data.Count);

            Console.WriteLine("waiting 1 sec");
            Thread.Sleep(1000);
            data = GlobalData.CountryList();
            Console.WriteLine(data.Count);

            Console.WriteLine("waiting 5 sec");
            Thread.Sleep(5000);
            data = GlobalData.CountryList();
            Console.WriteLine(data.Count);

        }
    }
}

 

Other tutorials are:

5MinThrowVsThrowEx
5Min Usefull Attributes
5MinIValidatableObject
5MinAsyncException
5MinAsync
5Min iMacrosAHK
5min Zip
5MinPSR
5MinParseWebPage
5MinFileHelpers
5Min Logging
5min Send emails and SMTP4Dev
5Min Memory Profiler ( User Object and/or memory leaks)
5min SFHB
5min – .TT files in Visual Studio

The full list is at https://www.youtube.com/playlist?list=PL4aSKgR4yk4OnmJW6PlBuDOXdYk6zTGps  .

EF Record and play – conclusions

 

Part 1 : What is EF record and play : http://msprogrammer.serviciipeweb.ro/2014/11/29/entity-framework-6-record-and-play-1-of-5/ 

Part 2: EF Record and play use: Testing : http://msprogrammer.serviciipeweb.ro/2014/12/08/entity-framework-6-record-and-play-use-unit-testing-part-2-of-5/

Part 3: EF Record and play use: Make demo: http://msprogrammer.serviciipeweb.ro/2014/12/14/entity-framework-6-record-and-play-use-making-demos-part-3-of-5/ 

Part 4: EF Record and play use: Record user Sql when a bug occurs: http://msprogrammer.serviciipeweb.ro/2014/12/26/ef-record-and-play-use-recording-user-sql-when-a-bug-occurred-part-4-of-5/

Part 5: EF record and play: conclusions: http://msprogrammer.serviciipeweb.ro/2015/01/05/ef-record-and-play-conclusions/

 

 

I have show to you how easy you can record and play again Entity Framework 6 actions. Also, some possible uses as testing, making demos and record user Sql.

One possible use that can be done is Profiling  – analyzing sql – for example, just like MiniProfiler or EFProfiler does. Read this article about profiling : http://msdn.microsoft.com/en-us/magazine/gg490349.aspx

I hope you enjoy this piece of code.

Source code is available at https://github.com/ignatandrei/EFRecordAndPlay/wiki/
There is also a NuGet package at https://www.nuget.org/packages/EFRecordAndPlay/

Video at https://www.youtube.com/playlist?list=PL4aSKgR4yk4Mi1eLKArsgoqQRluxXv2-Y

EF Record and play use: Recording user sql when a bug occurred (part 4 of 5)

 

Part 1 : What is EF record and play : http://msprogrammer.serviciipeweb.ro/2014/11/29/entity-framework-6-record-and-play-1-of-5/ 

Part 2: EF Record and play use: Testing : http://msprogrammer.serviciipeweb.ro/2014/12/08/entity-framework-6-record-and-play-use-unit-testing-part-2-of-5/

Part 3: EF Record and play use: Make demo: http://msprogrammer.serviciipeweb.ro/2014/12/14/entity-framework-6-record-and-play-use-making-demos-part-3-of-5/ 

Part 4: EF Record and play use: Record user Sql when a bug occurs: http://msprogrammer.serviciipeweb.ro/2014/12/26/ef-record-and-play-use-recording-user-sql-when-a-bug-occurred-part-4-of-5/

Part 5: EF record and play: conclusions: http://msprogrammer.serviciipeweb.ro/2015/01/05/ef-record-and-play-conclusions/

 

In this video tutorial I will show how to see the sql when a database bug occurs.

Let’s suppose that a department should have the name not null – and , by mistake, we have not prevented the registering of a null in the name of the department

We put those lines

 
 #region set record EF
            record = new InterceptionRecordOrPlay(@"DemoEmployeeAndDepartment.zip", ModeInterception.Record);

            DbInterception.Add(record);
            #endregion 

We register the null name department and  in the DemoEmployeeAndDepartment.zip we see the error

<?xml version="1.0"?>
<InterceptionData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <CommandText>INSERT [Department]([NameDepartment])
VALUES (NULL)
; SELECT [Id]
FROM [Department]
WHERE [Id] = CAST (@@IDENTITY AS int)</CommandText>
  <NonQuery>0</NonQuery>
  <ThrownExc>
    <ExceptionType>System.Data.SqlServerCe.SqlCeException</ExceptionType>
    <ExceptionMessage>The column cannot contain null values. [ Column name = NameDepartment,Table name = Department ]</ExceptionMessage>
  </ThrownExc>
</InterceptionData>

Source code is available at https://github.com/ignatandrei/EFRecordAndPlay/wiki/
There is also a NuGet package at https://www.nuget.org/packages/EFRecordAndPlay/

Entity Framework 6 Record and play use : Making demos ( part 3 of 5)

 

Part 1 : What is EF record and play : http://msprogrammer.serviciipeweb.ro/2014/11/29/entity-framework-6-record-and-play-1-of-5/ 

Part 2: EF Record and play use: Testing : http://msprogrammer.serviciipeweb.ro/2014/12/08/entity-framework-6-record-and-play-use-unit-testing-part-2-of-5/

Part 3: EF Record and play use: Make demo: http://msprogrammer.serviciipeweb.ro/2014/12/14/entity-framework-6-record-and-play-use-making-demos-part-3-of-5/ 

Part 4: EF Record and play use: Record user Sql when a bug occurs: http://msprogrammer.serviciipeweb.ro/2014/12/26/ef-record-and-play-use-recording-user-sql-when-a-bug-occurred-part-4-of-5/

Part 5: EF record and play: conclusions: http://msprogrammer.serviciipeweb.ro/2015/01/05/ef-record-and-play-conclusions/

 

 

Let’s assume you want to register a demo for a application ( Desktop or Web).

You do arrange the database and you make some repetition before. However, the database must be re-arranged the next time you will make the demo.

For this case you can use EF 6 Record and play – it will make a .zip file with containing resultsets and you can make indefinitely the repetition of the demo – without the real database.

 

Let’s see in action here

 

 

The code is simple

 Database.SetInitializer<ContextForDatabase>(null);
            #region set record EF
            record = new InterceptionRecordOrPlay(@"DemoEmployeeAndDepartment.zip", ModeInterception.Play);

            DbInterception.Add(record);
            #endregion

Source code is available at  https://github.com/ignatandrei/EFRecordAndPlay/wiki/

There is also a NuGet package at https://www.nuget.org/packages/EFRecordAndPlay/

Entity Framework 6 Record and play use : Unit Testing ( part 2 of 5)

 

Part 1 : What is EF record and play : http://msprogrammer.serviciipeweb.ro/2014/11/29/entity-framework-6-record-and-play-1-of-5/ 

Part 2: EF Record and play use: Testing : http://msprogrammer.serviciipeweb.ro/2014/12/08/entity-framework-6-record-and-play-use-unit-testing-part-2-of-5/

Part 3: EF Record and play use: Make demo: http://msprogrammer.serviciipeweb.ro/2014/12/14/entity-framework-6-record-and-play-use-making-demos-part-3-of-5/ 

Part 4: EF Record and play use: Record user Sql when a bug occurs: http://msprogrammer.serviciipeweb.ro/2014/12/26/ef-record-and-play-use-recording-user-sql-when-a-bug-occurred-part-4-of-5/

Part 5: EF record and play: conclusions: http://msprogrammer.serviciipeweb.ro/2015/01/05/ef-record-and-play-conclusions/

 

Let’s suppose that we have a program that have Departments and Employees.

And we want to make sure that, when we add an employee, the department must exists.

We can ensure this from database ( by foreign key) but we can pro-actively search for the department and throw a more meaningful validation .

More, I like more validation than errors.

So, let’s suppose that in the Validation for the Employee we must check in the database for the IdDepartment to see if there is such a department.

How could we make a test for that runs without a database ?

With some trick:  we first Record with a database  – then we can Play the file – and we do not need anymore the database. The test is self contained. 

 

Let’s see in action here

 

Database.SetInitializer<ContextForDatabase>(null);
            #region set record EF
            var record = new InterceptionRecordOrPlay(@"VerifyIValidatableWorks.zip", ModeInterception.Play);

            DbInterception.Add(record);
            #endregion
            var e= new Employee();
            e.ValidateEmployee = true;
            e.IDDepartment = 60000;
            var err= e.Validate(null).ToArray();
            Assert.IsNotNull(err);
            Assert.AreEqual(1, err.Length);

Source code is available at  https://github.com/ignatandrei/EFRecordAndPlay/wiki/

 

There is also a NuGet package at https://www.nuget.org/packages/EFRecordAndPlay/

Andrei Ignat weekly software news(mostly .NET)

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