I Will Get That Job

My preparation for coding interviews

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The Top 5 .NET Memory Management Misconceptions - Simple Talk

.NET Memory management is an impressive and comprehensive system, but it's not flawless, and it’s occasionally a little anti-intuitive. As a result, there are a few common misconceptions which need to be dispelled before you can really get the most out of the .NET framework.

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Where to use internals

regarding MSDN

A common use of internal access is in component-based development because it enables a group of components to cooperate in a private manner without being exposed to the rest of the application code. For example, a framework for building graphical user interfaces could provide Control and Form classes that cooperate using members with internal access. Since these members are internal, they are not exposed to code that is using the framework.

You can also use the internal modifier along with the InternalsVisibleTo assembly level attribute to create "friend" assemblies that are granted special access to the target assembly internal classes.

This can be useful for creation of unit testing assemblies that are then allowed to call internal members of the assembly to be tested. Of course no other assemblies are granted this level of access, so when you release your system, encapsulation is maintained.

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calling controller as browser in test

[TestMethod]        public async Task TestMethod1()        {            var applicationBasePath = @"the path to WebApplication3"            var builder = new WebHostBuilder()                .UseEnvironment("Developpement")                .UseKestrel()                .UseContentRoot(applicationBasePath)                                .UseStartup<WebApplication3.Startup>();            var server = new TestServer(builder)            {                BaseAddress = new Uri("http://localhost:1987"),            };            var client = server.CreateClient();            var res = await client.GetAsync("/Home/About");        }

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Skype interview 2017

1 kolo.

yield   (kde se pouziva - tate machine, generovane IEnumerable kolekce)

managed heap,stack

Jak funguje GC

Dictionary, searching in dictionary (modulo)

networking - osi model , RPC,  TCP (sitova zarizeni, komunikace po siti ), jake problemy nastavaji pri toku v sitich (zdoublene pakety, chybejici pakety, spatne poradi - resenim je packet counter).         

2. kolo    

struktura Heap + popsat heapsort

naimplementovat producer, consumer (AutoResetEvent)    

co je to semafor    Dependency  injection    

lock vs monitor    Co se deje, pokud zavolam monitor.Wait uvnitr locku

Inheritance x Composition    (SOLID)

relacni db x objektove db    

horizontalni skalovani   

Co je Divide and conquer 

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Find k-th shortest number

The example below finds k-th shortest number with complexity O(N). Alghoritm has two parts: 

SwapTillEnd - Moves the elements in a single position starting by index 

Main Cycle - iterates through main array 

k-th shortest element is last element in temporary array.

//Swaps elements from index till end of array public static void SwapTillEnd<T>(this T[] arr, int index) { T tmp = arr[index]; for (int i = index; i < arr.Length-1; i++) { T x = arr[i + 1]; arr[i + 1] = tmp; tmp = x; } }

public static void Main(string[] args) { int[] arr = { 10, 5, -1, 20, 12, 2, 2, 3, 5, 3, 4, 4, 7, 8, 9, -2, 0, 0, 1 }; int k = 4; var temp = new int[k]; //copy array because I don't want to manage <empty or Add > // corner cases Array.Copy(arr, temp, k); foreach (var item in arr) { for (int i = 0; i < temp.Length; i++) { if (item < temp[i]) { temp.SwapTillEnd(i); temp[i] = item; //don't forget break, break; } } } }

And result...

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Find the kth smallest number in list (QuickSelect)

It is simple. Get the list and sort all items Theta(N*log2(N)) and take k-th item.  Another way is to take incomplete quicksort and sort well only items <= k-th item.

The good explination is in this example and here.

Complexity

Work = n + n/2 + n/4 + …

To sum it all up, we can use the similarity rule:

Work/2 = n/2 + n/4 + n/8 + …

Hence:

Work – (Work/2) = n Work/2 = n Work = 2n Work = O(n)

public static class ArrayExtensions { public static T GetNthElement<T>(this T[] source, int nth) where T: IComparable<T> { if (source == null) throw new ArgumentNullException(); if (source.Count() == 0) throw new ArgumentOutOfRangeException(); var count = source.Count(); var from = 0; var to = count - 1; while(from < to) { var left = from; var right = to; int pivot = (left + right) / 2; var pivotValue = source[pivot]; while (left < right) { if(source[left].CompareTo(pivotValue) >= 0) { source.Swap(left, right); right--; } else { left++; } } if (source[left].CompareTo(pivotValue) > 0) left--; if (nth <= left) { to = left; } else { from = left + 1; } } return source[nth]; } private static void Swap<T>(this T[] array,int index1, int index2) { if (index1 == index2) return; var temp = array[index1]; array[index1] = array[index2]; array[index2] = temp; } }

[TestMethod] public void Find3rdLowerElement3() { var l = new int[] { 2, 1, 51, 52, 56, 35, 20 }; var x = l.GetNthElement(6); Assert.AreEqual(x, 56); x = l.GetNthElement(0); Assert.AreEqual(x, 1); x = l.GetNthElement(1); Assert.AreEqual(x, 2); }

Result StandardOutput: 5,9,2,1,0,3,5,2 LEFT:0 RIGHT:7 PIVOT:3 2,9,2,1,0,3,5,5 LEFT:0 RIGHT:6 PIVOT:3 5,9,2,1,0,3,2,5 LEFT:0 RIGHT:5 PIVOT:3 3,9,2,1,0,5,2,5 LEFT:0 RIGHT:4 PIVOT:3 0,9,2,1,3,5,2,5 LEFT:0 RIGHT:3 PIVOT:3 0,9,2,1,3,5,2,5 LEFT:1 RIGHT:3 PIVOT:3 0,1,2,9,3,5,2,5 LEFT:1 RIGHT:2 PIVOT:3 nth:3 pivot:3 from:0 to:7 -> nth:3 pivot:3 from:1 to:7 0,2,1,9,3,5,2,5 LEFT:1 RIGHT:7 PIVOT:4 0,2,1,9,3,5,2,5 LEFT:2 RIGHT:7 PIVOT:4 0,2,1,9,3,5,2,5 LEFT:3 RIGHT:7 PIVOT:4 0,2,1,5,3,5,2,9 LEFT:3 RIGHT:6 PIVOT:4 0,2,1,2,3,5,5,9 LEFT:3 RIGHT:5 PIVOT:4 0,2,1,2,3,5,5,9 LEFT:4 RIGHT:5 PIVOT:4 nth:3 pivot:4 from:1 to:7 -> nth:3 pivot:4 from:1 to:3 0,2,1,2,5,3,5,9 LEFT:1 RIGHT:3 PIVOT:2 0,2,1,2,5,3,5,9 LEFT:1 RIGHT:2 PIVOT:2 nth:3 pivot:2 from:1 to:3 -> nth:3 pivot:2 from:2 to:3 0,1,2,2,5,3,5,9 LEFT:2 RIGHT:3 PIVOT:2 nth:3 pivot:2 from:2 to:3 -> nth:3 pivot:2 from:3 to:3

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Current Versions in APRIL 2017

IIS 10 is included in Windows Server 2016 and Windows 10. This version includes support for HTTP/2.

.NET 4.7 

C# 7.0 released with VS2017 (March 2017) Major new features: tuples, ref locals and ref return, pattern matching (including pattern-based switch statements), inline out parameter declarations, local functions, binary literals, digit separators, and arbitrary async returns.

SQL Server 2014

C# 1.0 released with .NET 1.0 and VS2002 (January 2002) C# 1.2 (bizarrely enough); released with .NET 1.1 and VS2003 (April 2003). First version to call Dispose on IEnumerators which implemented IDisposable. A few other small features. C# 2.0 released with .NET 2.0 and VS2005 (November 2005). Major new features: generics, anonymous methods, nullable types, iterator blocks C# 3.0 released with .NET 3.5 and VS2008 (November 2007). Major new features: lambda expressions, extension methods, expression trees, anonymous types, implicit typing (var), query expressions C# 4.0 released with .NET 4 and VS2010 (April 2010). Major new features: late binding (dynamic), delegate and interface generic variance, more COM support, named arguments, tuple data type and optional parameters C# 5.0 released with .NET 4.5 and VS2012 (August 2012). Major features: async programming, caller info attributes. Breaking change: loop variable closure. C# 6.0 released with .NET 4.6 and VS2015 (July 2015). Implemented by Roslyn. Features: initializers for automatically implemented properties, using directives to import static members, exception filters, indexed members and element initializers, await in catch and finally, extension Add methods in collection initializers. C# 7.0 released with VS2017 (March 2017) Major new features: tuples, ref locals and ref return, pattern matching (including pattern-based switch statements), inline out parameter declarations, local functions, binary literals, digit separators, and arbitrary async returns.

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Web role vs Worker role

Very good article describing Azure in general is here

An application that runs on Windows Azure is referred to as a hosted service. Typically, a hosted service contains different computational resources that collectively process information and interact with each other and the external world. Hosted services in Windows Azure are said to contain roles, and there are currently two roles available: a worker role and a web role.

Worker roles are general-purpose code hosts. They are frequently used for long-running tasks that are non-interactive, but you can host any type of workload in them. Worker roles are general enough to host even complete application platforms such as Microsoft Internet Information Services (IIS) or Apache Tomcat. Windows Azure initiates worker roles and, like Windows services, they run all the time.

You can think of web roles as special cases of worker roles that have IIS 7 enabled by default. Therefore, they can host web applications and web services. Figure 1 illustrates web and worker roles.

Web roles and worker roles

Typically, a web role instance accepts incoming HTTP or HTTPS requests over ports 80 and 443. These public ports are referred to as public endpoints. All public endpoints are automatically load balanced at the network level. Both worker roles and web roles can make outbound TCP connections and can also open endpoints for incoming connections. In addition to the load-balanced public endpoints, instances can open internal endpoints. These internal endpoints are neither load-balanced, nor publically visible to the Internet. Instead, internal endpoints can be used for synchronous communication among instances and roles.

The VMs that run both web role and worker role instances also run a Windows Azure agent. This agent exposes an API that lets an instance interact with the Windows Azure FC. For example, an instance can use the agent to enumerate the public and internal endpoints in the VM instance it's running in or to discover run-time configuration settings.

An application deployed in a web role can be implemented with ASP.NET, Windows Communication Foundation (WCF), or any technology that works with IIS. For example, you can host a Hypertext Preprocessor (PHP) application on Windows Azure because IIS supports it through Fast CGI, which is a protocol that interfaces interactive applications with a web server. Most web role applications are optimized for workloads that follow a request-reply pattern, where the time between a request and a response is ideally very short.

A key consideration for the scalability of web roles is session management. In standard ASP.NET applications, there is some way to store session state. For example, an online store may keep track of a shopping cart. Similar to web farms, storing session state in memory on each server instance is a problem for web role–based websites because there's no guarantee that users will be directed to the same web role instance each time they make a request. Instead, you maintain state information in someplace other than the web role instance such as Windows Azure storage, SQL Azure, in a cookie that you pass back to the client, or in hidden form elements.

One of the most common application patterns in Windows Azure is for a web role to receive incoming requests and then use Windows Azure queues to pass them to the worker role to process. The worker role periodically looks in the queue for messages to see if there is any work to do. If there is, it performs the task. The web role typically retrieves completed work from persistent storage, such as a blob or a table. Figure 2 illustrates this typical design pattern.

Typical application pattern for web roles and worker roles

This is a simple and common interaction between a web role and a worker role, but there are many other possibilities. For example, you can use WCF to link web roles and worker roles.

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IaaS, Paas, SaaS

IaaS :  Infrastructure as a Service; Infrsatracture, Harddrives, Virtual Machines - scalable (Docker support on Linux, Windows, workflows to upload)

PaaS : Platform as a Service; Push the application to platform but not managing underlying infrastructure

SaaS : Software as a Service - Office 365, API’s that you use in your own applications (credit card verification)

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Describe how would you do logging and tracing in Azure ?

Application diagnostics allows you to capture information produced by a web application. ASP.NET applications can use the System.Diagnostics.Trace class to log information to the application diagnostics log. 

System.Diagnostics.Trace.TraceError("If you're seeing this, something bad happened");

In The classic portal Web app Configure tab you can choose verbosity Level (informational, warning and error) and configure  storage where is log writtn to: 

File system - stores the application diagnostics information to the web app file system. These files can be accessed by FTP, or downloaded as a Zip archive by using the Azure PowerShell or Azure Command-Line Interface (Azure CLI).

Table storage - stores the application diagnostics information in the specified Azure Storage Account and table name.

Blob storage - stores the application diagnostics information in the specified Azure Storage Account and blob container.

Retention period - by default, logs are not automatically deleted from blob storage. Select set retention and enter the number of days to keep logs if you wish to automatically delete logs.

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What is Partition key, Row key

I used all from this article. 

RowKey in Table Storage

A RowKey in Table Storage is a very simple thing: it’s your “primary key” within a partition. PartitionKey + RowKey form the composite unique identifier for an entity. Within one PartitionKey, you can only have unique RowKeys. If you use multiple partitions, the same RowKey can be reused in every partition.

So in essence, a RowKey is just the identifier of an entity within a partition.

PartitionKey in Table Storage

In Table Storage, you have to decide on the PartitionKey yourself. In essence, you are responsible for the throughput you’ll get on your system. If you put every entity in the same partition (by using the same partition key), you’ll be limited to the size of the storage machines for the amount of storage you can use. Plus, you’ll be constraining the maximal throughput as there’s lots of entities in the same partition.

Performance

The fastest way of querying? Specifying both PartitionKey and RowKey. By doing this, table storage will immediately know which partition to query and can simply do an ID lookup on RowKey within that partition.

Less fast but still fast enough will be querying by specifying PartitionKey: table storage will know which partition to query.

Less fast: querying on only RowKey. Doing this will give table storage no pointer on which partition to search in, resulting in a query that possibly spans multiple partitions, possibly multiple storage nodes as well. Wihtin a partition, searching on RowKey is still pretty fast as it’s a unique index.

Slow: searching on other properties (again, spans multiple partitions and properties).

Deciding on PartitionKey and RowKey

Let’s use three tables: Customer, Order and Orderline.

An ideal setup may be this one, depending on how you want to query everything:

Customer (PK: sales region, RK: customer id) – it enables fast searches on region and on customer id Order (PK: customer id, RK; order id) – it allows me to quickly fetch all orders for a specific customer (as they are colocated in one partition), it still allows fast querying on a specific order id as well) Orderline (PK: order id, RK: order line id) – allows fast querying on both order id as well as order line id.

Of course, depending on the system you are building, the following may be a better setup:

Customer (PK: customer id, RK: display name) – it enables fast searches on customer id and display name Order (PK: customer id, RK; order id) – it allows me to quickly fetch all orders for a specific customer (as they are colocated in one partition), it still allows fast querying on a specific order id as well) Orderline (PK: order id, RK: item id) – allows fast querying on both order id as well as the item bought, of course given that one order can only contain one order line for a specific item (PK + RK should be unique)

You see? Choose them wisely, depending on your queries. And maybe an important sidenote: don’t be afraid of denormalizing your data and storing data twice in a different format, supporting more query variations.

There’s one additional “index”

That’s right! People have been asking Microsoft for a secondary index. And it’s already there… The table name itself! Take our customer – order – orderline sample again…

Having a Customer table containing all customers may be interesting to search within that data. But having an Orders table containing every order for every customer may not be the ideal solution. Maybe you want to create an order table per customer? Doing that, you can easily query the order id (it’s the table name) and within the order table, you can have more detail in PK and RK.

And there's one more: your account name. Split data over multiple storage accounts and you have yet another "partition".

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Table Storage vs. Blob storage in Azure

Blob : 200GB - 1TB (1TB per blob; 100TB by Account). Storing Audio, Video, files, DB backups, VM backups etc.. Access through HTTP and HTTPS. Storage has containers (logical + physical grouping for containers). The number of containers is not limited. Containers has public or private restrictions. Blobs are accessible through HTTP or API:  http://<accountName>.blob.core.Windows.net/<container>/<blob> Microsoft.WindowsAzure.Storage.dll

//UPLOAD TO FILE, the code should be called asynchronously var accnt = CloudStorageAccount.Parse(connstr.Name,key,credentials) var client = accnt.CreateCloudBlobClient(); var container client.GetContainerReference(containerName); container.CreateIfNotExist(); var blob = container.GetBlobReference("file.txt"); using(var stream = new MemoryStream("file Content".AsByteArray())){ blob.uploadFromStream(stream); }

Table : 100 TB key-value pairs. Is faster than SQL Server Azure

public class StorageFactory { private static readonly ILog _Log = LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType.Name); public ICloudTable CreateTable(string connectionString, string name) { _Log.InfoFormat("Creating {0} table", name); var storageAccount = CloudStorageAccount.Parse(connectionString); var tableClient = storageAccount.CreateCloudTableClient(); var table = tableClient.GetTableReference(name); table.CreateIfNotExists(); _Log.InfoFormat("Created {0} table", name); return new CloudTableWrapper(table); } }

var storageFactory = new StorageFactory(); var accountsTable = storageFactory.CreateTable(cfg.ConnectionString, "Accounts"); var betsTable = storageFactory.CreateTable(cfg.ConnectionString, "Bets"); var repository = new BettingRepository(accountsTable, betsTable);

public sealed class BettingRepository : IAccountRepository { private readonly ICloudTable _accountTable; private readonly ICloudTable _betTable; public BettingRepository(ICloudTable accountTable, ICloudTable betsTable) { if (accountTable == null) throw new ArgumentNullException("accountTable"); _accountTable = accountTable; _betTable = betsTable; } public async Task<IEnumerable<BettingAccount>> LoadAccountsAsync() { BettingAccountEntity[] entities = await _accountTable.ExecuteQueryAsync(new TableQuery<BettingAccountEntity>()); return entities.Select(e => e.ToAccount()); } public async Task UpsertAccountAsync(BettingAccount account) { await _accountTable.ExecuteAsync(TableOperationWrapper.Insert(BettingAccountEntity.ConvertFrom(account))); } public async Task<IEnumerable<Bet>> LoadBetsAsync() { var entities = await _betTable.ExecuteQueryAsync(new TableQuery<BetEntity>()); return entities.Select(e => e.ToBet()); } public async Task<bool> BetExists(string rowKey) { var result = await _betTable.ExecuteAsync(TableOperationWrapper.Retrieve<BetEntity>("[email protected]", rowKey)); return result.Result != null; } public async Task UpsertBetAsync(Bet bet) { await _betTable.ExecuteAsync(TableOperationWrapper.InsertOrUpdate(BetEntity.ConvertFrom(bet))); } }

You should store metadata in Table and video in BLOBS. regarding this: https://docs.microsoft.com/en-us/rest/api/storageservices/fileservices/understanding-the-table-service-data-model PartitionKey: max 1KB, Rowkey: max 1KB, Timestamp: internally to provide optimistic concurrency and data: An entity can have up to 255 properties.MAximum size of property is 64kb. Whole row may not exceed 1MB.

CDN  cache blobs and static content.Maximizes performance and splits the traffic.  It is useful when we know where are users physically sitting. Works for public containers where anonymous access is enabled. 

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What happened if you throw an exception while processing an Event?

Nothing special, program will be stopped on place where Exception is thrown:

class ExceptionInEventHandlerTest { private event EventHandler LocalEvent; static void Main(string[] args) { var instance = new ExceptionInEventHandlerTest(); instance.LocalEvent += Program_LocalEvent; instance.LocalEvent(instance, new EventArgs()); Console.WriteLine("Program is over press something"); Console.ReadLine(); } private static void Program_LocalEvent(object sender, EventArgs e) { Console.WriteLine("Hello from EventHandler"); throw new Exception("planned exception for test purposes"); Console.WriteLine("End of EventHandler"); } }

and result :

Hello from EventHandler Neošetřená výjimka: System.Exception: planned exception for test purposes v ExceptionInEventHandler.ExceptionInEventHandlerTest.Program_LocalEvent(Object sender, EventArgs e) v C:\Temp\ExceptionInEventHandler\ExceptionInEventHandler\Program.cs:řádek 24 v ExceptionInEventHandler.ExceptionInEventHandlerTest.Main(String[] args) v C:\Temp\ExceptionInEventHandler\ExceptionInEventHandler\Program.cs:řádek 16 Press any key to continue . . .

Following code shows how exception bubbles back to the caller:

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; namespace ExceptionInEventHandler { class ExceptionInEventHandlerTest { private event EventHandler LocalEvent; static void Main(string[] args) { try { var instance = new ExceptionInEventHandlerTest(); instance.LocalEvent += Program_LocalEvent; instance.LocalEvent(instance, new EventArgs()); Console.WriteLine("Program is over press something"); } catch (Exception ex) { Console.WriteLine("Handled exception occured: {0}",ex.Message); } Console.ReadLine(); } private static void Program_LocalEvent(object sender, EventArgs e) { Console.WriteLine("Hello from EventHandler"); throw new Exception("planned exception for test purposes"); Console.WriteLine("End of EventHandler"); } } }

and result :

Hello from EventHandler Handled exception occured: planned exception for test purposes

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What Is Semaphore And How To Use It?

Semaphore lets you set a limit on how many threads have access to a critical section ( A critical section is a piece of code that accesses a shared resource (data structure or device) but only limited number of threads can access it).   It's often described as a nightclub (the semaphore) where the visitors (threads) stands in a queue outside the nightclub waiting for someone to leave in order to gain entrance.

WaitOne() method when entering an area of code that you want restricted to a certain number of threads. When processing finishes, call the Release() method to release the slot back to the pool. 

The count on a semaphore is decremented each time a thread enters the semaphore, and incremented when a thread releases the semaphore. When the count is zero, subsequent requests block until other threads release the semaphore. When all threads have released the semaphore, the count is at the maximum value specified when the semaphore was created. There is no guaranteed order  in which blocked threads enter the semaphore.

Example

class Program { int result = 0; AutoResetEvent event1 = new AutoResetEvent(false); AutoResetEvent event2 = new AutoResetEvent(false); AutoResetEvent event3 = new AutoResetEvent(false); void Work1() { result = 1; event1.Set(); } void Work2() { result = 2; event2.Set(); } void Work3() { WaitHandle.WaitAll(new WaitHandle[] { event1, event2 }); result = 3; event3.Set(); } static void Main(string[] args) { Program a = new Program(); Thread worker1 = new Thread(a.Work1); Thread worker2 = new Thread(a.Work2); Thread worker3 = new Thread(a.Work3); worker3.Start(); worker1.Start(); worker2.Start(); WaitHandle.WaitAny(new WaitHandle[] { a.event3 }); Console.WriteLine(a.result); } }

We run code by

class Program { static void Main(string[] args) { var semaphore = new SemaphoreTest(); semaphore.Start(); Console.ReadKey(); } }

And result says that threads were running in the batches (check seconds)

1.0007945 processing T1 1.0018164 processing T2 1.0018283 processing T3 2.0019704 processing T6 2.0019806 processing T5 2.0022565 processing T4 3.0023643 processing T8 3.0023743 processing T7 3.003351 processing T9 4.0027151 processing T1 4.0037565 processing T3 4.0037705 processing T2 Press any key to continue . . .

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What is Race Condition ?

The race condition occurs when when two or more threads modifies shared data in insufficient order. 

 Because the thread scheduling algorithm can swap between threads at any point, we cannot know the order at which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm.

Example

class Program { int result = 0; void Work1() { result = 1; } void Work2() { result = 2; } void Work3() { result = 3; } static void Main(string[] args) { Program a = new Program(); Thread worker1 = new Thread(a.Work1); Thread worker2 = new Thread(a.Work2); Thread worker3 = new Thread(a.Work3); worker1.Start(); worker2.Start(); worker3.Start(); Console.WriteLine(a.result); } }

At the example above you might expecting stable result 3. After several runs I got 3,3,2,3,2,3 but result might be 1,2,3 or even 0. The reason for this is that the operating system decides which thread gets executed first. And the order in which we start the threads is not all that important. So thread that is given least priority by the operating system gets executed last.

The sample code returning 3,3,3,3,.. in row looks like this:

class Program { int result = 0; AutoResetEvent event1 = new AutoResetEvent(false); AutoResetEvent event2 = new AutoResetEvent(false); AutoResetEvent event3 = new AutoResetEvent(false); void Work1() { result = 1; event1.Set(); } void Work2() { result = 2; event2.Set(); } void Work3() { WaitHandle.WaitAll(new WaitHandle[] { event1, event2 }); result = 3; event3.Set(); } static void Main(string[] args) { Program a = new Program(); Thread worker1 = new Thread(a.Work1); Thread worker2 = new Thread(a.Work2); Thread worker3 = new Thread(a.Work3); worker3.Start(); worker1.Start(); worker2.Start(); WaitHandle.WaitAny(new WaitHandle[] { a.event3 }); Console.WriteLine(a.result); } }

Awoiding Race Conditions

Use synchronisation objects (locks, Mutexes, ManualResetEvents, semaphores etc...) to make the code thread safe.

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You want to use class as a key in Dictionary, what you have to do?

To use custom class as key you need override virtual methods GetHashCode() and Equals(). See the example below:

>class KeyClass { public KeyClass(string a) { A = a; } string A { get; set; } public override bool Equals(object obj) { return A == ((KeyClass)obj).A; } public override int GetHashCode() { return A.Length == 1 ? 5 : 0; } } static void Main(string[] args) { var d = new Dictionary<KeyClass, string>(); d.Add(new KeyClass("hello"), "new KeyClass(\"hello\")"); d.Add(new KeyClass("f"), "foo"); d.Add(new KeyClass(string.Empty), "new KeyClass(string.Empty)"); Console.WriteLine(d[new KeyClass("f")]); Console.WriteLine(d[new KeyClass("hello")]); Console.WriteLine(d[new KeyClass("something else")]); }

And expected output

foo new KeyClass("hello") Neošetřená výjimka: System.Collections.Generic.KeyNotFoundException: Daný klíč není ve slovníku k dispozici. v System.Collections.Generic.Dictionary`2.get_Item(TKey key) v DictionaryTest.Program.Main(String[] args) v C:\Temp\PripravaNa\DictionaryTest\DictionaryTest\Program.cs:řádek 39 Press any key to continue . . .

If we override only one of the methods we get KeyNotFound exception on line 

Console.WriteLine(d[new KeyClass("f")]);

Please look at following articles:

C# Object Equality Test

C# HashCode

About Dictionary and string collisions