60 .NET Interview Questions Devs Must Focus On (ANSWERED)

string is an alias in C# for System.String. So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, it's generally recommended to use string any time you're referring to an object.

string place = "world";

Likewise, it's generally recommended to use String if you need to refer specifically to the class.

string greet = String.Format("Hello {0}!", place);

Yes. This might surprise many, but ASP.NET Core works with .NET framework and this is officially supported by Microsoft.

ASP.NET Core works with:

• .NET Core framework
• .NET framework

• Model — It’s a business entity and it is used to represent the application data.
• Controller — Request sent by the user always scatters through controller and it’s responsibility is to redirect to the specific view using View() method.
• View — It’s the presentation layer of MVC.

A type is said to be nullable if it can be assigned a value or can be assigned null, which means the type has no value whatsoever. By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.

For example, you can store any value from -2,147,483,648 to 2,147,483,647 or null in a Nullable<Int32> variable. Similarly, you can assign true, false, or null in a Nullable<bool> variable.

The reference types do not contain the actual data stored in a variable, but they contain a reference to the variables.

In other words, they refer to a memory location. Using multiple variables, the reference types can refer to a memory location. If the data in the memory location is changed by one of the variables, the other variable automatically reflects this change in value. Example of built-in reference types are: object, dynamic, and string.

Generics allow you to delay the specification of the data type of programming elements in a class or a method, until it is actually used in the program. In other words, generics allow you to write a class or method that can work with any data type.

Consider:

class DataStore<T>
{
    public T Data { get; set; }
}

DataStore<string> store = new DataStore<string>();

Application pools allow you to isolate your applications from one another, even if they are running on the same server. This way, if there is an error in one app, it won't take down other applications.

Additionally, applications pools allow you to separate different apps which require different levels of security.

1. .NET Standard solves the code sharing problem for .NET developers across all platforms by bringing all the APIs that you expect and love across the environments that you need: desktop applications, mobile apps & games, and cloud services:
2. .NET Standard is a set of APIs that all .NET platforms have to implement. This unifies the .NET platforms and prevents future fragmentation.
3. .NET Standard 2.0 will be implemented by .NET Framework, .NET Core, and Xamarin. For .NET Core, this will add many of the existing APIs that have been requested.
4. .NET Standard 2.0 includes a compatibility shim for .NET Framework binaries, significantly increasing the set of libraries that you can reference from your .NET Standard libraries.
5. .NET Standard will replace Portable Class Libraries (PCLs) as the tooling story for building multi-platform .NET libraries.

The CLR stands for Common Language Runtime and it is an Execution Environment. It works as a layer between Operating Systems and the applications written in .NET languages that conforms to the Common Language Specification (CLS). The main function of Common Language Runtime (CLR) is to convert the Managed Code into native code and then execute the program.

When we compile our .NET code then it is not directly converted to native/binary code; it is first converted into intermediate code known as MSIL code which is then interpreted by the CLR. MSIL is independent of hardware and the operating system. Cross language relationships are possible since MSIL is the same for all .NET languages. MSIL is further converted into native code.

Boxing and Unboxing both are used for type conversion but have some difference:

• Boxing - Boxing is the process of converting a value type data type to the object or to any interface data type which is implemented by this value type. When the CLR boxes a value means when CLR is converting a value type to Object Type, it wraps the value inside a System.Object and stores it on the heap area in application domain.

• Unboxing - Unboxing is also a process which is used to extract the value type from the object or any implemented interface type. Boxing may be done implicitly, but unboxing have to be explicit by code.

The concept of boxing and unboxing underlines the C# unified view of the type system in which a value of any type can be treated as an object.

• Managed Code - The code, which is developed in .NET framework is known as managed code. This code is directly executed by CLR with the help of managed code execution. Any language that is written in .NET Framework is managed code.
• Unmanaged Code - The code, which is developed outside .NET framework is known as unmanaged code. Applications that do not run under the control of the CLR are said to be unmanaged, and certain languages such as C++ can be used to write such applications, which, for example, access low - level functions of the operating system. Background compatibility with the code of VB, ASP and COM are examples of unmanaged code.

The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events.

An Abstract class is a class which is denoted by abstract keyword and can be used only as a Base class.

abstract class Shape
{
    public abstract int GetArea();
}

class Square : Shape
{
    int side;

    public Square(int n) => side = n;

    // GetArea method is required to avoid a compile-time error.
    public override int GetArea() => side * side;

    static void Main()
    {
        var sq = new Square(12);
        Console.WriteLine($"Area of the square = {sq.GetArea()}");
    }
}
// Output: Area of the square = 144

Abstract classes have the following features:

• An abstract class cannot be instantiated.

• An abstract class may contain abstract methods and accessors.

• It is not possible to modify an abstract class with the sealed modifier because the two modifiers have opposite meanings. The sealed modifier prevents a class from being inherited and the abstract modifier requires a class to be inherited.

• A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.

Use that rule of thumb:

• If you found it in the Microsoft .NET Framework: it's managed.
• If you went poking around MSDN yourself, it's unmanaged.

Anything you've used P/Invoke calls to get outside of the nice comfy world of everything available to you in the .NET Framwork is unmanaged – and you're now responsible for cleaning it up.

In short:

• Disconnected - Make Connection , Fetch Data , Close Connection
• Connected - Make Connection , Keep Connection alive , Close Connection when close is called.

The ADO.NET architecture, in which connection must be kept open till the end to retrieve and access data from database is called as connected architecture. Connected architecture is built on the these types - connection, command, datareader

The ADO.NET architecture, in which connection will be kept open only till the data retrieved from database, and later can be accessed even when connection to database is closed is called as disconnected architecture. Disconnected architecture of ADO.net is built on these types - connection, dataadapter, commandbuilder and dataset and dataview.

ASP.NET, at its most basic level, provides a means for you to provide general HTML markup combined with server side "controls" within the event-driven programming model that can be leveraged with VB, C#, and so on. You define the page(s) of a site, drop in the controls, and provide the programmatic plumbing to make it all work.

ASP.NET MVC is an application framework based on the Model-View-Controller architectural pattern. This is what might be considered a "canned" framework for a specific way of implementing a web site, with a page acting as the "controller" and dispatching requests to the appropriate pages in the application. The idea is to "partition" the various elements of the application, eg business rules, presentation rules, and so on.

Think of the former as the "blank slate" for implementing a site architecture you've designed more or less from the ground up. MVC provides a mechanism for designing a site around a pre-determined "pattern" of application access, if that makes sense. There's more technical detail to it than that, to be sure, but that's the nickel tour for the purposes of the question.

• Use Controller to render your normal views.
• ApiController action only returns data that is serialized and sent to the client.

Consider:

public class TweetsController : Controller {
  // GET: /Tweets/
  [HttpGet]
  public ActionResult Index() {
    return Json(Twitter.GetTweets(), JsonRequestBehavior.AllowGet);
  }
}

or

public class TweetsController : ApiController {
  // GET: /Api/Tweets/
  public List<Tweet> Get() {
    return Twitter.GetTweets();
  }
}

Precision is the main difference.

• Float - 7 digits (32 bit)
• Double -15-16 digits (64 bit)
• Decimal -28-29 significant digits (128 bit)

As for what to use when:

• For values which are "naturally exact decimals" it's good to use decimal. This is usually suitable for any concepts invented by humans: financial values are the most obvious example, but there are others too. Consider the score given to divers or ice skaters, for example.

• For values which are more artefacts of nature which can't really be measured exactly anyway, float/double are more appropriate. For example, scientific data would usually be represented in this form. Here, the original values won't be "decimally accurate" to start with, so it's not important for the expected results to maintain the "decimal accuracy". Floating binary point types are much faster to work with than decimals.

Class and struct both are the user defined data type but have some major difference:

Struct

• The struct is value type in C# and it inherits from System.Value Type.
• Struct is usually used for smaller amounts of data.
• Struct can't be inherited to other type.
• A structure can't be abstract.

Class

• The class is reference type in C# and it inherits from the System.Object Type.
• Classes are usually used for large amounts of data.
• Classes can be inherited to other class.
• A class can be abstract type.
• We can create a default constructor.

Finally block will be executed irrespective of exception. So while executing the code in try block when exception is occurred, control is returned to catch block and at last finally block will be executed. So closing connection to database / releasing the file handlers can be kept in finally block.

Below are the processed followed in the sequence:

• App initialization
• Routing
• Instantiate and execute controller
• Locate and invoke controller action
• Instantiate and render view

• Windows Communication Foundation is designed to exchange standard SOAP-based messages using variety of transport protocols like HTTP, TCP, NamedPipes or MSMQ, etc.
• On the other hand, ASP.NET API is a framework for building non-SOAP based services over HTTP only.

Anonymous types allow us to create a new type without defining them. This is way to defining read only properties into a single object without having to define type explicitly. Here Type is generating by the compiler and it is accessible only for the current block of code. The type of properties is also inferred by the compiler.

Consider:

var anonymousData = new
{  
     ForeName = "Jignesh",  
     SurName = "Trivedi"
};  

Console.WriteLine("First Name : " + anonymousData.ForeName); 

• Lazy Loading: It is a process to delay the loading of related objects until it is required.
• Eager Loading: It occurs when you query for an object and all of the related objects are also returned. In eager loading, related objects are loaded automatically with its parent object
• Explicit Loading: Explicitly loading takes place when you have disabled Lazy loading, and you still want to lazy loading. For this, we have to call the load method on the related entities.

• Managed code is not compiled to machine code but to an intermediate language which is interpreted and executed by some service on a machine and is therefore operating within a (hopefully!) secure framework which handles dangerous things like memory and threads for you. It runs on the CLR (Common Language Runtime), which, among other things, offers services like garbage collection, run-time type checking, and reference checking. So, think of it as, "My code is managed by the CLR."

• Unmanaged code is compiled to machine code and therefore executed by the OS directly. It therefore has the ability to do damaging/powerful things Managed code does not. This is how everything used to work, so typically it's associated with old stuff like .dlls

• Thread represents an actual OS-level thread, with its own stack and kernel resources. Thread allows the highest degree of control; you can Abort() or Suspend() or Resume() a thread, you can observe its state, and you can set thread-level properties like the stack size, apartment state, or culture. ThreadPool is a wrapper around a pool of threads maintained by the CLR.

• The Task class from the Task Parallel Library offers the best of both worlds. Like the ThreadPool, a task does not create its own OS thread. Instead, tasks are executed by a TaskScheduler; the default scheduler simply runs on the ThreadPool. Unlike the ThreadPool, Task also allows you to find out when it finishes, and (via the generic Task) to return a result.

View state is a kind of hash map (or at least you can think of it that way) that ASP.NET uses to store all the temporary information about a page - like what options are currently chosen in each select box, what values are there in each text box, which panel are open, etc. You can also use it to store any arbitrary information.

The entire map is serialized and encoded and kept in a hidden variable (__VIEWSTATE form field) that's posted back to the server whenever you take any action on the page that requires a server round trip. This is how you can access the values on the controls from the server code. If you change any value in the server code, that change is made in the view state and sent back to the browser.

Just be careful about how much information you store in the view state, though... it can quickly become bloated and slow to transfer each time to the server and back.

It's not encrypted at all. Just base encoded, which easily reversible.

Unit of Work is referred to as a single transaction that involves multiple operations of insert/update/delete and so on kinds. To say it in simple words, it means that for a specific user action (say registration on a website), all the transactions like insert/update/delete and so on are done in one single transaction, rather than doing multiple database transactions.

• Once a class is defined as a sealed class, the class cannot be inherited.
• Structs are also sealed.

A Destructor is used to clean up the memory and free the resources. But in C# this is done by the garbage collector on its own. System.GC.Collect() is called internally for cleaning up. The answer to second question is "almost never".

Typically one only creates a destructor when your class is holding on to some expensive unmanaged resource that must be cleaned up when the object goes away. It is better to use the disposable pattern to ensure that the resource is cleaned up. A destructor is then essentially an assurance that if the consumer of your object forgets to dispose it, the resource still gets cleaned up eventually.

If you make a destructor be extremely careful and understand how the garbage collector works. Destructors are really weird:

• They don't run on your thread; they run on their own thread. Don't cause deadlocks!
• An unhandled exception thrown from a destructor is bad news. It's on its own thread; who is going to catch it?
• A destructor may be called on an object after the constructor starts but before the constructor finishes. A properly written destructor will not rely on invariants established in the constructor.
• A destructor can "resurrect" an object, making a dead object alive again. That's really weird. Don't do it.
• A destructor might never run; you can't rely on the object ever being scheduled for finalization. It probably will be, but that's not a guarantee.

.NET as whole now has 2 flavors:

• .NET Framework
• .NET Core

.NET Core and the .NET Framework have (for the most part) a subset-superset relationship. .NET Core is named “Core” since it contains the core features from the .NET Framework, for both the runtime and framework libraries. For example, .NET Core and the .NET Framework share the GC, the JIT and types such as String and List.

.NET Core was created so that .NET could be open source, cross platform and be used in more resource-constrained environments.

• Compatibility: Libraries that target .NET Standard will run on any .NET Standard compliant runtime, such as .NET Core, .NET Framework, Mono/Xamarin. On the other hand, libraries that target .NET Core can only run on the .NET Core runtime.

• API Surface Area: .NET Standard libraries come with everything in NETStandard.Library whereas .NET Core libraries come with everything in Microsoft.NETCore.App. The latter includes approximately 20 additional libraries, some of which we can add manually to our .NET Standard library (such as System.Threading.Thread) and some of which are not compatible with the .NET Standard (such as Microsoft.NETCore.CoreCLR).

The == Operator (usually means the same as ReferenceEquals, could be overrided) compares the reference identity while the Equals() (virtual Equals()) method compares if two objects are equivalent.

There are some differences between Abstract Class and Interface which are listed below:

• interfaces can have no state or implementation
• a class that implements an interface must provide an implementation of all the methods of that interface
• abstract classes may contain state (data members) and/or implementation (methods)
• abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)
• interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes - they permit an implementation of multiple inheritance that removes many of the problems of general MI).

Consider using abstract classes if :

1. You want to share code among several closely related classes.
2. You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
3. You want to declare non-static or non-final fields.

Consider using interfaces if :

1. You expect that unrelated classes would implement your interface. For example, many unrelated objects can implement Serializable interface.
2. You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
3. You want to take advantage of multiple inheritances of type.

To connect to the database server is recommended to use Windows Authentication, commonly known as integrated security. To specify the Windows authentication, you can use any of the following two key-value pairs with the data provider. NET Framework for SQL Server:

Integrated Security = true;
Integrated Security = SSPI;

However, only the second works with the data provider .NET Framework OleDb. If you set Integrated Security = true for ConnectionString an exception is thrown.

ViewBag is a wrapper around ViewData, which allows to create dynamic properties.

Advantage of viewbag over viewdata will be:

• In ViewBag no need to typecast the objects as in ViewData.
• ViewBag will take advantage of dynamic keyword which is introduced in version 4.0. But before using ViewBag we have to keep in mind that ViewBag is slower than ViewData.

• ref tells the compiler that the object is initialized before entering the function, while
• out tells the compiler that the object will be initialized inside the function.

So while ref is two-ways, out is out-only.

The "primary" use of the IDisposable interface is to clean up unmanaged resources. Note the purpose of the Dispose pattern is to provide a mechanism to clean up both managed and unmanaged resources and when that occurs depends on how the Dispose method is being called.