C Sharp Interview Questions

C# INTERVIEW QUESTIONS

What’s the implicit name of the parameter that gets passed into the class’ set method?

Value, and it’s datatype depends on whatever variable we’re changing.

How do you inherit from a class in C#?

Place a colon and then the name of the base class. Notice that it’s double colon in C++.

Does C# support multiple inheritance?

No, use interfaces instead.

When you inherit a protected class-level variable, who is it available to? Classes in the same namespace.

Are private class-level variables inherited?

Yes, but they are not accessible, so looking at it you can honestly say that they are not inherited. But they are.

Describe the accessibility modifier protected internal.

It’s available to derived classes and classes within the same Assembly (and naturally from the base class it’s declared in).

C# provides a default constructor for me. I write a constructor that takes a string as a parameter, but want to keep the no parameter one. How many constructors should I write?

Two. Once you write at least one constructor, C# cancels the freebie constructor, and now you have to write one yourself, even if there’s no implementation in it.

What’s the top .NET class that everything is derived from?

System.Object.

How’s method overriding different from overloading?

When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.

What does the keyword virtual mean in the method definition?

The method can be over-ridden.

Can you declare the override method static while the original method is non-static?

No, you can’t, the signature of the virtual method must remain the same, only the keyword virtual is changed to keyword override.

Can you override private virtual methods?

No, moreover, you cannot access private methods in inherited classes, have to be protected in the base class to allow any sort of access.

Can you prevent your class from being inherited and becoming a base class for some other classes?

Yes, that’s what keyword sealed in the class definition is for. The developer trying to derive from your class will get a message: cannot inherit from Sealed class WhateverBaseClassName. It’s the same concept as final class in Java.

Can you allow class to be inherited, but prevent the method from being over-ridden?

Yes, just leave the class public and make the method sealed.

What’s an abstract class?

A class that cannot be instantiated. A concept in C++ known as pure virtual method. A class that must be inherited and have the methods over-ridden. Essentially, it’s a blueprint for a class without any implementation.

When do you absolutely have to declare a class as abstract (as opposed to free-willed educated choice or decision based on UML diagram)?

When at least one of the methods in the class is abstract. When the class itself is inherited from an abstract class, but not all base abstract methods have been over-ridden.

What’s an interface class?

It’s an abstract class with public abstract methods all of which must be implemented in the inherited classes.

Why can’t you specify the accessibility modifier for methods inside the interface?

They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it’s public by default.

Can you inherit multiple interfaces?

Yes, why not.

And if they have conflicting method names?

It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.

What’s the difference between an interface and abstract class?

In the interface all methods must be abstract, in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes.

How can you overload a method?

Different parameter data types, different number of parameters, different order of parameters.

If a base class has a bunch of overloaded constructors, and an inherited class has another bunch of overloaded constructors, can you enforce a call from an inherited constructor to an arbitrary base constructor?

Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

What’s the difference between System.String and System.StringBuilder classes?

System.String is immutable, System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

Is it namespace class or class namespace?

The .NET class library is organized into namespaces. Each namespace contains a functionally related group of classes so natural namespace comes first.

What’s the advantage of using System.Text.StringBuilder over System.String? StringBuilder is more efficient in the cases, where a lot of manipulation is done to the text. Strings are immutable, so each time it’s being operated on, a new instance is created.

Can you store multiple data types in System.Array? No.

What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?

The first one performs a deep copy of the array, the second one is shallow.

How can you sort the elements of the array in descending order?

By calling Sort() and then Reverse() methods.

What’s the .NET datatype that allows the retrieval of data by a unique key? HashTable.

What’s class SortedList underneath?

A sorted HashTable.

Will finally block get executed if the exception had not occurred? Yes.

What’s the C# equivalent of C++ catch (…), which was a catch-all statement for any possible exception?

A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.

Can multiple catch blocks be executed?

No, once the proper catch code fires off, the control is transferred to the finally block (if there are any), and then whatever follows the finally block.

Why is it a bad idea to throw your own exceptions?

Well, if at that point you know that an error has occurred, then why not write the proper code to handle that error instead of passing a new Exception object to the catch block? Throwing your own exceptions signifies some design flaws in the project.

What’s a delegate?

A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.

What’s a multicast delegate?

It’s a delegate that points to and eventually fires off several methods.

How’s the DLL Hell problem solved in .NET?

Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.

What are the ways to deploy an assembly?

An MSI installer, a CAB archive, and XCOPY command.

What’s a satellite assembly?

When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.

What namespaces are necessary to create a localized application? System.Globalization, System.Resources.

What’s the difference between // comments, /* */ comments and /// comments? Single-line, multi-line and XML documentation comments.

How do you generate documentation from the C# file commented properly with a command-line compiler?

Compile it with a /doc switch.

What’s the difference between <c> and <code> XML documentation tag?

Single line code example and multiple-line code example.

Is XML case-sensitive?

Yes, so <Student> and <student> are different elements.

What debugging tools come with the .NET SDK?

CorDBG – command-line debugger, and DbgCLR – graphic debugger. Visual Studio .NET uses the DbgCLR. To use CorDbg, you must compile the original C# file using the /debug switch.

What does the This window show in the debugger?

It points to the object that’s pointed to by this reference. Object’s instance data is shown.

What does assert() do?

In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.

What’s the difference between the Debug class and Trace class? Documentation looks the same.

 Use Debug class for debug builds, use Trace class for both debug and release builds.

Why are there five tracing levels in System.Diagnostics.TraceSwitcher?

The tracing dumps can be quite verbose and for some applications that are constantly running you run the risk of overloading the machine and the hard drive there. Five levels range from None to Verbose, allowing to fine-tune the tracing activities.

Where is the output of TextWriterTraceListener redirected?

To the Console or a text file depending on the parameter passed to the constructor.

How do you debug an ASP.NET Web application?

Attach the aspnet_wp.exe process to the DbgClr debugger.

What are three test cases you should go through in unit testing?

Positive test cases (correct data, correct output), negative test cases (broken or missing data, proper handling), exception test cases (exceptions are thrown and caught properly).

Can you change the value of a variable while debugging a C# application?

Yes, if you are debugging via Visual Studio.NET, just go to Immediate window.

Explain the three services model (three-tier application).

Presentation (UI), business (logic and underlying code) and data (from storage or other sources).

What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET?

SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix, but it’s a .NET layer on top of OLE layer, so not the fastest thing in the world. ODBC.NET is a deprecated layer provided for backward compatibility to ODBC engines.

What’s the role of the DataReader class in ADO.NET connections?

It returns a read-only dataset from the data source when the command is executed.

What is the wildcard character in SQL? Let’s say you want to query database with LIKE for all employees whose name starts with La.

The wildcard character is %, the proper query with LIKE would involve ‘La%’.

Explain ACID rule of thumb for transactions.

Transaction must be Atomic (it is one unit of work and does not dependent on previous and following transactions), Consistent (data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t), Isolated (no transaction sees the intermediate results of the current transaction), Durable (the values persist if the data had been committed even if the system crashes right after).

What connections does Microsoft SQL Server support?

Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and passwords).

Which one is trusted and which one is untrusted?

Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.

Why would you use untrusted verificaion?

Web Services might use it, as well as non-Windows applications.

What does the parameter Initial Catalog define inside Connection String?

The database name to connect to.

What’s the data provider name to connect to Access database?

Microsoft.Access.

What does Dispose method do with the connection object?

Deletes it from the memory.

What is a pre-requisite for connection pooling?

Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings.

How big is the datatype int in .NET?

32 bits.

How big is the char?

16 bits (Unicode).

How do you initiate a string without escaping each backslash?

Put an @ sign in front of the double-quoted string.

What are valid signatures for the Main function?

• public static void Main()
• public static int Main()
• public static void Main( string[] args )
• public static int Main(string[] args )

How do you initialize a two-dimensional array that you don’t know the dimensions of?

• int [, ] myArray; //declaration
• myArray= new int [5, 8]; //actual initialization

What’s the access level of the visibility type internal?

Current assembly.

What’s the difference between struct and class in C#?

• Structs cannot be inherited.
• Structs are passed by value, not by reference.
• Struct is stored on the stack, not the heap.

Explain encapsulation.

The implementation is hidden, the interface is exposed.

What data type should you use if you want an 8-bit value that’s signed?

sbyte.

Speaking of Boolean data types, what’s different between C# and C/C++?

There’s no conversion between 0 and false, as well as any other number and true, like in C/C++.

Where are the value-type variables allocated in the computer RAM? Stack.

Where do the reference-type variables go in the RAM?

The references go on the stack, while the objects themselves go on the heap.

What is the difference between the value-type variables and reference-type variables in terms of garbage collection?

The value-type variables are not garbage-collected, they just fall off the stack when they fall out of scope, the reference-type objects are picked up by GC when their references go null.

How do you convert a string into an integer in .NET?

Int32.Parse(string)

How do you box a primitive data type variable?

Assign it to the object, pass an object.

Why do you need to box a primitive variable?

To pass it by reference.

What’s the difference between Java and .NET garbage collectors?

Sun left the implementation of a specific garbage collector up to the JRE developer, so their performance varies widely, depending on whose JRE you’re using. Microsoft standardized on their garbage collection.

How do you enforce garbage collection in .NET?

System.GC.Collect();

Can you declare a C++ type destructor in C# like ~MyClass()?

Yes, but what’s the point, since it will call Finalize(), and Finalize() has no guarantees when the memory will be cleaned up, plus, it introduces additional load on the garbage collector.

What’s different about namespace declaration when comparing that to package declaration in Java?

No semicolon.

What’s the difference between const and readonly?

You can initialize readonly variables to some runtime values. Let’s say your program uses current date and time as one of the values that won’t change. This way you declare

public readonly string DateT = new DateTime().ToString().

What does a character do?

On most systems, produces a rather annoying beep.

Can you create enumerated data types in C#?

Yes.

What’s different about switch statements in C#?

No fall-throughs allowed.

What happens when you encounter a continue statement inside the for loop?

The code for the rest of the loop is ignored, the control is transferred back to the beginning of the loop.

Is goto statement supported in C#? How about Java?

Gotos are supported in C#to the fullest. In Java goto is a reserved keyword that provides absolutely no functionality.

Describe the compilation process for .NET code?

Source code is compiled and run in the .NET Framework using a two-stage process. First, source code is compiled to Microsoft intermediate language (MSIL) code using a .NET Framework-compatible compiler, such as that for Visual Basic .NET or Visual C#. Second, MSIL code is compiled to native code.

Name any 2 of the 4 .NET authentification methods.

ASP.NET, in conjunction with Microsoft Internet Information Services (IIS), can authenticate user credentials such as names and passwords using any of the following authentication methods:

• Windows: Basic, digest, or Integrated Windows Authentication (NTLM or Kerberos).
• Microsoft Passport authentication
• Forms authentication
• Client Certificate authentication

Describe the .NET Framework compile process.

Source code is compiled and run in the .NET Framework using a two-stage process. First, source code is compiled to Microsoft intermediate language (MSIL) code using a .NET Framework–compatible compiler, such as that for Visual Basic .NET or Visual C#. Second, MSIL code is compiled to native code.

How do you turn off SessionState in the web.config file?

In the system.web section of web.config, you should locate the httpmodule tag and you simply disable session by doing a remove tag with attribute name set to session.

<httpModules>
<remove name="Session” />
</httpModules>

What is main difference between Global.asax and Web.Config?

ASP.NET uses the global.asax to establish any global objects that your Web application uses. The .asax extension denotes an application file rather than .aspx for a page file. Each ASP.NET application can contain at most one global.asax file. The file is compiled on the first page hit to your Web application. ASP.NET is also configured so that any attempts to browse to the global.asax page directly are rejected. However, you can specify application-wide settings in the web.config file. The web.config is an XML-formatted text file that resides in the Web site’s root directory. Through Web.config you can specify settings like custom 404 error pages, authentication and authorization settings for the Web site, compilation options for the ASP.NET Web pages, if tracing should be enabled, etc.