C# Interview Questions

This is a list of questions I have gathered from other sources and created myself over a period of time from my experience, many of which I felt where incomplete or simply wrong.  I have finally taken the time to go through each question and correct them to the best of my ability.  However, please feel free to post feedback to challenge, improve, or suggest new questions.  I want to thank those of you that have contributed quality questions and corrections thus far.

There are some question in this list that I do not consider to be good questions for an interview.  However, they do exist on other lists available on the Internet so I felt compelled to keep them easy access.

General Questions

  1. Does C# support multiple-inheritance?
  2. Who is a protected class-level variable available to?
    It is available to any sub-class (a class inheriting this class).
  3. Are private class-level variables inherited?
    Yes, but they are not accessible.  Although they are not visible or accessible via the class interface, they are inherited. 
  4. Describe the accessibility modifier “protected internal”.
    It is available to classes that are within the same assembly and derived from the specified base class. 
  5. What’s the top .NET class that everything is derived from?
  6. What does the term immutable mean?
    The data value may not be changed.  Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
  7. What’s the difference between System.String and System.Text.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. 
  8. What’s the advantage of using System.Text.StringBuilder over System.String?
    StringBuilder is more efficient in cases where there is a large amount of string manipulation.  Strings are immutable, so each time a string is changed, a new instance in memory is created.
  9. Can you store multiple data types in System.Array?
  10. What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
    The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array.  The CopyTo() method copies the elements into another existing array.  Both perform a shallow copy.  A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array.  A deep copy (which neither of these methods performs) would create a new instance of each element’s object, resulting in a different, yet identacle object.
  11. How can you sort the elements of the array in descending order?
    By calling Sort() and then Reverse() methods. 
  12. What’s the .NET collection class that allows an element to be accessed using a unique key?
  13. What class is underneath the SortedList class?
    A sorted HashTable. 
  14. Will the finally block get executed if an exception has not occurred?­
  15. What’s the C# syntax to catch 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 {}. 
  16. Can multiple catch blocks be executed for a single try statement?
    No.  Once the proper catch block processed, control is transferred to the finally block (if there are any). 
  17. Explain the three services model commonly know as a three-tier application.
    Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources). 

Class Questions

  1. What is the syntax to inherit from a class in C#?
    Place a colon and then the name of the base class.
    Example: class MyNewClass : MyBaseClass 
  2. Can you prevent your class from being inherited by another class?
    Yes.  The keyword “sealed” will prevent the class from being inherited. 
  3. Can you allow a class to be inherited, but prevent the method from being over-ridden?
    Yes.  Just leave the class public and make the method sealed. 
  4. What’s an abstract class?
    A class that cannot be instantiated.  An abstract class is a class that must be inherited and have the methods overridden.  An abstract class is essentially a blueprint for a class without any implementation. 
  5. When do you absolutely have to declare a class as abstract?
    1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
    When at least one of the methods in the class is abstract. 
  6. What is an interface class?
    Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes. 
  7. Why can’t you specify the accessibility modifier for methods inside the interface?
    They all must be public, and are therefore public by default. 
  8. Can you inherit multiple interfaces?
    Yes.  .NET does support multiple interfaces. 
  9. What happens if you inherit multiple interfaces and 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.
    To Do: Investigate 
  10. What’s the difference between an interface and abstract class?
    In an interface class, all methods are abstract – there is no implementation.  In an abstract class some methods can be concrete.  In an interface class, no accessibility modifiers are allowed.  An abstract class may have accessibility modifiers. 
  11. What is the difference between a Struct and a Class?
    Structs are value-type variables and are thus saved on the stack, additional overhead but faster retrieval.  Another difference is that structs cannot inherit. 

Method and Property Questions

  1. What’s the implicit name of the parameter that gets passed into the set method/property of a class?
    Value.  The data type of the value parameter is defined by whatever data type the property is declared as. 
  2. What does the keyword “virtual” declare for a method or property?
    The method or property can be overridden. 
  3. How is method overriding different from method overloading?
    When overriding a method, you change the behavior of the method for the derived class.  Overloading a method simply involves having another method with the same name within the class. 
  4. Can you declare an override method to be static if the original method is not static?
    No.  The signature of the virtual method must remain the same.  (Note: Only the keyword virtual is changed to keyword override) 
  5. What are the different ways a method can be overloaded?
    Different parameter data types, different number of parameters, different order of parameters. 
  6. If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific 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.

Events and Delegates

  1. What’s a delegate?
    A delegate object encapsulates a reference to a method. 
  2. What’s a multicast delegate?
    A delegate that has multiple handlers assigned to it.  Each assigned handler (method) is called.

XML Documentation Questions

  1. Is XML case-sensitive?
  2. What’s the difference between // comments, /* */ comments and /// comments?
    Single-line comments, multi-line comments, and XML documentation comments. 
  3. How do you generate documentation from the C# file commented properly with a command-line compiler?
    Compile it with the /doc switch.

Debugging and Testing Questions

  1. What debugging tools come with the .NET SDK?
    1.   CorDBG – command-line debugger.  To use CorDbg, you must compile the original C# file using the /debug switch.
    2.   DbgCLR – graphic debugger.  Visual Studio .NET uses the DbgCLR. 
  2. What does assert() method 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. 
  3. 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. 
  4. Why are there five tracing levels in System.Diagnostics.TraceSwitcher?
    The tracing dumps can be quite verbose.  For applications that are constantly running you run the risk of overloading the machine and the hard drive.  Five levels range from None to Verbose, allowing you to fine-tune the tracing activities. 
  5. Where is the output of TextWriterTraceListener redirected?
    To the Console or a text file depending on the parameter passed to the constructor. 
  6. How do you debug an ASP.NET Web application?
    Attach the aspnet_wp.exe process to the DbgClr debugger. 
  7. What are three test cases you should go through in unit testing?
    1.       Positive test cases (correct data, correct output).
    2.       Negative test cases (broken or missing data, proper handling).
    3.       Exception test cases (exceptions are thrown and caught properly). 
  8. 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. 

ADO.NET and Database Questions

  1. What is the role of the DataReader class in ADO.NET connections?
    It returns a read-only, forward-only rowset from the data source.  A DataReader provides fast access when a forward-only sequential read is needed.  
  2. 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.  OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET. 
  3. 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%’. 
  4. Explain ACID rule of thumb for transactions.
    A transaction must be:
    1.       Atomic – it is one unit of work and does not dependent on previous and following transactions.
    2.       Consistent – data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.
    3.       Isolated – no transaction sees the intermediate results of the current transaction).
    4.       Durable – the values persist if the data had been committed even if the system crashes right after. 
  5. What connections does Microsoft SQL Server support?
    Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password). 
  6. Between Windows Authentication and SQL Server Authentication, 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. 
  7. What does the Initial Catalog parameter define in the connection string?
    The database name to connect to. 
  8. What does the Dispose method do with the connection object?
    Deletes it from the memory.
    To Do: answer better.  The current answer is not entirely correct. 
  9. 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.  The connection string must be identical.

Assembly Questions

  1. How is 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. 
  2. What are the ways to deploy an assembly?
    An MSI installer, a CAB archive, and XCOPY command. 
  3. What is 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. 
  4. What namespaces are necessary to create a localized application?
    System.Globalization and System.Resources.
  5. What is the smallest unit of execution in .NET?
    an Assembly.
  6. When should you call the garbage collector in .NET?
    As a good rule, you should not call the garbage collector.  However, you could call the garbage collector when you are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory.  However, this is usually not a good practice.
  7. How do you convert a value-type to a reference-type?
    Use Boxing.
  8. What happens in memory when you Box and Unbox a value-type?
    Boxing converts a value-type to a reference-type, thus storing the object on the heap.  Unboxing converts a reference-type to a value-type, thus storing the value on the stack.

ASP.NET Interview Questions

ASP.NET Interview Questions

This is a list of questions I have gathered and created over a period of time from my experience, many of which I felt where incomplete or simply wrong.  I have finally taken the time to go through each question and correct them to the best of my ability.  However, please feel free to post feedback to challenge, improve, or suggest new questions.  I want to thank those of you that have contributed quality questions and corrections thus far.

There are some questions in this list that I do not consider to be good questions for an interview.  However, they do exist on other lists available on the Internet so I felt compelled to keep them here for easy access.

  1. Describe the role of inetinfo.exe, aspnet_isapi.dll andaspnet_wp.exe in the page loading process.
    inetinfo.exe is theMicrosoft IIS server running, handling ASP.NET requests among other things.When an ASP.NET request is received (usually a file with .aspx extension), the ISAPI filter aspnet_isapi.dll takes care of it by passing the request tothe actual worker process aspnet_wp.exe.
  2. What’s the difference between Response.Write() andResponse.Output.Write()?
    Response.Output.Write() allows you to write formatted output. 
  3. What methods are fired during the page load?
    Init() – when the page is instantiated
    Load() – when the page is loaded into server memory
    PreRender() – the brief moment before the page is displayed to the user as HTML
    Unload() – when page finishes loading. 
  4. When during the page processing cycle is ViewState available?
    After the Init() and before the Page_Load(), or OnLoad() for a control. 
  5. What namespace does the Web page belong in the .NET Framework class hierarchy?
  6. Where do you store the information about the user’s locale?
  7. What’s the difference between Codebehind="MyCode.aspx.cs" andSrc="MyCode.aspx.cs"?
    CodeBehind is relevant to Visual Studio.NET only. 
  8. What’s a bubbled event?
    When you have a complex control, like DataGrid, writing an event processing routine for each object (cell, button, row, etc.) is quite tedious. The controls can bubble up their eventhandlers, allowing the main DataGrid event handler to take care of its constituents. 
  9. Suppose you want a certain ASP.NET function executed on MouseOver for a certain button.  Where do you add an event handler?
    Add an OnMouseOver attribute to the button.  Example: btnSubmit.Attributes.Add("onmouseover","someClientCodeHere();"); 
  10. What data types do the RangeValidator control support?
    Integer, String, and Date. 
  11. Explain the differences between Server-side and Client-side code?
    Server-side code executes on the server.  Client-side code executes in the client’s browser. 
  12. What type of code (server or client) is found in a Code-Behind class?
    The answer is server-side code since code-behind is executed on the server.  However, during the code-behind’s execution on the server, it can render client-side code such as JavaScript to be processed in the clients browser.  But just to be clear, code-behind executes on the server, thus making it server-side code. 
  13. Should user input data validation occur server-side or client-side?  Why?
    All user input data validation should occur on the server at a minimum.  Additionally, client-side validation can be performed where deemed appropriate and feasable to provide a richer, more responsive experience for the user. 
  14. What is the difference between Server.Transfer and Response.Redirect?  Why would I choose one over the other?
    Server.Transfer transfers page processing from one page directly to the next page without making a round-trip back to the client’s browser.  This provides a faster response with a little less overhead on the server.  Server.Transfer does not update the clients url history list or current url.  Response.Redirect is used to redirect the user’s browser to another page or site.  This performas a trip back to the client where the client’s browser is redirected to the new page.  The user’s browser history list is updated to reflect the new address. 
  15. Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?
    Valid answers are:
    A DataSet can represent an entire relational database in memory, complete with tables, relations, and views.
    ·  A DataSet is designed to work without any continuing connection to the original data source.
    ·  Data in a DataSet is bulk-loaded, rather than being loaded on demand.
    ·  There’s no concept of cursor types in a DataSet.
    ·  DataSets have no current record pointer You can use For Each loops to move through the data.
    ·  You can store many edits in a DataSet, and write them to the original data source in a single operation.
    ·  Though the DataSet is universal, other objects in ADO.NET come in different versions for different data sources. 
  16. What is the Global.asax used for?
    The Global.asax (including the Global.asax.cs file) is used to implement application and session level events. 
  17. What are the Application_Start and Session_Start subroutines used for?
    This is where you can set the specific variables for the Application and Session objects. 
  18. Can you explain what inheritance is and an example of when you might use it?
    When you want to inherit (use the functionality of) another class.  Example: With a base class named Employee, a Manager class could be derived from the Employee base class. 
  19. Whats an assembly?
    Assemblies are the building blocks of the .NET framework. Overview of assemblies from MSDN 
  20. Describe the difference between inline and code behind.
    Inline code written along side the html in a page. Code-behind is code written in a separate file and referenced by the .aspx page. 
  21. Explain what a diffgram is, and a good use for one?
    The DiffGram is one of the two XML formats that you can use to render DataSet object contents to XML.  A good use is reading database data to an XML file to be sent to a Web Service. 
  22. Whats MSIL, and why should my developers need an appreciation of it if at all?
    MSIL is the Microsoft Intermediate Language. All .NET compatible languages will get converted to MSIL.  MSIL also allows the .NET Framework to JIT compile the assembly on the installed computer. 
  23. Which method do you invoke on the DataAdapter control to load your generated dataset with data?
    The Fill() method. 
  24. Can you edit data in the Repeater control?
    No, it just reads the information from its data source. 
  25. Which template must you provide, in order to display data in a Repeater control?
  26. How can you provide an alternating color scheme in a Repeater control?
    Use the AlternatingItemTemplate. 
  27. What property must you set, and what method must you call in your code, in order to bind the data from a data source to the Repeater control?
    You must set the DataSource property and call the DataBind method. 
  28. What base class do all Web Forms inherit from?
    The Page class. 
  29. Name two properties common in every validation control?
    ControlToValidate property and Text property. 
  30. Which property on a Combo Box do you set with a column name, prior to setting the DataSource, to display data in the combo box?
    DataTextField property. 
  31. Which control would you use if you needed to make sure the values in two different controls matched?
    CompareValidator control. 
  32. How many classes can a single .NET DLL contain?
    It can contain many classes.

Web Service Questions

  1. What is the transport protocol you use to call a Web service?
    SOAP (Simple Object Access Protocol) is the preferred protocol. 
  2. True or False: A Web service can only be written in .NET?
  3. What does WSDL stand for?
    Web Services Description Language. 
  4. Where on the Internet would you look for Web services?
  5. True or False: To test a Web service you must create a Windows application or Web application to consume this service?
    False, the web service comes with a test page and it provides HTTP-GET method to test.

State Management Questions

  1. What is ViewState?
    ViewState allows the state of objects (serializable) to be stored in a hidden field on the page.  ViewState is transported to the client and back to the server, and is not stored on the server or any other external source.  ViewState is used the retain the state of server-side objects between postabacks. 
  2. What is the lifespan for items stored in ViewState?
    Item stored in ViewState exist for the life of the current page.  This includes postbacks (to the same page). 
  3. What does the "EnableViewState" property do?  Why would I want it on or off?
    It allows the page to save the users input on a form across postbacks.  It saves the server-side values for a given control into ViewState, which is stored as a hidden value on the page before sending the page to the clients browser.  When the page is posted back to the server the server control is recreated with the state stored in viewstate. 
  4. What are the different types of Session state management options available with ASP.NET?
    ASP.NET provides In-Process and Out-of-Process state management.  In-Process stores the session in memory on the web server.  This requires the a "sticky-server" (or no load-balancing) so that the user is always reconnected to the same web server.  Out-of-Process Session state management stores data in an external data source.  The external data source may be either a SQL Server or a State Server service.  Out-of-Process state management requires that all objects stored in session are serializable.