Theory Questions in C# Interview

Also check Practical Interview Questions that can be asked from C# Programmer

  • 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?


  • 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?


  • What’s class SortedList underneath?

A sorted HashTable.

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


  • 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 verification?

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?


  • What does Dispose method do with the connection object?

Deletes it from the memory.

  • What is a prerequisite for connection pooling?

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

  • What is CSHARP?

C# (pronounced “see sharp”) is a one of Dot Net programming language and supports functional, generic, object-oriented (class-based), and component-oriented programming disciplines. Microsoft has introduced C# on June 2000 and updating/Introducing new features with different versions.

The most recent version is C# 4.0, which was released in April 12, 2010.

  • Explain different C# Types used to create a csharp program?

Basically c# compiler supplies intrinsic data types (int, double, string etc) and programmers can define user defined data types(class, struct, enum etc).
All above C# types fall into any one of the following categories

  1. Value types
  2. Reference types
  • Explain Value type and reference type?

ValueTypeVariables of value types directly contain their data.
Value types store in stack.
ReferenceTypeVariables of reference types store references to their data
Reference types store in Heap Memory.

  • Tell me the three important Object Oriented programming features supported by C sharp?


Could you able to apply inheritance to structure type in C#?

No, we could not apply inheritance to structure. Structure types are always sealed.

  • What are the main features involved in C#2.0 Version?

C# Generics and Iterators has come in C# 2.0 version with these features C# Collections has great benefits. Some of the other features are mentioned below

  1. Partial Classes
  2. Anonymous Methods
  3. Nullable Types
  4. CoVariance and ContraVariance in Delegates
  • Could remember some of the new features included in C#3.0 Version?

C# 3.0 has following good features and these features works with Visual studio 2008.

  1. Object and Collection Initializers
  2. Implicitly Typed Local Variables
  3. Implicitly Typed Arrays
  4. Extension Methods
  5. Lambda Expressions
  6. Anonymous Types
  7. Query Expressions(LINQ)
  8. Automatically Implemented Properties
  • What are the new features included in C#4.0 version?

C# 4.0 has released recently in 2010 and all these features could work with visual studio 2010 because VS2010 can be targeted to .net framework 4.0 version. C# 4.0 mainly focused on dynamic programming and contains following features.

  1. Dynamic Binding
  2. Name and optional Arguments
  3. Improvements in CoVariance and ContraVariance
  4. COM specific interop features
  • Do C# support multiple inheritance?


  • Who is protected class level variable available to?

It is available to any subclass

  • Describe the accessibility modifier a protected internal?

It is available to classes that are within the same assembly and derived from specified base class.

  • What is the top .net class that everything is derived from?


  • What does term immutable mean?

The data value may not be changed.

  • What is the difference between System.Array.Copyto() and System.Array.Clone()?

The clone() method returns a new array object containing all the element in original array.
The copyTo() method copies the elements into another existing array. Both perform a shallow copy
A shallow copy means contents contain reference to same object as the element in original array. A deep copy would create a new instance of each element object, resulting in different.

  • What is the .net collection class that allows an element to be accessed using a unique key?


  • Explain three services model commonly known as a three tier application.

Presentation Logic(UI), Business Logic and Data Logic