Twenty C# questions explained

#​1 When do you use structs vs classes?

• structs are value types that can contain data and functions
• structs are value types and do not require heap allocation. 
• structs directly store their data in the struct, classes store a reference to a dynamically allocated object. 
• structs are useful for small data structures 
• structs can affect performance
• Constructors are invoked with the new operator, but that does not allocate memory on the heap
• A struct constructor simply returns the struct value itself (typically in a temporary location on the stack), and this value is then copied as necessary
• With classes, multiple variables may have a reference to the same object
• It is possible for operations on one variable to affect the object referenced by the other variable.
• With structs, the variables each have their own copy of the data, and it is not possible for operations on one to affect the other. 
• structs do not support user-specified inheritance, and they implicitly inherit from type object

call to functions

#​2 How Does One Parse XML Files?

• Reference System.Xml
• Use XmlReader to parse the text string
• You will need to have an idea of the XML elements and attributes for this example
• Use the XmlReader methods to read the XML data

#​3 What is the difference between String and string ?

• No difference but code conventions state that upper case String should be used for class methods.

#​4 How do I get the application’s path in a C# console app?

• Use reflection to get the executing assembly path
• Pass that the IO.Path.GetDirectoryName

#​5 How do you call a base constructor in C#?

• Method 1 - Traditional

public InheritTest(string newName) : base(newName) {     }  

• Method 2 - New

public InheritTest(string name) : base(ModifyBase(name)) {     }  
private static string ModifyBase(string newName) {  return newName.ToUpper();  }

#​6 How would you count the occurrences of a string within a string?

• Single character search

- foreach

• String search

- RegEx

#​7 How to Check if a Number is a Power of 2?

• Use binary & operator
• Converts values to binary
• Performs logical AND operation on values

1000 - 8

0111 - 7

0000 – 0

1001 – 9

1000 – 8

1000 - 8

#​8 What is the difference between break and continue in a loop?

• Break leaves immediately
• Continue allows loop to continue
• Place the word loop after each keyword to make sense of it

break loop

continue loop

#​9 What is the difference between abstract and virtual functions?

• Abstract methods have no implementation and MUST be overridden
• Virtual methods MAY have an implementation but are not required to and then CAN be overridden.
• You cannot call base.method() for abstract but you can for virtual methods

#​10 What is the difference between ref and out keywords?

• ref causes an argument to be passed by reference not by value.  It provides a reference to the source value and changes made in the method will be made to the source object

- Arguments passed as ref must be initialized before they are passed.

• out also causes an argument to be passed by reference.  Changes made to the parameter are also changed in the source.

- Arguments passed using out do not have to be initialized first.
- The called method must assign a value prior to the method return statement

#​11 Explain how to encrypt/decrypt a string in .NET?

• Generate secret key
• Encrypt string
• Decrypt string using same key and encryption provider

#​12 How do I get the index of the current iteration of a foreach loop?

• Maintain a counter and increment it manually
• Some collection types provide IndexOf()

#​13 How do I get my own IP address in C#?

• Check private internal IP address

- Use Dns.GetHostEntry, IPAddress, and AddressList

• Check public external IP address

- Make external calls

#​14 How do I calculate someone's age in C#?

• Use the DateTime class
• Get the current date
• Subtract the birth year from the current year
• Allow for actual birth month

#​15 How do I get string value of an enum?

• Use Enum.GetName()
• Use ToString()

#​16 How do I  make a textbox that only accepts numbers?

• Use NumericUpDown
• Handle the KeyPress Event
• Handle the Text_Changed Event

#​17 How do I  round a decimal value to two places for output?

• Use string format specifiers
• ("#.##")
• ("{0:0.00}", value)
• ("n2")
• ("{0:c}", value)

#​18 How do I remove duplicates from an array?

• Use LINQ
• Use a List

#​19 How do I sort a dictionary by value?

• Use LINQ
• Sorts into a new object

#​20 How do I return multiple values from a function in C#?

• Using out parameters
• Using arrays or structs
• Consider finding min and max values in an array

source: Microsoft MVA