C# Interview Questions & Answers

String :
1. Is string reference type / value type?
Ans: It's a reference type.

More about string:

The string type represents a sequence of zero or more Unicode characters. string is an alias for String in the .NET Framework.

The equality operators (== and !=) are defined to compare the values of string objects, not references.

The String object is immutable. Every time you use one of the methods in the System.String class, you create a new string object in memory, which requires a new allocation of space for that new object. For example, when you write this code, the compiler actually creates a new string object to hold the new sequence of characters, and that new object is assigned to b. The string "h" is then eligible for garbage collection.

string b = "h";

b += "ello";

2. Difference between .ToString() and Convert.ToString().

Ans: The basic difference between .ToString() and Convert.ToString()

is Convert.ToString() handle null values and .ToString() throw a null reference exception.

//Returns a null reference exception for str.

string str;

object i = null;

str = i.ToString();

//Returns an empty string for str and does not throw an exception. If you dont

string str;

object i = null;

str = Convert.ToString(i);

1.

Which of the following statements are true about the C#.NET code given below? String str1, str2; str1 = "Hi"; str2 = "Hi"; String objects cannot be created without using new. Only one object will get created. s1 and s2 both will refer to the same object. Two objects will get created, one pointed to by s1 and another pointed to by s2. s1 and s2 are references to the same object.

Ans: 2. Only one object will get created.

3.The string built using the String class are immutable (unchangeable), whereas, the ones built- using the StringBuilder class are mutable.

Ans: True.

The String object is immutable. Every time you use one of the methods in the System.String class, you create a new string object in memory, which requires a new allocation of space for that new object. In situations where you need to perform repeated modifications to a string, the overhead associated with creating a new String object can be costly. TheSystem.Text.StringBuilder class can be used when you want to modify a string without creating a new object. For example, using the StringBuilder class can boost performance when concatenating many strings together in a loop.

4. Which of the following statements about a String is correct?

A. A String is created on the stack. B. Whether a String is created on the stack or the heap depends on the length of the String. C. A String is a primitive. D. A String can be created by using the statement String s1 = new String; E. A String is created on the heap. A String has a zero-based index.

Ans: E. A String is created on the heap. A String has a zero-based index.

5.What is difference between value and reference types?

Value Type: Variables that are based on value types directly contain values. Assigning one value type variable to another copies the contained value.

• Value type are stored on stack
• Value types derive from System.ValueType
• Value type consists of primitive data types, structures, enumerations.
• Value types can not contain the value null.However, the nullable types feature does allow for value types to be assigned to null.

Reference Type: Reference types store the address of their data on the stack. The actual data that the address refers to is stored in an area of memory called the heap. Because reference types represent the address of the data rather than the data itself, assigning a reference variable to another doesn’t copy the data.

• Reference type are stored on heap.
• Reference types derive from System.Object
• Reference type consists of class, array, interface, delegates.
• Reference types can contain the value null.

            object obj1 = "String1";

            // create a new string; assign obj1 the reference to that new string "String1"

            object obj2 = obj1;

            // copy the reference from obj1 and assign into obj2; obj2 now refers to

            // the same string instance

6. What is difference between a.equals(b) and a==b?

In .Net Framework data types can be classified according to whether a variable of a particular type stores its own data or a pointer to the data. If the variable stores its own data, it is a value type and if it holds a pointer to data elsewhere in memory it is a reference type. You can assign either a reference type or a value type to a variable of the Object data type

The Equals() and == operator are used for comparison and both returns the boolean value (true/false).

Value Type

For Value Type == and Equals() works in the same way, both compare two object by value.

 int a = 5;

 int b = 5;

a==b and a.Equals(b) returns true , because in this case both compare two object by value. 

Reference Type

In case of Reference Type both works in different way.

            StringBuilder sb1 = new StringBuilder("Asp.Net");

            StringBuilder sb2 = new StringBuilder("Asp.Net");

sb1 == sb2 returns false and sb1.Equals(sb2) returns true.

== operator compares reference returns true when both references point to the same object and Equals() compares object by value and it will return true if the references refers object which are equivalent.