MuleSoft Most Frequently Asked Latest C# Interview Questions Answers
Are C# References The Same As C++ References?
Not quite. The basic idea is the same, but one significant difference is that C# references can be null . So you cannot rely on a C# reference pointing to a valid object. In that respect a C# reference is more like a C++ pointer than a C++ reference. If you try to use a null reference, a NullReferenceException is thrown.
For example, look at the following method
void displayStringLength( string s )
{
Console.WriteLine( "String is length {0}", s.Length );
}
The problem with this method is that it will throw a NullReferenceException if called like this
string s = null;
displayStringLength( s );
Of course for some situations you may deem a NullReferenceException to be a perfectly acceptable outcome, but in this case it might be better to re-write the method like this
void displayStringLength( string s )
{
if( s == null )
Console.WriteLine( "String is null" );
else
Console.WriteLine( "String is length {0}", s.Length );
}
When Should I Throw An Exception?
This is the subject of some debate, and is partly a matter of taste. However, it is accepted by many that exceptions should be thrown only when an 'unexpected' error occurs. How do you decide if an error is expected or unexpected? This is a judgement call, but a straightforward example of an expected error is failing to read from a file because the seek pointer is at the end of the file, whereas an example of an unexpected error is failing to allocate memory from the heap.
Does C# Have A 'throws' Clause?
No, unlike Java, C# does not require (or even allow) the developer to specify the exceptions that a method can throw.
How Can I Check The Type Of An Object At Runtime?
You can use the is keyword. For example
using System;
class CApp
{
public static void Main()
{
string s = "fred";
long i = 10;
Console.WriteLine( "{0} is {1}an integer", s, (IsInteger(s) ? "" : "not ") );
Console.WriteLine( "{0} is {1}an integer", i, (IsInteger(i) ? "" : "not ") );
}
static bool IsInteger( object obj )
{
if( obj is int || obj is long )
return true;
else
return false;
}
}
produces the output
fred is not an integer
10 is an integer
Can I Get The Name Of A Type At Runtime?
Yes, use the GetType method of the object class (which all types inherit from). For example
using System;
class CTest
{
class CApp
{
public static void Main()
{
long i = 10;
CTest ctest = new CTest();
DisplayTypeInfo( ctest );
DisplayTypeInfo( i );
}
static void DisplayTypeInfo( object obj )
{
Console.WriteLine( "Type name = {0}, full type name = {1}", obj.GetType(), obj.GetType().FullName );
}
}
}
produces the following output
Type name = CTest, full type name = CTest
Type name = Int64, full type name = System.Int64
How Do I Do A Case-insensitive String Comparison?
Use the String.Compare function. Its third parameter is a boolean which specifies whether case should be ignored or not.
"fred" == "Fred" // false
System.String.Compare( "fred", "Fred", true ) // true
Structs Are Largely Redundant In C++. Why Does C# Have Them?
In C++, a struct and a class are pretty much the same thing. The only difference is the default visibility level (public for structs, private for classes). However, in C# structs and classes are very different. In C#, structs are value types (instances stored directly on the stack, or inline within heap-based objects), whereas classes are reference types (instances stored on the heap, accessed indirectly via a reference). Also structs cannot inherit from structs or classes, though they can implement interfaces. Structs cannot have destructors. A C# struct is much more like a C struct than a C++ struct.
Does C# Support Multiple Inheritance (mi)?
No, though it does support implementation of multiple interfaces on a single class or struct.
Is A C# Interface The Same As A C++ Abstract Class?
No, not quite. An abstract class in C++ cannot be instantiated, but it can (and often does) contain
implementation code and/or data members. A C# interface cannot contain any implementation code or data members - it is simply a group of method names & signatures. A C# interface is more like a COM interface than a C++ abstract class.
Are C# Constructors The Same As C++ Constructors?
Very similar, but there are some significant differences. First, C# supports constructor chaining. This means one constructor can call another
class Person
{
public Person( string name, int age ) { ... }
public Person( string name ) : this( name, 0 ) {}
public Person() : this( "", 0 ) {}
}
Another difference is that virtual method calls within a constructor are routed to the most derived implementation - see Can I Call a virtual method from a constructor.
Error handling is also somewhat different. If an exception occurs during construction of a C# object, the destuctor (finalizer) will still be called. This is unlike C++ where the destructor is not called if construction is not completed. (Thanks to Jon Jagger for pointing this out.)
Finally, C# has static constructors. The static constructor for a class runs before the first instance of the class is created.
Also note that (like C++) some C# developers prefer the factory method pattern over constructors. See Brad Wilson's article.
Are C# Destructors The Same As C++ Destructors?
No. They look the same but they are very different. The C# destructor syntax (with the familiar ~ character) is just syntactic sugar for an override of the System.Object Finalize method. This Finalize method is called by the garbage collector when it determines that an object is no longer referenced, before it frees the memory associated with the object. So far this sounds like a C++ destructor. The difference is that the garbage collector makes no guarantees about when this procedure happens. Indeed, the algorithm employed by the CLR garbage collector means that it may be a long time after the application has finished with the object. This lack of certainty is often termed 'non-deterministic finalization', and it means that C# destructors are not suitable for releasing scarce resources such as database connections, file handles etc.
To achieve deterministic destruction, a class must offer a method to be used for the purpose. The standard approach is for the class to implement the IDisposable interface. The user of the object must call the Dispose() method when it has finished with the object. C# offers the 'using' construct to make this easier.
If C# Destructors Are So Different To C++ Destructors, Why Did Ms Use The Same Syntax?
Presumably they wanted C++ programmers to feel at home. I think they made a mistake.
Are All Methods Virtual In C#?
No. Like C++, methods are non-virtual by default, but can be marked as virtual.
Are C# References The Same As C++ References?
Not quite. The basic idea is the same, but one significant difference is that C# references can be null . So you cannot rely on a C# reference pointing to a valid object. In that respect a C# reference is more like a C++ pointer than a C++ reference. If you try to use a null reference, a NullReferenceException is thrown.
For example, look at the following method
void displayStringLength( string s )
{
Console.WriteLine( "String is length {0}", s.Length );
}
The problem with this method is that it will throw a NullReferenceException if called like this
string s = null;
displayStringLength( s );
Of course for some situations you may deem a NullReferenceException to be a perfectly acceptable outcome, but in this case it might be better to re-write the method like this
void displayStringLength( string s )
{
if( s == null )
Console.WriteLine( "String is null" );
else
Console.WriteLine( "String is length {0}", s.Length );
}
When Should I Throw An Exception?
This is the subject of some debate, and is partly a matter of taste. However, it is accepted by many that exceptions should be thrown only when an 'unexpected' error occurs. How do you decide if an error is expected or unexpected? This is a judgement call, but a straightforward example of an expected error is failing to read from a file because the seek pointer is at the end of the file, whereas an example of an unexpected error is failing to allocate memory from the heap.
Does C# Have A 'throws' Clause?
No, unlike Java, C# does not require (or even allow) the developer to specify the exceptions that a method can throw.
How Can I Check The Type Of An Object At Runtime?
You can use the is keyword. For example
using System;
class CApp
{
public static void Main()
{
string s = "fred";
long i = 10;
Console.WriteLine( "{0} is {1}an integer", s, (IsInteger(s) ? "" : "not ") );
Console.WriteLine( "{0} is {1}an integer", i, (IsInteger(i) ? "" : "not ") );
}
static bool IsInteger( object obj )
{
if( obj is int || obj is long )
return true;
else
return false;
}
}
produces the output
fred is not an integer
10 is an integer
 |
| MuleSoft Most Frequently Asked Latest C# Interview Questions Answers |
Can I Get The Name Of A Type At Runtime?
Yes, use the GetType method of the object class (which all types inherit from). For example
using System;
class CTest
{
class CApp
{
public static void Main()
{
long i = 10;
CTest ctest = new CTest();
DisplayTypeInfo( ctest );
DisplayTypeInfo( i );
}
static void DisplayTypeInfo( object obj )
{
Console.WriteLine( "Type name = {0}, full type name = {1}", obj.GetType(), obj.GetType().FullName );
}
}
}
produces the following output
Type name = CTest, full type name = CTest
Type name = Int64, full type name = System.Int64
How Do I Do A Case-insensitive String Comparison?
Use the String.Compare function. Its third parameter is a boolean which specifies whether case should be ignored or not.
"fred" == "Fred" // false
System.String.Compare( "fred", "Fred", true ) // true
Structs Are Largely Redundant In C++. Why Does C# Have Them?
In C++, a struct and a class are pretty much the same thing. The only difference is the default visibility level (public for structs, private for classes). However, in C# structs and classes are very different. In C#, structs are value types (instances stored directly on the stack, or inline within heap-based objects), whereas classes are reference types (instances stored on the heap, accessed indirectly via a reference). Also structs cannot inherit from structs or classes, though they can implement interfaces. Structs cannot have destructors. A C# struct is much more like a C struct than a C++ struct.
Does C# Support Multiple Inheritance (mi)?
No, though it does support implementation of multiple interfaces on a single class or struct.
Is A C# Interface The Same As A C++ Abstract Class?
No, not quite. An abstract class in C++ cannot be instantiated, but it can (and often does) contain
implementation code and/or data members. A C# interface cannot contain any implementation code or data members - it is simply a group of method names & signatures. A C# interface is more like a COM interface than a C++ abstract class.
Are C# Constructors The Same As C++ Constructors?
Very similar, but there are some significant differences. First, C# supports constructor chaining. This means one constructor can call another
class Person
{
public Person( string name, int age ) { ... }
public Person( string name ) : this( name, 0 ) {}
public Person() : this( "", 0 ) {}
}
Another difference is that virtual method calls within a constructor are routed to the most derived implementation - see Can I Call a virtual method from a constructor.
Error handling is also somewhat different. If an exception occurs during construction of a C# object, the destuctor (finalizer) will still be called. This is unlike C++ where the destructor is not called if construction is not completed. (Thanks to Jon Jagger for pointing this out.)
Finally, C# has static constructors. The static constructor for a class runs before the first instance of the class is created.
Also note that (like C++) some C# developers prefer the factory method pattern over constructors. See Brad Wilson's article.
Are C# Destructors The Same As C++ Destructors?
No. They look the same but they are very different. The C# destructor syntax (with the familiar ~ character) is just syntactic sugar for an override of the System.Object Finalize method. This Finalize method is called by the garbage collector when it determines that an object is no longer referenced, before it frees the memory associated with the object. So far this sounds like a C++ destructor. The difference is that the garbage collector makes no guarantees about when this procedure happens. Indeed, the algorithm employed by the CLR garbage collector means that it may be a long time after the application has finished with the object. This lack of certainty is often termed 'non-deterministic finalization', and it means that C# destructors are not suitable for releasing scarce resources such as database connections, file handles etc.
To achieve deterministic destruction, a class must offer a method to be used for the purpose. The standard approach is for the class to implement the IDisposable interface. The user of the object must call the Dispose() method when it has finished with the object. C# offers the 'using' construct to make this easier.
If C# Destructors Are So Different To C++ Destructors, Why Did Ms Use The Same Syntax?
Presumably they wanted C++ programmers to feel at home. I think they made a mistake.
Are All Methods Virtual In C#?
No. Like C++, methods are non-virtual by default, but can be marked as virtual.