Following are some C Sharp interview questions which I have found on Internet and decided to share it will all of you.
What is C#?
C# is a programming language designed by Microsoft. It is loosely based on C/C++, and bears a striking similarity to Java. Microsoft describe C# as follows:
"C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. C# (pronounced 'C sharp') is firmly planted in the C and C++ family tree of languages, and will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++."
What is C#?
C# is a programming language designed by Microsoft. It is loosely based on C/C++, and bears a striking similarity to Java. Microsoft describe C# as follows:
"C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. C# (pronounced 'C sharp') is firmly planted in the C and C++ family tree of languages, and will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++."
Does C# have its own class library?
Not exactly. The .NET Framework has a comprehensive class library, which C# can make use of. C# does not have its own class library.
What standard types does C# use?
C# supports a very similar range of basic types to C++, including int, long, float, double, char, string, arrays, structs and classes. However, don't assume too much. The names may be familiar, but many of the details are different. For example, a long is 64 bits in C#, whereas in C++ the size of a long depends on the platform (typically 32 bits on a 32-bit platform, 64 bits on a 64-bit platform). Also classes and structs are almost the same in C++ - this is not true for C#. Finally, chars and strings in .NET are 16-bit (Unicode/UTF-16), not 8-bit like C++.
Is it true that all C# types derive from a common base class?
Yes and no. All types can be treated as if they derive from object (System.Object), but in order to treat an instance of a value type (e.g. int, float) as object-derived, the instance must be converted to a reference type using a process called 'boxing'. In theory a developer can forget about this and let the run-time worry about when the conversion is necessary, but in reality this implicit conversion can have side-effects that may trip up the unwary.
So I can pass an instance of a value type to a method that takes an object as a parameter?
Yes.
Not exactly. The .NET Framework has a comprehensive class library, which C# can make use of. C# does not have its own class library.
What standard types does C# use?
C# supports a very similar range of basic types to C++, including int, long, float, double, char, string, arrays, structs and classes. However, don't assume too much. The names may be familiar, but many of the details are different. For example, a long is 64 bits in C#, whereas in C++ the size of a long depends on the platform (typically 32 bits on a 32-bit platform, 64 bits on a 64-bit platform). Also classes and structs are almost the same in C++ - this is not true for C#. Finally, chars and strings in .NET are 16-bit (Unicode/UTF-16), not 8-bit like C++.
Is it true that all C# types derive from a common base class?
Yes and no. All types can be treated as if they derive from object (System.Object), but in order to treat an instance of a value type (e.g. int, float) as object-derived, the instance must be converted to a reference type using a process called 'boxing'. In theory a developer can forget about this and let the run-time worry about when the conversion is necessary, but in reality this implicit conversion can have side-effects that may trip up the unwary.
So I can pass an instance of a value type to a method that takes an object as a parameter?
Yes.
What are the fundamental differences between value types and reference types?
C# divides types into two categories - value types and reference types. Most of the intrinsic types (e.g. int, char) are value types. Structs are also value types. Reference types include classes, arrays and strings. The basic idea is straightforward - an instance of a value type represents the actual data, whereas an instance of a reference type represents a pointer or reference to the data.
Does C# support multiple inheritance (MI)?
No, though it does support implementation of multiple interfaces on a single class or struct.
How do I declare a pure virtual function in C#?
Use the abstract modifier on the method. The class must also be marked as abstract (naturally). Note that abstract methods cannot have an implementation (unlike pure virtual C++ methods).
Can I call a virtual method from a constructor/destructor?
Yes, but it's generally not a good idea. The mechanics of object construction in .NET are quite different from C++, and this affects virtual method calls in constructors.
C++ constructs objects from base to derived, so when the base constructor is executing the object is effectively a base object, and virtual method calls are routed to the base class implementation. By contrast, in .NET the derived constructor is executed first, which means the object is always a derived object and virtual method calls are always routed to the derived implementation. (Note that the C# compiler inserts a call to the base class constructor at the start of the derived constructor, thus preserving standard OO semantics by creating the illusion that the base constructor is executed first.)
The same issue arises when calling virtual methods from C# destructors. A virtual method call in a base destructor will be routed to the derived implementation.
Should I make my destructor virtual?
A C# destructor is really just an override of the System.Object Finalize method, and so is virtual by definition.
Can I use exceptions in C#?
Yes, in fact exceptions are the recommended error-handling mechanism in C# (and in .NET in general). Most of the .NET framework classes use exceptions to signal errors.
What types of object can I throw as exceptions?
Only instances of the System.Exception classes, or classes derived from System.Exception. This is in sharp contrast with C++ where instances of almost any type can be thrown.
Can I define my own exceptions?
Yes, just derive your exception class from System.Exception.
Does the System.Exception class have any cool features?
Yes - the feature which stands out is the StackTrace property. This provides a call stack which records where the exception was thrown from.
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.
Can I get the name of a type at runtime?C# divides types into two categories - value types and reference types. Most of the intrinsic types (e.g. int, char) are value types. Structs are also value types. Reference types include classes, arrays and strings. The basic idea is straightforward - an instance of a value type represents the actual data, whereas an instance of a reference type represents a pointer or reference to the data.
Does C# support multiple inheritance (MI)?
No, though it does support implementation of multiple interfaces on a single class or struct.
How do I declare a pure virtual function in C#?
Use the abstract modifier on the method. The class must also be marked as abstract (naturally). Note that abstract methods cannot have an implementation (unlike pure virtual C++ methods).
Can I call a virtual method from a constructor/destructor?
Yes, but it's generally not a good idea. The mechanics of object construction in .NET are quite different from C++, and this affects virtual method calls in constructors.
C++ constructs objects from base to derived, so when the base constructor is executing the object is effectively a base object, and virtual method calls are routed to the base class implementation. By contrast, in .NET the derived constructor is executed first, which means the object is always a derived object and virtual method calls are always routed to the derived implementation. (Note that the C# compiler inserts a call to the base class constructor at the start of the derived constructor, thus preserving standard OO semantics by creating the illusion that the base constructor is executed first.)
The same issue arises when calling virtual methods from C# destructors. A virtual method call in a base destructor will be routed to the derived implementation.
Should I make my destructor virtual?
A C# destructor is really just an override of the System.Object Finalize method, and so is virtual by definition.
Can I use exceptions in C#?
Yes, in fact exceptions are the recommended error-handling mechanism in C# (and in .NET in general). Most of the .NET framework classes use exceptions to signal errors.
What types of object can I throw as exceptions?
Only instances of the System.Exception classes, or classes derived from System.Exception. This is in sharp contrast with C++ where instances of almost any type can be thrown.
Can I define my own exceptions?
Yes, just derive your exception class from System.Exception.
Does the System.Exception class have any cool features?
Yes - the feature which stands out is the StackTrace property. This provides a call stack which records where the exception was thrown from.
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.
Yes, use the GetType method of the object class (which all types inherit from).
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.
Does C# support a variable number of arguments?
Yes, using the params keyword. The arguments are specified as a list of arguments of a specific type, e.g. int. For ultimate flexibility, the type can be object. The standard example of a method which uses this approach is System.Console.WriteLine().
Does C# do array bounds checking?
Yes. An IndexOutOfRange exception is used to signal an error.
How can I make sure my C# classes will interoperate with other .NET languages?
Make sure your C# code conforms to the Common Language Subset (CLS). To help with this, add the [assembly:CLSCompliant(true)] global attribute to your C# source files. The compiler will emit an error if you use a C# feature which is not CLS-compliant.
What is the difference between == and object.Equals?
For value types, == and Equals() usually compare two objects by value. For example:
int y = 10;
Console.WriteLine( x == y );
Console.WriteLine( x.Equals(y) );
will display:
True
True
However things are more complex for reference types. Generally speaking, for reference types == is expected to perform an identity comparison, i.e. it will only return true if both references point to the same object. By contrast, Equals() is expected to perform a value comparison, i.e. it will return true if the references point to objects that are equivalent.
What are the new features in C# 2.0?
Support for all of the new framework features such as generics, anonymous methods, partial classes, iterators and static classes.
Delegate inference is a new feature of the C# compiler which makes delegate usage a little simpler. It allows you to write this:
Thread t = new Thread(ThreadFunc);
instead of this:
Thread t = new Thread( new ThreadStart(ThreadFunc) );
Another minor but welcome addition is the explicit global namespace, which fixes a hole in namespace usage in C# 1.x. You can prefix a type name with global:: to indicate that the type belongs to the global namespace, thus avoiding problems where the compiler infers the namespace and gets it wrong.
Finally C# 2.0 includes some syntactic sugar for the new System.Nullable type. You can use T? as a synonym for System.Nullable
Are C# generics the same as C++ templates?
No, not really. There are some similarities, but there are also fundamental differences.
Shahid Riaz Bhatti
MCP, MCAD
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