C# Interview Questions and Answers

C# Interview Questions and Answers

What's C# ?

C# (pronounced C-sharp) is a new object oriented language from Microsoft and is derived from C and C++. It also borrows a lot of concepts from Java too including garbage collection.

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.

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 verificaion?

 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? 

Microsoft.Access.

  

What does Dispose method do with the connection object? 

Deletes it from the memory.

What is a pre-requisite for connection pooling? 

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

Is it possible to inline assembly or IL in C# code? 

- No.

Is it possible to have different access modifiers on the get/set methods of a property? 

- No. The access modifier on a property applies to both its get and set accessors. What you need to do if you want them to be different is make the property read-only (by only providing a get accessor) and create a private/internal set method that is separate from the property.

Is it possible to have a static indexer in C#? allowed in C#. 

- No. Static indexers are not

If I return out of a try/finally in C#, does the code in the finally-clause run? 

-Yes. The code in the finally always runs. If you return out of the try block, or even if you do a goto out of the try, the finally block always runs:
using System; 
class main
{
public static void Main()
{
try
{
Console.WriteLine(\"In Try block\");
return;
}
finally
{
Console.WriteLine(\"In Finally block\");
}
}
} 

Both In Try block and In Finally block will be displayed. Whether the return is in the try block or after the try-finally block, performance is not affected either way. The compiler treats it as if the return were outside the try block anyway. If it’s a return without an expression (as it is above), the IL emitted is identical whether the return is inside or outside of the try. If the return has an expression, there’s an extra store/load of the value of the expression (since it has to be computed within the try block). 

I was trying to use an out int parameter in one of my functions. How should I declare the variable that I am passing to it?

You should declare the variable as an int, but when you pass it in you must specify it as ‘out’, like the following: int i; foo(out i); where foo is declared as follows: 
[return-type] foo(out int o) { }

How does one compare strings in C#? 

In the past, you had to call .ToString() on the strings when using the == or != operators to compare the strings’ values. That will still work, but the C# compiler now automatically compares the values instead of the references when the == or != operators are used on string types. If you actually do want to compare references, it can be done as follows: if ((object) str1 == (object) str2) { } Here’s an example showing how string compares work:
using System; 
public class StringTest
{
public static void Main(string[] args)
{
Object nullObj = null; Object realObj = new StringTest();
int i = 10;
Console.WriteLine(\"Null Object is [\" + nullObj + \"]\n\"
+ \"Real Object is [\" + realObj + \"]\n\"
+ \"i is [\" + i + \"]\n\");
// Show string equality operators
string str1 = \"foo\";
string str2 = \"bar\";
string str3 = \"bar\";
Console.WriteLine(\"{0} == {1} ? {2}\", str1, str2, str1 == str2 );
Console.WriteLine(\"{0} == {1} ? {2}\", str2, str3, str2 == str3 );
}
}

Output:

Null Object is []
Real Object is [StringTest]
i is [10]
foo == bar ? False
bar == bar ? True

How do you mark a method obsolete?

[Obsolete] public int Foo() {...}
or
[Obsolete(\"This is a message describing why this method is obsolete\")] public int Foo() {...}
Note: The O in Obsolete is always capitalized.

How do you implement thread synchronization (Object.Wait, Notify,and CriticalSection) in C#? 

You want the lock statement, which is the same as Monitor Enter/Exit: 
lock(obj) { // code }

translates to

try {
CriticalSection.Enter(obj);
// code
}
finally
{
CriticalSection.Exit(obj);
}

How do you directly call a native function exported from a DLL? 

Here’s a quick example of the DllImport attribute in action:

using System.Runtime.InteropServices; \
class C
{
[DllImport(\"user32.dll\")]
public static extern int MessageBoxA(int h, string m, string c, int type);
public static int Main()
{
return MessageBoxA(0, \"Hello World!\", \"Caption\", 0);
}
}

This example shows the minimum requirements for declaring a C# method that is implemented in a native DLL. The method C.MessageBoxA() is declared with the static and external modifiers, and has the DllImport attribute, which tells the compiler that the implementation comes from the user32.dll, using the default name of MessageBoxA. For more information, look at the Platform Invoke tutorial in the documentation.

How do I simulate optional parameters to COM calls?

You must use the Missing class and pass Missing.Value (in System.Reflection) for any values that have optional parameters. 

What do you know about .NET assemblies? 

Assemblies are the smallest units of versioning and deployment in the .NET application. Assemblies are also the building blocks for programs such as Web services, Windows services, serviced components, and .NET remoting applications.

What’s the difference between private and shared assembly? 

Private assembly is used inside an application only and does not have to be identified by a strong name. Shared assembly can be used by multiple applications and has to have a strong name. 

What’s a strong name? 

A strong name includes the name of the assembly, version number, culture identity, and a public key token.

How can you tell the application to look for assemblies at the locations other than its own install? 

Use the directive in the XML .config file for a given application.
< probing privatePath=c:\mylibs; bin\debug />
should do the trick. Or you can add additional search paths in the Properties box of the deployed application.

How can you debug failed assembly binds? 

Use the Assembly Binding Log Viewer (fuslogvw.exe) to find out the paths searched. 

Where are shared assemblies stored? 

Global assembly cache.

How can you create a strong name for a .NET assembly? 

With the help of Strong Name tool (sn.exe). 

Where’s global assembly cache located on the system? 

Usually C:\winnt\assembly or C:\windows\assembly.

Can you have two files with the same file name in GAC? 

Yes, remember that GAC is a very special folder, and while normally you would not be able to place two files with the same name into a Windows folder, GAC differentiates by version number as well, so it’s possible for MyApp.dll and MyApp.dll to co-exist in GAC if the first one is version 1.0.0.0 and the second one is 1.1.0.0.

What is delay signing? 

Delay signing allows you to place a shared assembly in the GAC by signing the assembly with just the public key. This allows the assembly to be signed with the private key at a later stage, when the development process is complete and the component or assembly is ready to be deployed. This process enables developers to work with shared assemblies as if they were strongly named, and it secures the private key of the signature from being accessed at different stages of development.

Is there an equivalent of exit() for quitting a C# .NET application? 

Yes, you can use System.Environment.Exit(int exitCode) to exit the application or Application.Exit() if it's a Windows Forms app. 

Can you prevent your class from being inherited and becoming a base class for some other classes? 

Yes, that is what keyword sealed in the class definition is for. The developer trying to derive from your class will get a message: cannot inherit from Sealed class WhateverBaseClassName. It is the same concept as final class in Java.

Is XML case-sensitive? 

Yes, so and are different elements.

If a base class has a bunch of overloaded constructors, and an inherited class has another bunch of overloaded constructors, can you enforce a call from an inherited constructor to an arbitrary base constructor? 

Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

I was trying to use an "out int" parameter in one of my functions. How should I declare the variable that I am passing to it? 

You should declare the variable as an int, but when you pass it in you must specify it as 'out', like the following:
int i;
foo(out i);
where foo is declared as follows:
[return-type] foo(out int o) { }

How do I make a DLL in C#? 

You need to use the /target:library compiler option. 

How do I simulate optional parameters to COM calls? 

You must use the Missing class and pass Missing.Value (in System.Reflection) for any values that have optional parameters.

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

Yes.

What is the C# equivalent of C++ catch (…), which was a catch-all statement for any possible exception? Does C# support try-catch-finally blocks? 

Yes. Try-catch-finally blocks are supported by the C# compiler. Here's an example of a try-catch-finally block: using System; 
public class TryTest
{
static void Main()
{
try
{
Console.WriteLine("In Try block");
throw new ArgumentException();
}
catch(ArgumentException n1)
{
Console.WriteLine("Catch Block");
}
finally
{
Console.WriteLine("Finally Block");
}
}
}
Output: In Try Block
Catch Block
Finally Block

If I return out of a try/finally in C#, does the code in the finally-clause run? Yes. The code in the finally always runs. If you return out of the try block, or even if you do a "goto" out of the try, the finally block always runs, as shown in the following 
example: using System;
class main
{
public static void Main()
{
try
{
Console.WriteLine("In Try block");
return;
}
finally
{
Console.WriteLine("In Finally block");
}
}
}

Both "In Try block" and "In Finally block" will be displayed. Whether the return is in the try block or after the try-finally block, performance is not affected either way. The compiler treats it as if the return were outside the try block anyway. If it's a return without an expression (as it is above), the IL emitted is identical whether the return is inside or outside of the try. If the return has an expression, there's an extra store/load of the value of the expression (since it has to be computed within the try block).

Is there regular expression (regex) support available to C# developers?

Yes. The .NET class libraries provide support for regular expressions. Look at the documentation for the System.Text.RegularExpressions namespace.

Is there a way to force garbage collection? 

Yes. Set all references to null and then call System.GC.Collect(). If you need to have some objects destructed, and System.GC.Collect() doesn't seem to be doing it for you, you can force finalizers to be run by setting all the references to the object to null and then calling System.GC.RunFinalizers().

Does C# support properties of array types? 

Yes. Here's a simple example: using System;
class Class1
{
private string[] MyField;
public string[] MyProperty
{
get { return MyField; }
set { MyField = value; }
}
}
class MainClass
{
public static int Main(string[] args)
{
Class1 c = new Class1();
string[] arr = new string[] {"apple", "banana"};
c.MyProperty = arr;
Console.WriteLine(c.MyProperty[0]); // "apple"
return 0;
}
}

What connections does Microsoft SQL Server support? 

Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and passwords) 

What is 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.

How is method overriding different from overloading? 

When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.

When do you absolutely have to declare a class as abstract (as opposed to free-willed educated choice or decision based on UML diagram)? 

When at least one of the methods in the class is abstract. When the class itself is inherited from an abstract class, but not all base abstract methods have been over-ridden.

Why would you use untrusted verification? 

Web Services might use it, as well as non-Windows applications.

What is the implicit name of the parameter that gets passed into the class set method? 

Value, and its datatype depends on whatever variable we are changing.

How do I register my code for use by classic COM clients? 

Use the regasm.exe utility to generate a type library (if needed) and the necessary entries in the Windows Registry to make a class available to classic COM clients. Once a class is registered in the Windows Registry with regasm.exe, a COM client can use the class as though it were a COM class. 

How do I do implement a trace and assert? 

Use a conditional attribute on the method, as shown below:
class Debug
{
[conditional("TRACE")]
public void Trace(string s)
{
Console.WriteLine(s);
}
}
class MyClass
{
public static void Main()
{
Debug.Trace("hello");
}
}

In this example, the call to Debug.Trace() is made only if the preprocessor symbol TRACE is defined at the call site. You can define preprocessor symbols on the command line by using the /D switch. The restriction on conditional methods is that they must have void return type.

How do I create a multi language, multi file assembly?

Unfortunately, this is currently not supported in the IDE. To do this from the command line, you must compile your projects into netmodules (/target:module on the C# compiler), and then use the command line tool al.exe (alink) to link these netmodules together.

C# provides a default constructor for me. I write a constructor that takes a string as a parameter, but want to keep the no parameter one. How many constructors should I write? 

Two. Once you write at least one constructor, C# cancels the freebie constructor, and now you have to write one yourself, even if there is no implementation in

What is the equivalent to regsvr32 and regsvr32 /u a file in .NET development? 

Try using RegAsm.exe. The general syntax would be: RegAsm. A good description of RegAsm and its associated switches is located in the .NET SDK docs. Just search on "Assembly Registration Tool".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 hasnot), 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).

Where is the output of TextWriterTraceListener redirected? 

To the Console or a text file depending on the parameter passed to the constructor. 

How do I create a multilanguage, single-file assembly? 

This is currently not supported by Visual Studio .NET.

Why cannot you specify the accessibility modifier for methods inside the interface? 

They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it is public by default.

Is it possible to restrict the scope of a field/method of a class to the classes in the same namespace? 

There is no way to restrict to a namespace. Namespaces are never units of protection. But if you're using assemblies, you can use the 'internal' access modifier to restrict access to only within the assembly. 

Why do I get a syntax error when trying to declare a variable called checked? 

The word checked is a keyword in C#.

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.

What is the syntax for calling an overloaded constructor within a constructor (this() and constructorname() does not compile)? 

The syntax for calling another constructor is as follows:
class B
{
B(int i)
{ }
}
class C : B
{
C() : base(5) // call base constructor B(5)
{ }
C(int i) : this() // call C()
{ }
public static void Main() {}
}

Why do I get a "CS5001: does not have an entry point defined" error when compiling? 

The most common problem is that you used a lowercase 'm' when defining the Main method. The correct way to implement the entry point is as follows:
class test
{
static void Main(string[] args) {}
} 

What does the keyword virtual mean in the method definition? 

The method can be over-ridden.

What is 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 do I declare inout arguments in C#? 

The equivalent of inout in C# is ref. , as shown in the following
example: public void MyMethod (ref String str1, out String str2)
{
...
}
When calling the method, it would be called like this: String s1;
String s2;
s1 = "Hello";
MyMethod(ref s1, out s2);
Console.WriteLine(s1);
Console.WriteLine(s2);
Notice that you need to specify ref when declaring the function and calling it. 

Is there a way of specifying which block or loop to break out of when working with nested loops? 

The easiest way is to use goto: using System;
class BreakExample
{
public static void Main(String[] args)
{
for(int i=0; i<3 i="" span="">
{
Console.WriteLine("Pass {0}: ", i);
for( int j=0 ; j<100 j="" span="">
{
if ( j == 10) goto done;
Console.WriteLine("{0} ", j);
}
Console.WriteLine("This will not print");
}
done:
Console.WriteLine("Loops complete.");
}
}

What is the difference between const and static read-only? 

The difference is that static read-only can be modified by the containing class, but const can never be modified and must be initialized to a compile time constant. To expand on the static read-only case a bit, the containing class can only modify it: -- in the variable declaration (through a variable initializer). 
-- in the static constructor (instance constructors if it's not static). 

What does the parameter Initial Catalog define inside Connection String? 

The database name to connect to.

What is the difference between System.String and System.StringBuilder classes? 

System.String is immutable; System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

What is the top .NET class that everything is derived from? 

System.Object. 

Can you allow class to be inherited, but prevent the method from being over-ridden? 

Yes, just leave the class public and make the method sealed

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.

Are private class-level variables inherited? 

Yes, but they are not accessible, so looking at it you can honestly say that they are not inherited. But they are.

Can you inherit multiple interfaces? 

Yes. .NET does support multiple interfaces.

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

What namespaces are necessary to create a localized application?

 
System.Globalization, System.Resources.

Does Console.WriteLine() stop printing when it reaches a NULL character within a string? 

Strings are not null terminated in the runtime, so embedded nulls are allowed. Console.WriteLine() and all similar methods continue until the end of the string.

What is 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 is being operated on, a new instance is created.

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 is 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.

Is there any sample C# code for simple threading? 

Some sample code follows: using System;
using System.Threading;
class ThreadTest
{
public void runme()
{
Console.WriteLine("Runme Called");
}
public static void Main(String[] args)
{
ThreadTest b = new ThreadTest();
Thread t = new Thread(new ThreadStart(b.runme));
t.Start();
}
}

What is the difference between // comments, /* */ comments and /// comments? 

Single-line, multi-line and XML documentation comments.

What is the difference between and XML documentation tag? 

Single line code example and multiple-line code example.

Explain the three services model (three-tier application). Presentation (UI), business (logic and underlying code) and data (from storage or other sources). 

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).

How do you inherit from a class in C#? 

Place a colon and then the name of the base class. Notice that it is double colon in C++.

How do I port "synchronized" functions from Visual J++ to C#? 

Original Visual J++ code: public synchronized void Run()
{
// function body
}
Ported C# code: class C
{
public void Run()
{
lock(this)
{
// function body
}
}
public static void Main() {}

}

Can I define a type that is an alias of another type (like typedef in C++)?

Not exactly. You can create an alias within a single file with the "using" directive: using System; using Integer = System.Int32; // alias
But you can't create a true alias, one that extends beyond the file in which it is declared. Refer to the C# spec for more info on the 'using' statement's scope. 

Is it possible to have different access modifiers on the get/set methods of a property? 

No. The access modifier on a property applies to both its get and set accessors. What you need to do if you want them to be different is make the property read-only (by only providing a get accessor) and create a private/internal set method that is separate from the property.

Is it possible to have a static indexer in C#? 

No. Static indexers are not allowed in C#.

How is 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.

Why does DllImport not work for me?

All methods marked with the DllImport attribute must be marked as public static extern.

What is a delegate? 

A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.

What is the difference between an interface and abstract class?

 
In the interface all methods must be abstract; in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes. 

What is an abstract class? 

A class that cannot be instantiated. A concept in C++ known as pure virtual method. A class that must be inherited and have the methods over-ridden. Essentially, it is a blueprint for a class without any implementation. 

Who is a protected class-level variable available to? 

It is available to any sub-class (a class inheriting this class).

What’s the top .NET class that everything is derived from? 

System.Object.

What does the term immutable mean? 

The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.

What’s the difference between System.String and System.Text.StringBuilder classes? 

System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

What’s the advantage of using System.Text.StringBuilder over System.String? 

StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created. 

How can you sort the elements of the array in descending order? 

By calling Sort() and then Reverse() methods.

What’s the .NET collection class that allows an element to be accessed using a unique key? 

HashTable.

What class is underneath the SortedList class? 

A sorted HashTable.

Will the finally block get executed if an exception has not occurred?

Yes.

What’s the C# syntax to catch 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 for a single try statement? 

No. Once the proper catch block processed, control is transferred to the finally block (if there are any).

Explain the three services model commonly know as a three-tier application. 

Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).

What is the syntax to inherit from a class in C#?

Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass

Can you prevent your class from being inherited by another class? 

Yes. The keyword “sealed” will prevent the class from being inherited.

Can you allow a class to be inherited, but prevent the method from being over-ridden? 

Yes. Just leave the class public and make the method sealed.

What’s an abstract class? 

A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.

When do you absolutely have to declare a class as abstract? 

1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
2. When at least one of the methods in the class is abstract.

What is an interface class? 

Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.

Why can’t you specify the accessibility modifier for methods inside the interface? 

They all must be public, and are therefore public by default.

What’s the difference between an interface and abstract class? 

In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.

What is the difference between a Struct and a Class? 

Struts are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that struts cannot inherit.

What’s the implicit name of the parameter that gets passed into the set method/property of a class? 

Value. The data type of the value parameter is defined by whatever data type the property is declared as.

What does the keyword “virtual” declare for a method or property? 

The method or property can be overridden.

How is method overriding different from method overloading? 

When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.

Can you declare an override method to be static if the original method is not static? 

No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override) 

What are the different ways a method can be overloaded? 

Different parameter data types, different number of parameters, different order of parameters.