As developers knows that many arithmetic operations on primitives types could result in an overflow:

Byte b=100;
b=(Byte) (b+200);       //b now contains 44 (2c in hex)

In some cases silent overflow can give undesirable results, and if not detected causes the application to behave in strange and unusual ways. Many languages handles overflows in different ways. Like C and C++ allows overflow and is not considered as an error and allow the value to wrap; allows to application continues running. Microsoft visual basic always considers overflows to be an errors and throws an exception when it detects.

The common language runtime (CLR) offers IL (Intermediate language) instructions that allow the compiler to choose the desired behavior. as you know the C# compiler generated IL has an add instruction as it the default behavior for C# to generate silent overflow. C# compiler use the /checked+ compiler switch to control the overflow, which tell the C# compiler to uses the safe version of the add with overflow check , add.ovf which will prevent this kind of silent overflow and throw OverflowException if overflow occur.

C# allows the programmer to decide how overflow should be handled. as you know, By default overflow checking is turned off. this means that the compiler generates IL code by using the versions of the add, subtract, multiply, and conversion instructions that don't include overflow checking. as a result, the code runs faster but developers must be assured that overflows won't occur or that their code is designed to anticipate these overflows.

If overflow occur, the CLR throws an OverflowException. You should design your application's code to handle this exception. Rather than have overflow checking turned off on or off globally, C# allows this flexibility by offering checked and unchecked operators. Simply checked operator tells the C# compiler to use the safe IL instruction for this operation , and the unchecked use the normal IL instructions (which is the normal behavior in C#).

Here is an example that use checked operator:

Byte b=100;
b=checked((Byte)(b+200));                  //OverflowException is thrown

If the Byte cast outside the checked operator, the exception wouldn't be occurred.

b=(Byte)checked((b+200));                 //no OverflowException

In addition to the checked and unchecked operators, C# also offers checked and unchecked statements. the statements cause all expressions within a block to be checked or unchecked:

checked{
 Byte b=100;
 b=(Byte((b=200);
}

Now, try to turn on the compiler's switch /checked+ switch for debug build. Your application will run slowly because the system will be checking for overflow on any code that you didn't explicitly mark as checked and unchecked. If an exception occurs, you will be able to easily detect and be able to fix the bug in you application.

A String represents an immutable ordered set of characters. The String type is derived from Object, making it a reference type, and therefore, String objects (its array of characters) always live in the heap, never on a thread's stack. The String  type also implements several interface (IComparable/ IComparable<String>, ICloneable, IConvertible, IEnumerable/ IEnumerable<Char>, and IEquatable<String>). The String class is sealed no inheritance allowed and string is an alias for System.String in the .NET Framework.

Once created, a string can never get longer, get shorter, or have any of its characters changed. It allows you  to perform operations on a string without actually changing the string. If you perform a lot of string manipulations, you end up creating a lot of String objects on the heap, which causes more frequent garbage collections, thus hurting your application's performance. To perform a lot of string manipulations efficiently, use the StringBuilder class.

You can concatenate several strings to form a single string by using the C# + (plus) operator: 

String  sObj=”Hi” + “  “ + “Gentleman”; 

In this example all strings are literal strings so C# compiler concatenates them at compile time and end up just one string “Hi Gentleman” in the module's metadata. Using the + (plus)  operator on nonliteral strings causes the concatenation to be performed at run time. To concatenate several strings together at run time, avoid using the + operator as it creates multiple string objects on the garbage-collected heap. Instead, use the System.Text.StringBuilder type.

C# also offers a special way to declare a string in which all characters between quotes are considered part of the string. These special declarations are called verbatim strings and are typically used when specifying the path of a file or directory or when working with regular expressions. 

// Specifying the pathname of an application 
String file = "C:\\Windows\\System32\\pbrush.exe"; 

// Specifying the pathname of an application by using a verbatim string 
String file = @"C:\Windows\System32\pbrush.exe";

The @ symbol before the string tells the compiler that the string is a verbatim string. In effect, this  tells the compiler to treat backslash characters as backslash characters instead of escape characters.