C# 0 The Complete Reference

Using checked and unchecked

A special feature in C# relates to the generation of overflow exceptions in arithmetic 

computations. As you know, it is possible for some types of arithmetic computations to 

produce a result that exceeds the range of the data type involved in the computation. When 

this occurs, the result is said to 

overflow.

 For example, consider the following sequence:

byte a, b, result;

a = 127;

b = 127;

result = (byte)(a * b);

Here, the product of 

a

 and 

b

 exceeds the range of a 

byte

 value. Thus, the result overflows 

the type of the result.

C# allows you to specify whether your code will raise an exception when overflow 

occurs by using the keywords 

checked

 and 

unchecked

. To specify that an expression be 

checked for overflow, use 

checked

. To specify that overflow be ignored, use 

unchecked

.

In this case, the result is truncated to fit into the target type of the expression.

The

checked

 keyword has these two general forms. One checks a specific expression 

and is called the 

operator form

 of 

checked

. The other checks a block of statements and is 

called the 

statement form.

checked (

expr

)

checked {

    // statements to be checked

}

Here, 

expr

 is the expression being checked. If a checked expression overflows, then an 

OverflowException

 is thrown.

The

unchecked

 keyword also has two general forms. The first is the operator form, 

which ignores overflow for a specific expression. The second ignores overflow for a block 

of statements.

unchecked (

expr

)

unchecked {

    // statements for which overfl ow is ignored

}

Here, 

expr

 is the expression that is not being checked for overflow. If an unchecked 

expression overflows, then truncation will occur.

Here is a program that demonstrates both 

checked

 and 

unchecked

:

// Using checked and unchecked.

using System;

class CheckedDemo {

  static void Main() {

    byte a, b;

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