Microsoft .NET 面试题与答案

Question: What is checked { } and unchecked { }?

Answer: By default C# does not check for overflow (unless using constants), we can use
checked to raise an exception. E.g.:
static short x = 32767; // Max short value
static short y = 32767;
// Using a checked expression
public static int myMethodCh()
{
int z = 0;
try
{
z = checked((short)(x + y));
//z = (short)(x + y);
}
catch (System.OverflowException e)
{
System.Console.WriteLine(e.ToString());
}
return z; // Throws the exception OverflowException
}
This code will raise an exception, if we remove unchecked as in:
//z = checked((short)(x + y));
z = (short)(x + y);
Then the cast will raise no overflow exception and z will be assigned –2. unchecked can
be used in the opposite way, to say avoid compile time errors with constanst overflow.
E.g. the following will cause a compiler error:
const short x = 32767; // Max short value
const short y = 32767;
public static int myMethodUnch()
{
int z = (short)(x + y);
return z; // Returns -2
}
The following will not:
const short x = 32767; // Max short value
const short y = 32767;
public static int myMethodUnch()
{
int z = unchecked((short)(x + y));
return z; // Returns -2
}