checked and unchecked keywords

Daily Knowledge Drop

The checked and unchecked C# keywords, can be used to control overflow checking when working with integral type arithmetic operations and conversions:

• checked: is used to explicitly enable overflow checking
• unchecked: is used to suppress overflow-checking

checked

The checked keyword is used to explicitly enable overflow checking. Consider the following:

int maxInt = Int32.MaxValue;
Console.WriteLine(maxInt);

maxInt = maxInt + 10;
Console.WriteLine(maxInt);

Here an int is being defined with its value set to the maximum possible value an int can have. It's then being increased by 10.

The output to the above is as follows:

2147483647
-2147483639

By default the overflow does not cause an exception, and the addition operation does not produce a accurate outcome.

To force an exception, the checked keyword can be used:

// wrap the entire block
checked
{
    int maxInt = Int32.MaxValue;
    Console.WriteLine(maxInt);

    maxInt = maxInt + 10;
    Console.WriteLine(maxInt);
}

// wrap just the specific operation
int maxInt2 = Int32.MaxValue;
Console.WriteLine(maxInt2);

maxInt2 = checked(maxInt2 + 10);
Console.WriteLine(maxInt2);

Now when either of the above sections of checked code is executed, an exception occurs:

2147483647
Unhandled exception. System.OverflowException: 
        Arithmetic operation resulted in an overflow.
   at Program.<Main>$(String[] args) in C:\Development\Projects\
        Blog\CheckedUnchecked\CheckedUnchecked\Program.cs:line 9

checked can be used to explicitly force operations which cause an overflow to throw an exception.

unchecked

The unchecked keyword is used to suppress overflow checking. Consider the following:

int int1 = 2147483647 + 10;
Console.WriteLine(int1);

The above will not compile, as the compiler can determine that this will result in an overflow. The error received when trying to compile is:

    error CS0220: The operation overflows at compile time in checked mode

The compiler can be instructed to ignore this, using the unchecked keyword:

// wrap the entire block
unchecked
{
    int int1 = 2147483647 + 10;
    Console.WriteLine(int1);
}

// wrap just the specific operation
int int2 = unchecked(2147483647 + 10);
Console.WriteLine(int2);

Now when either of the above sections of unchecked code is compiled, it will be successful, but result in the overflow occurring when executed:

-2147483639
-2147483639

unchecked can be used to explicitly suppress operations which cause an overflow from throwing an exception.

Notes

Very niche keywords, but the check specifically could be of use if the application is dealing with integral types who's value frequently approaches the limits. Rather have an exception be thrown, and handle it, than have calculation which result in overflow and incorrect results.

References

Checked and Unchecked (C# Reference)

Daily Drop 71: 11-05-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.