Numbers - Discussion
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A narrowing conversion occurs when a larger type is cast down to a smaller type. For instance, consider casting a value of type Int32 to a value of type Int16. If the Int32 value is smaller or equal to the Int16.MaxValue field and the Int32 value is higher or equal to the Int16.MinValue field, the cast will occur without error or loss of information. Loss of information occurs when the Int32 value is larger than the Int16. MaxValue field or the Int32 value is lower than the Int16.MinValue field. In either of these cases, the most-significant bits of the Int32 value would be truncated and discarded, changing the value after the cast.
If a loss of information occurs in an unchecked context, it will occur silently without the application noticing. This problem can cause some very insidious bugs that are hard to track down. To prevent this, check the value to be converted to determine whether it is within the lower and upper bounds of the type that it will be cast to. If the value is outside these bounds, then code can be written to handle this situation. This code could force the cast not to occur and/or possibly to inform the application of the casting problem. This solution can aid in the prevention of hard-to-find arithmetic bugs from appearing in your applications.
You should understand that both techniques shown in the Solution section are valid. However, the technique you use will depend on whether you expect to hit the overflowcase on a regular basis or only occasionally. If you expect to hit the overflow case quite often, you might want to choose the second technique of manually testing the numeric value. Otherwise, it might be easier to use the checked keyword, as in the first technique.
Note: In C#, code can run in either a checked or unchecked context; by default, the code runs in an unchecked context. In a checked context, any arithmetic and conversions involving integral types are examined to determine whether an overflow condition exists. If so, an OverflowException is thrown. In an unchecked context, no OverflowException will be thrown when an overflow condition exists.
A checked context can be set up by using the /checked{+} compiler switch, by setting the Check for Arithmetic Overflow/Underflow project property to true, or by using the checked keyword. An unchecked context can be set up using the /checked-compiler switch, by setting the Check for Arithmetic Overflow/Underflow project property to false, or by using the unchecked keyword.
Notice that floating-point and decimal types are not included in the code that handles the conversions to integral types in this recipe. The reason is that a conversion from any integral type to a float, double,or decimal will not lose any information; therefore, it is redundant to check these conversions.
In addition, you should be aware of the following when performing a conversion:
- Casting from a float, double, or decimal type to an integral type results in the truncation of the fractional portion of this number.
- Casting from a float or double to a decimal results in the float or double being rounded to 28 decimal places.
- Casting from a double to a float results in the double being rounded to the nearest float value.
- Casting from a decimal to a float or double results in the decimal being rounded to the resulting type (float or double).
- Casting from int, uint, or long to a float could result in the loss of precision, but never magnitude.
- Casting from long to a double could result in the loss of precision, but never magnitude.
See Also: See the “checked” keyword and “Checked and Unchecked” topics in the MSDN documentation.
