Csharp: Fix typo in LeapYear qhelp

[igfoo]

This PR corrects a typo in the UnsafeYearConstruction.qhelp file where "add/substract" is changed to "add/subtract".

[Copilot]

Pull Request Overview

This PR fixes a spelling error in the C# documentation for the UnsafeYearConstruction query help file.

[Copilot]

The typo has been correctly fixed from 'substract' to 'subtract'.

[github-actions]

QHelp previews:

csharp/ql/src/Likely Bugs/LeapYear/UnsafeYearConstruction.qhelp

Unsafe year argument for 'DateTime' constructor

When creating a System.DateTime object by setting the year, month, and day in the constructor by performing an arithmetic operation on a different DateTime object, there is a risk that the date you are setting is invalid.

On a leap year, such code may throw an ArgumentOutOfRangeException with a message of "Year, Month, and Day parameters describe an unrepresentable DateTime."

Recommendation

Creating a System.DateTime object based on a different System.DateTime object, use the appropriate methods to manipulate the date instead of arithmetic.

Example

In this example, we are incrementing/decrementing the current date by one year when creating a new System.DateTime object. This may work most of the time, but on any given February 29th, the resulting value will be invalid.

usingSystem;publicclassUnsafeYearConstructionBad{publicUnsafeYearConstructionBad(){DateTimeStart;DateTimeEnd;varnow=DateTime.UtcNow;// the base-date +/- n years may not be a valid date.Start=newDateTime(now.Year-1,now.Month,now.Day,0,0,0,DateTimeKind.Utc);End=newDateTime(now.Year+1,now.Month,now.Day,0,0,1,DateTimeKind.Utc);}}

To fix this bug, we add/subtract years to the current date by calling AddYears method on it.

usingSystem;publicclassUnsafeYearConstructionGood{publicUnsafeYearConstructionGood(){DateTimeStart;DateTimeEnd;varnow=DateTime.UtcNow;Start=now.AddYears(-1).Date;End=now.AddYears(-1).Date.AddSeconds(1);}}

References

• System.DateTime Struct.