CA1806 should analyze calls to date and time methods

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Analyzer

Diagnostic ID: CA1806: Do not ignore method results

Describe the improvement

DateTime is similar to String insofar as it's immutable and has methods for changing the value which return a new instance.

CA1806 currently checks that the return value from certain String methods are not ignored. This analyzer could also check that the return value from certain DateTime methods are not ignored.

DateTimeOffset, DateOnly, TimeOnly and TimeSpan could also be supported.

Describe suggestions on how to achieve the rule

On DateTime and DateTimeOffset, the return value of these methods should not be ignored:

• Add(TimeSpan)
• AddDays(Double)
• AddHours(Double)
• AddMicroseconds(Double)
• AddMilliseconds(Double)
• AddMinutes(Double)
• AddMonths(Int32)
• AddSeconds(Double)
• AddTicks(Int64)
• AddYears(Int32)
• Subtract(DateTime)
• Subtract(TimeSpan)
• ToLocalTime()
• ToUniversalTime()

On DateOnly, the return value of these methods should not be ignored:

• AddDays(Int32)
• AddMonths(Int32)
• AddYears(Int32)

On TimeOnly, the return value of these methods should not be ignored:

• Add(TimeSpan)
• Add(TimeSpan, Int32)
• AddHours(Double)
• AddHours(Double, Int32)
• AddMinutes(Double)
• AddMinutes(Double, Int32)

On TimeSpan, the return value of these methods should not be ignored:

• Add(TimeSpan)
• Divide(Double)
• Divide(TimeSpan)
• Multiply(Double)
• Multiply(TimeSpan)
• Negate(Double)
• Subtract(TimeSpan)

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Possibly related to or superseded by dotnet/runtime#34098