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V3154. The 'a % b' expression always ev…
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V3154. The 'a % b' expression always evaluates to 0.

Aug 05 2020

A modulo operation performed on an integer type always returns 0. This happens because its right operand is represented by the value 1 (-1), which is most likely a typo.

A common pattern of this error is checking if no remainder is left after dividing a value by 2. To do this, you use the modulo operation and compare the result with 0 or 1. Making a typo at this point is easy because since you anticipate the value 1, you may accidentally divide by 1 too. For example:

if ((x % 1) == 1)
{
    ....
}

The programmer wrote the value 1 instead of 2, which will result in the 'x % 1' expression always evaluating to 0 no matter the value of 'x'. Therefore, the condition will always be false.

Fixed code:

if ((x % 2) == 1)
{
    ....
}

The following example was taken from a real-life application:

const int SyncReportFrequency = 1;
....
private void TimerOnElapsed(object sender, ElapsedEventArgs e)
{
    if (_reportId % SyncReportFrequency == 0)
    {
        WriteSyncReport();
    }
}

The 'SyncReportFrequency ' is a constant equal to 1, so regardless of the value of the '_reportId' variable, the condition of the 'if' statement will always be true.

It is either that the developer made a typo in this code or the condition is superfluous. Fixed version:

const int SyncReportFrequency = 2;
....
private void TimerOnElapsed(object sender, ElapsedEventArgs e)
{
    if (_reportId % SyncReportFrequency == 0)
    {
        WriteSyncReport();
    }
}

This diagnostic is classified as:

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