New rule for checking an enum flag is set verifies enum value

[alexangas]

Description

Where an attempt is made to check if an enum value has a flag set, then SonarC# should verify that this can be done correctly.

This issue suggests a new rule to cover this, based off these remarks in MS API documentation.

Repro steps

Given one of the following code patterns is used to check if a flag is set:

enumValue.HasFlag(Colors.SomeValue);
(enumValue & Colors.SomeValue) == Colors.SomeValue;

Then this rule would be triggered if the target enum declaration does not have the FlagsAttribute, or if one of the values in the enum declaration is zero or not a power of two.

Related example problematic enum declaration:

// [Flags] // (missing FlagsAttribute)
public enum Colors
{
    None = 0, // Value of 0
    Red = 1,
    Orange = 2,
    Yellow = 3 // Value is not a power of 2
}

Related information

• SonarC# Version: 7.2
• Visual Studio Version: N/A
• If running through the Scanner for MSBuild, its version: N/A

[Corniel]

I was thinking of this as well. There are according to me, 3 scenario's that are none-compliant, if you use Enum.HasFlag(Enum):

• Enum type used, is not decorated with the FlagsAttribute
• Enum types don't match
• the value to test the flag for, represents the 0 value

Like this code:

public class HasFlag
{
    [Flags]
    public enum Flagged
    {
        None = 0,
        UnionJack = 1,
        StarSpangledBanner = 2,
    }

    public void Test(Flagged flags, LoaderOptimization optimization)
    {
        var noFlag = optimization.HasFlag(LoaderOptimization.MultiDomain); // none-compliant. LoaderOptimization is not a flags enum.
        var hasNone = flags.HasFlag(Flagged.None); //none-compliant, None is not a flag value.
        var hasOther = flags.HasFlag(LoaderOptimization.SingleDomain); // not-compliant, enum types don't match
        var hasUS = flags.HasFlag(Flagged.StarSpangledBanner); // compliant
    }
}