HasFlag not recognizing role assignment

问题

I'm using an Enum decorated with [Flags] to control autoization within my MVC2 app. Below is my code examples:

[Flags]
public enum SecurityRoles
{
    None = 0,
    Executive = 1,
    BackOffice = 2,
    AccountManager = 4,
    Consultant = 8,
    Administrator = 16
}

[TestMethod]
public void MultipleSelectionsTest()
{
    var requiredRoles = SecurityRoles.Executive | SecurityRoles.BackOffice;
    var user1Roles = SecurityRoles.Executive | SecurityRoles.Administrator | SecurityRoles.BackOffice | SecurityRoles.Consultant;
    var user1HasAccess = user1Roles.HasFlag(requiredRoles);

    var user2Roles = SecurityRoles.Administrator | SecurityRoles.BackOffice | SecurityRoles.Consultant;
    var user2HasAccess = user2Roles.HasFlag(requiredRoles);


    Assert.IsTrue(user1HasAccess);  //returns true
    Assert.IsTrue(user2HasAccess);  //returns false
}

As you can see, user2Roles containes BackOffice role and requiredRoles also contains BackOffice role, however, user2HasAccess is false. Why is that? What am I missing? user1HasAccess is true.

回答1:

Correct me if I'm wrong (because I could be), but when you perform Enum flag checks, .NET is essentially doing binary arithmetic on an integer which represents the sum of the flags.

So having SecurityRoles.Administrator | SecurityRoles.BackOffice | SecurityRoles.Consultant is the same as having a value of 26 or 11010 in binary.

When you make the call to Enum.HasFlags, the operation that's being performed is return thisInstance & flag == flag

So if you're checking the previously mentioned flag set against your required roles of SecurityRoles.Executive | SecurityRoles.BackOffice a value of 3 or 11 in binary, the math goes something like this:

11010 - 26 Administrator | BackOffice | Consultant
00011 -  3 Executive | BackOffice
----- 
00010 -  2 BackOffice which really doesn't mean anything useful

Then it would follow that 26 & 3 == 3 is false.

And for the sake of being thorough, given SecurityRoles.Executive | SecurityRoles.Administrator | SecurityRoles.BackOffice | SecurityRoles.Consultant a value of 27 or 11011 in binary, the math goes like this:

11011 - 26 Executive  | Administrator | BackOffice | Consultant
00011 -  3 Executive | BackOffice
----- 
00011 -  3 Executive | BackOffice

Then it would follow that 26 & 3 == 3 is true.

An extension method something like this, might be worthwhile (untested)

public static bool HasFlags(this Enum source, Enum[] flags) 
{
    return flags.Any(f => source.HasFlag(f));
}

回答2:

Thanks for your help Wes P. I was able to take your suggestion and combine it with the extension found here: link text and came up with my own extension to solve the problem

Here's me extension. It uses the GetFlags() method found in the extension methods i found in the link above.

public static bool HasFlags(this Enum userRoles, Enum requiredRoles)
        {
            var hasFlags = false;
            var userRolesList = userRoles.GetFlags();
            var requiredRolesList = requiredRoles.GetFlags();

            foreach (var role in userRolesList)
            {
                var role1 = role;
                hasFlags = requiredRolesList.Any(securityRole => role1.CompareTo(securityRole) == 0);

                if(hasFlags)
                    break;
            }

            return hasFlags;
        }

来源：https://stackoverflow.com/questions/4717550/hasflag-not-recognizing-role-assignment

标签

.net

authorization

bit-manipulation