How to Compare Flags in C#?

I have a flag enum below.

[Flags]
public enum FlagTest
{
    None = 0x0,
    Flag1 = 0x1,
    Flag2 = 0x2,
    Flag3 = 0x4
}

I cannot make the if statement evaluate to true.

FlagTest testItem = FlagTest.Flag1 | FlagTest.Flag2;

if (testItem == FlagTest.Flag1)
{
    // Do something,
    // however This is never true.
}

How can I make this true?

Solution 1:

In .NET 4 there is a new method Enum.HasFlag. This allows you to write:

if ( testItem.HasFlag( FlagTest.Flag1 ) )
{
    // Do Stuff
}

which is much more readable, IMO.

The .NET source indicates that this performs the same logic as the accepted answer:

public Boolean HasFlag(Enum flag) {
    if (!this.GetType().IsEquivalentTo(flag.GetType())) {
        throw new ArgumentException(
            Environment.GetResourceString(
                "Argument_EnumTypeDoesNotMatch", 
                flag.GetType(), 
                this.GetType()));
    }

    ulong uFlag = ToUInt64(flag.GetValue()); 
    ulong uThis = ToUInt64(GetValue());
    // test predicate
    return ((uThis & uFlag) == uFlag); 
}

Solution 2:

if ((testItem & FlagTest.Flag1) == FlagTest.Flag1)
{
     // Do something
}

(testItem & FlagTest.Flag1) is a bitwise AND operation.

FlagTest.Flag1 is equivalent to 001 with OP's enum. Now let's say testItem has Flag1 and Flag2 (so it's bitwise 101):

  001
 &101
 ----
  001 == FlagTest.Flag1

Solution 3:

For those who have trouble visualizing what is happening with the accepted solution (which is this),

if ((testItem & FlagTest.Flag1) == FlagTest.Flag1)
{
    // Do stuff.
}

testItem (as per the question) is defined as,

testItem 
 = flag1 | flag2  
 = 001 | 010  
 = 011

Then, in the if statement, the left hand side of the comparison is,

(testItem & flag1) 
 = (011 & 001) 
 = 001

And the full if statement (that evaluates to true if flag1 is set in testItem),

(testItem & flag1) == flag1
 = (001) == 001
 = true