问题
I'd like one general purpose function that could be used with any Flags style enum to see if a flag exists.
This doesn't compile, but if anyone has a suggestion, I'd appreciate it.
public static Boolean IsEnumFlagPresent<T>(T value,T lookingForFlag)
where T:enum
{
Boolean result = ((value & lookingForFlag) == lookingForFlag);
return result ;
}
回答1:
No, you can't do this with C# generics. However, you could do:
public static bool IsEnumFlagPresent<T>(T value, T lookingForFlag)
where T : struct
{
int intValue = (int) (object) value;
int intLookingForFlag = (int) (object) lookingForFlag;
return ((intValue & intLookingForFlag) == intLookingForFlag);
}
This will only work for enums which have an underlying type of int, and it's somewhat inefficient because it boxes the value... but it should work.
You may want to add an execution type check that T is actually an enum type (e.g. typeof(T).BaseType == typeof(Enum))
Here's a complete program demonstrating it working:
using System;
[Flags]
enum Foo
{
A = 1,
B = 2,
C = 4,
D = 8
}
class Test
{
public static Boolean IsEnumFlagPresent<T>(T value, T lookingForFlag)
where T : struct
{
int intValue = (int) (object) value;
int intLookingForFlag = (int) (object) lookingForFlag;
return ((intValue & intLookingForFlag) == intLookingForFlag);
}
static void Main()
{
Console.WriteLine(IsEnumFlagPresent(Foo.B | Foo.C, Foo.A));
Console.WriteLine(IsEnumFlagPresent(Foo.B | Foo.C, Foo.B));
Console.WriteLine(IsEnumFlagPresent(Foo.B | Foo.C, Foo.C));
Console.WriteLine(IsEnumFlagPresent(Foo.B | Foo.C, Foo.D));
}
}
回答2:
You're looking to replace one line of code with a function that wraps one line of code? I'd say to just use the one line of code...
回答3:
For what its worth, I recently read that this feature will be part of .NET 4.0. Specifically, it is implemented in the Enum.HasFlag() function.
回答4:
I have used this before:
public static bool In<T>(this T me, T values)
where T : struct, IConvertible
{
return (me.ToInt64(null) & values.ToInt64(null)) > 0;
}
What I like about it is you can use this clean syntax to call it since in 3.5 the compiler will can infer generic parameters.
AttributeTargets a = AttributeTargets.Class;
if (a.In(AttributeTargets.Class | AttributeTargets.Module))
{
// ...
}
回答5:
You can do this without generics:
static bool ContainsFlags(Enum value, Enum flag)
{
if (Enum.GetUnderlyingType(value.GetType()) == typeof(ulong))
return (Convert.ToUInt64(value) & Convert.ToUInt64(flag)) == Convert.ToUInt64(flag);
else
return (Convert.ToInt64(value) & Convert.ToInt64(flag)) == Convert.ToInt64(flag);
}
I'm converting to Int64 in this case, which should handle every case except ulong, which is why the extra check...
回答6:
Why not write an extension method for this? I did this in another post
public static class EnumerationExtensions {
public static bool Has<T>(this System.Enum type, T value) {
try {
return (((int)(object)type & (int)(object)value) == (int)(object)value);
}
catch {
return false;
}
}
//... etc...
}
//Then use it like this
bool hasValue = permissions.Has(PermissionTypes.Delete);
It could use a little refinement (since it assumes everything can be cast as an int), but it could get you started...
回答7:
Worth pointing out that simply providing some static overloads for all the integral types will work so long as you know you are working with a specific enum. They won't work if the consuming code is likewise operating on where t : struct
If you need to deal with arbitrary (struct) T
You cannot currently do a fast conversion of a generically typed struct into some alternate bitwise form (i.e. roughly speaking a reinterpret_cast) without using C++/CLI
generic <typename T>
where T : value class
public ref struct Reinterpret
{
private:
const static int size = sizeof(T);
public:
static int AsInt(T t)
{
return *((Int32*) (void*) (&t));
}
}
This will then let you write:
static void IsSet<T>(T value, T flags) where T : struct
{
if (!typeof(T).IsEnum)
throw new InvalidOperationException(typeof(T).Name +" is not an enum!");
Type t = Enum.GetUnderlyingType(typeof(T));
if (t == typeof(int))
{
return (Reinterpret.AsInt(value) & Reinterpret.AsInt(flags)) != 0
}
else if (t == typeof(byte))
{
return (Reinterpret.AsByte(value) & Reinterpret.AsByte(flags)) != 0
}
// you get the idea...
}
You cannot constrain to enums. But the mathematical validity of these methods do not change if they are used with non enum types so you could allow them if you can determine that they are convertible to a struct of the relevant size.
回答8:
Well, I don't believe there is a way to do this, as there are no constraints that apply to bitwise operators.
However... you can just cast your enum to int and do it.
public static Boolean IsEnumFlagPresent(int value,int lookingForFlag)
{
return ((value & lookingForFlag) == lookingForFlag);
}
This works, but may be confusing to someone.
回答9:
Question long over, but here's one for reference anyway:
public static bool HasFlag<TEnum>(this TEnum enumeratedType, TEnum value)
where TEnum : struct, IComparable, IFormattable, IConvertible
{
if (!(enumeratedType is Enum))
{
throw new InvalidOperationException("Struct is not an Enum.");
}
if (typeof(TEnum).GetCustomAttributes(
typeof(FlagsAttribute), false).Length == 0)
{
throw new InvalidOperationException("Enum must use [Flags].");
}
long enumValue = enumeratedType.ToInt64(CultureInfo.InvariantCulture);
long flagValue = value.ToInt64(CultureInfo.InvariantCulture);
if ((enumValue & flagValue) == flagValue)
{
return true;
}
return false;
}
回答10:
below is code that benchmarks 4 different methods. results are shown in code in comment "BENCHMARK: .. nSec".
"((enum & flag) != 0)' is 10x faster than HasFlag() function. if you are in a tight loop, then i think it is best to accept it.
public static int jumpCtr=0;
public static int ctr=0;
public static TestFlags gTestFlags = TestFlags.C;
[Flags] public enum TestFlags { A=1<<1, B=1<<2, C=1<<3 }
public static void Jump() { jumpCtr++; gTestFlags = (gTestFlags == TestFlags.B) ? TestFlags.C : TestFlags.B; }
// IsEnumFlagPresent() https://stackoverflow.com/questions/987607/c-flags-enum-generic-function-to-look-for-a-flag
[MethodImpl(MethodImplOptions.AggressiveInlining)] public static bool HasFlag_Faster<T>(T value, T lookingForFlag)
where T : struct
{
int intValue = (int) (object) value;
int intLookingForFlag = (int) (object) lookingForFlag;
return ((intValue & intLookingForFlag) != 0);
}
// IsEnumFlagPresent() https://stackoverflow.com/questions/987607/c-flags-enum-generic-function-to-look-for-a-flag
[MethodImpl(MethodImplOptions.AggressiveInlining)] public static bool HasFlag_Faster_Integer(int intValue, int intLookingForFlag)
{
return ((intValue & intLookingForFlag) != 0);
}
public static void Benchmark_HasFlag( )
{
if ( ! hwDvr._weAreOnGswCpu) { return; }
DateTime timer = DateTime.Now;
string a, b, c, d, e;
double base_nSecPerLoop, b_nSecPerLoop, c_nSecPerLoop, d_nSecPerLoop, e_nSecPerLoop;
int numOfLoops = (int) 1.0e6;
// ------------------------------------------------------
for (int i=0; i<numOfLoops;i++) {
Jump();
}
a = BenchMarkSystem_Helper.SimpleTimer_Loops( ref timer, numOfLoops, out base_nSecPerLoop);
// ------------------------------------------------------
// BENCHMARK: 50 nSec
for (int i=0; i<numOfLoops;i++) {
if (gTestFlags.HasFlag((TestFlags) TestFlags.C)) {
ctr++;
}
Jump();
}
b = BenchMarkSystem_Helper.SimpleTimer_Loops( ref timer, numOfLoops, out b_nSecPerLoop );
double b_diff = b_nSecPerLoop - base_nSecPerLoop;
// ------------------------------------------------------
// BENCHMARK: 3 nSec
for (int i=0; i<numOfLoops;i++) {
if ((gTestFlags & TestFlags.C) != 0) {
ctr++;
}
Jump();
}
c = BenchMarkSystem_Helper.SimpleTimer_Loops( ref timer, numOfLoops, out c_nSecPerLoop );
double c_diff = c_nSecPerLoop - base_nSecPerLoop;
// ------------------------------------------------------
// BENCHMARK: 64 nSec
for (int i=0; i<numOfLoops;i++) {
if (HasFlag_Faster<TestFlags>(value:gTestFlags, lookingForFlag: TestFlags.C)) {
ctr++;
}
Jump();
}
d = BenchMarkSystem_Helper.SimpleTimer_Loops( ref timer, numOfLoops, out d_nSecPerLoop );
double d_diff = d_nSecPerLoop - base_nSecPerLoop;
// ------------------------------------------------------
// BENCHMARK: 14 nSec
for (int i=0; i<numOfLoops;i++) {
if (HasFlag_Faster_Integer((int)gTestFlags, (int)TestFlags.C)) {
ctr++;
}
Jump();
}
e = BenchMarkSystem_Helper.SimpleTimer_Loops( ref timer, numOfLoops, out e_nSecPerLoop );
double e_diff = e_nSecPerLoop - base_nSecPerLoop;
int brkPt=0;
}
回答11:
Today, you can set the c# language version to >=7.3 and use the next code as the reference:
public static class EnumExt
{
public static IEnumerable<TEnum> Explode<TEnum>(this TEnum enumValue) where TEnum : Enum
{
var res = Enum.GetValues(enumValue.GetType())
.Cast<TEnum>()
.Where(x => enumValue.HasFlag(x));
return res;
}
public static string ExplodeToString<TEnum>(this TEnum enumValue, string delimeter = ",") where TEnum : Enum
{
return string.Join(delimeter, Explode(enumValue));
}
}
来源：https://stackoverflow.com/questions/987607/c-flags-enum-generic-function-to-look-for-a-flag