C# 'is' operator performance.

Question

I have a program that requires fast performance. Within one of its inner loops, I need to test the type of an object to see whether it inherits from a certain interface.

One way to do this would be with the CLR's built-in type-checking functionality. The most elegant method there probably being the 'is' keyword:

if (obj is ISpecialType)

Another approach would be to give the base class my own virtual GetType() function which returns a pre-defined enum value (in my case, actually, i only need a bool). That method would be fast, but less elegant.

I have heard that there is an IL instruction specifically for the 'is' keyword, but that doesn't mean it executes fast when translated into native assembly. Can anyone share some insight into the performance of 'is' versus the other method?

UPDATE: Thanks for all the informed answers! It seem a couple helpful points are spread out among the answers: Andrew's point about 'is' automatically performing a cast is essential, but the performance data gathered by Binary Worrier and Ian is also extremely useful. It would be great if one of the answers were edited to include all of this information.

Answer 1

Using is can hurt performance if, once you check the type, you cast to that type. is actually casts the object to the type you are checking so any subsequent casting is redundant.

If you are going to cast anyway, here is a better approach:

ISpecialType t = obj as ISpecialType;

if (t != null)
{
    // use t here
}

Answer 2

Andrew is correct. In fact with code analysis this gets reported by Visual Studio as an unnecessary cast.

One idea (without knowing what you're doing is a bit of a shot in the dark), but I've always been advised to avoid checking like this, and instead have another class. So rather than doing some checks and having different actions depending on the type, make the class know how to process itself...

e.g. Obj can be ISpecialType or IType;

both of them have a DoStuff() method defined. For IType it can just return or do custom stuff, whereas ISpecialType can do other stuff.

This then completely removes any casting, makes the code cleaner and easier to maintain, and the class knows how to do it's own tasks.

Answer 3

operator is is much faster then virtual functions

Answer 4

I'm with Ian, you probably don't want to do this.

However, just so you know, there is very little difference between the two, over 10,000,000 iterations

• The enum check comes in at 700 milliseconds (approx)
• The IS check comes in at 1000 milliseconds (approx)

I personally wouldn't fix this problem this way, but if I was forced to pick one method it would be the built in IS check, the performance difference isn't worth considering the coding overhead.

My base and derived classes

class MyBaseClass
{
    public enum ClassTypeEnum { A, B }
    public ClassTypeEnum ClassType { get; protected set; }
}

class MyClassA : MyBaseClass
{
    public MyClassA()
    {
        ClassType = MyBaseClass.ClassTypeEnum.A;
    }
}
class MyClassB : MyBaseClass
{
    public MyClassB()
    {
        ClassType = MyBaseClass.ClassTypeEnum.B;
    }
}

JubJub: As requested more info on the tests.

I ran both tests from a console app (a debug build) each test looks like the following

static void IsTest()
{
    DateTime start = DateTime.Now;
    for (int i = 0; i < 10000000; i++)
    {
        MyBaseClass a;
        if (i % 2 == 0)
            a = new MyClassA();
        else
            a = new MyClassB();
        bool b = a is MyClassB;
    }
    DateTime end = DateTime.Now;
    Console.WriteLine("Is test {0} miliseconds", (end - start).TotalMilliseconds);
}

Running in release, I get a difference of 60 - 70 ms, like Ian.

Answer 5

@Binary Worrier-

Your new operator allocations of classes are going to completely overshadow any performance differences in the 'is' operations.

Why don't you remove those new operations, by reusing two different pre-allocated instances, and then re-run the code and post your results.

Answer 6

Ok so I was chatting about this with someone and decided to test this more. As far as I can tell, the performance of as and is are both very good, compared to testing your own member or function to store type information.

I used Stopwatch, which I just learned may not be the most reliable approach, so I also tried UtcNow. Later, I also tried Processor time approach which seems similar to UtcNow including unpredictable create times. I also tried making the base class non-abstract with no virtuals but it didn't seem to have a significant effect.

I ran this on a Quad Q6600 with 16GB RAM. Even with 50mil iterations, the numbers still bounce around +/- 50 or so millisec so I wouldn't read too much into the minor differences.

It was interesting to see that x64 created faster but executed as/is slower than x86

x64 Release Mode:
Stopwatch:
As: 561ms
Is: 597ms
Base property: 539ms
Base field: 555ms
Base RO field: 552ms
Virtual GetEnumType() test: 556ms
Virtual IsB() test: 588ms
Create Time : 10416ms

UtcNow:
As: 499ms
Is: 532ms
Base property: 479ms
Base field: 502ms
Base RO field: 491ms
Virtual GetEnumType(): 502ms
Virtual bool IsB(): 522ms
Create Time : 285ms (This number seems unreliable with UtcNow. I also get 109ms and 806ms.)

x86 Release Mode:
Stopwatch:
As: 391ms
Is: 423ms
Base property: 369ms
Base field: 321ms
Base RO field: 339ms
Virtual GetEnumType() test: 361ms
Virtual IsB() test: 365ms
Create Time : 14106ms

UtcNow:
As: 348ms
Is: 375ms
Base property: 329ms
Base field: 286ms
Base RO field: 309ms
Virtual GetEnumType(): 321ms
Virtual bool IsB(): 332ms
Create Time : 544ms (This number seems unreliable with UtcNow.)

Here's most of the code:

    static readonly int iterations = 50000000;
    void IsTest()
    {
        Process.GetCurrentProcess().ProcessorAffinity = (IntPtr)1;
        MyBaseClass[] bases = new MyBaseClass[iterations];
        bool[] results1 = new bool[iterations];

        Stopwatch createTime = new Stopwatch();
        createTime.Start();
        DateTime createStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            if (i % 2 == 0) bases[i] = new MyClassA();
            else bases[i] = new MyClassB();
        }
        DateTime createStop = DateTime.UtcNow;
        createTime.Stop();


        Stopwatch isTimer = new Stopwatch();
        isTimer.Start();
        DateTime isStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] =  bases[i] is MyClassB;
        }
        DateTime isStop = DateTime.UtcNow; 
        isTimer.Stop();
        CheckResults(ref  results1);

        Stopwatch asTimer = new Stopwatch();
        asTimer.Start();
        DateTime asStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i] as MyClassB != null;
        }
        DateTime asStop = DateTime.UtcNow; 
        asTimer.Stop();
        CheckResults(ref  results1);

        Stopwatch baseMemberTime = new Stopwatch();
        baseMemberTime.Start();
        DateTime baseStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i].ClassType == MyBaseClass.ClassTypeEnum.B;
        }
        DateTime baseStop = DateTime.UtcNow;
        baseMemberTime.Stop();
        CheckResults(ref  results1);

        Stopwatch baseFieldTime = new Stopwatch();
        baseFieldTime.Start();
        DateTime baseFieldStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i].ClassTypeField == MyBaseClass.ClassTypeEnum.B;
        }
        DateTime baseFieldStop = DateTime.UtcNow;
        baseFieldTime.Stop();
        CheckResults(ref  results1);


        Stopwatch baseROFieldTime = new Stopwatch();
        baseROFieldTime.Start();
        DateTime baseROFieldStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i].ClassTypeField == MyBaseClass.ClassTypeEnum.B;
        }
        DateTime baseROFieldStop = DateTime.UtcNow;
        baseROFieldTime.Stop();
        CheckResults(ref  results1);

        Stopwatch virtMethTime = new Stopwatch();
        virtMethTime.Start();
        DateTime virtStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i].GetClassType() == MyBaseClass.ClassTypeEnum.B;
        }
        DateTime virtStop = DateTime.UtcNow;
        virtMethTime.Stop();
        CheckResults(ref  results1);

        Stopwatch virtMethBoolTime = new Stopwatch();
        virtMethBoolTime.Start();
        DateTime virtBoolStart = DateTime.UtcNow;
        for (int i = 0; i < iterations; i++)
        {
            results1[i] = bases[i].IsB();
        }
        DateTime virtBoolStop = DateTime.UtcNow;
        virtMethBoolTime.Stop();
        CheckResults(ref  results1);


        asdf.Text +=
        "Stopwatch: " + Environment.NewLine 
          +   "As:  " + asTimer.ElapsedMilliseconds + "ms" + Environment.NewLine
           +"Is:  " + isTimer.ElapsedMilliseconds + "ms" + Environment.NewLine
           + "Base property:  " + baseMemberTime.ElapsedMilliseconds + "ms" + Environment.NewLine + "Base field:  " + baseFieldTime.ElapsedMilliseconds + "ms" + Environment.NewLine + "Base RO field:  " + baseROFieldTime.ElapsedMilliseconds + "ms" + Environment.NewLine + "Virtual GetEnumType() test:  " + virtMethTime.ElapsedMilliseconds + "ms" + Environment.NewLine + "Virtual IsB() test:  " + virtMethBoolTime.ElapsedMilliseconds + "ms" + Environment.NewLine + "Create Time :  " + createTime.ElapsedMilliseconds + "ms" + Environment.NewLine + Environment.NewLine+"UtcNow: " + Environment.NewLine + "As:  " + (asStop - asStart).Milliseconds + "ms" + Environment.NewLine + "Is:  " + (isStop - isStart).Milliseconds + "ms" + Environment.NewLine + "Base property:  " + (baseStop - baseStart).Milliseconds + "ms" + Environment.NewLine + "Base field:  " + (baseFieldStop - baseFieldStart).Milliseconds + "ms" + Environment.NewLine + "Base RO field:  " + (baseROFieldStop - baseROFieldStart).Milliseconds + "ms" + Environment.NewLine + "Virtual GetEnumType():  " + (virtStop - virtStart).Milliseconds + "ms" + Environment.NewLine + "Virtual bool IsB():  " + (virtBoolStop - virtBoolStart).Milliseconds + "ms" + Environment.NewLine + "Create Time :  " + (createStop-createStart).Milliseconds + "ms" + Environment.NewLine;
    }
}

abstract class MyBaseClass
{
    public enum ClassTypeEnum { A, B }
    public ClassTypeEnum ClassType { get; protected set; }
    public ClassTypeEnum ClassTypeField;
    public readonly ClassTypeEnum ClassTypeReadonlyField;
    public abstract ClassTypeEnum GetClassType();
    public abstract bool IsB();
    protected MyBaseClass(ClassTypeEnum kind)
    {
        ClassTypeReadonlyField = kind;
    }
}

class MyClassA : MyBaseClass
{
    public override bool IsB() { return false; }
    public override ClassTypeEnum GetClassType() { return ClassTypeEnum.A; }
    public MyClassA() : base(MyBaseClass.ClassTypeEnum.A)
    {
        ClassType = MyBaseClass.ClassTypeEnum.A;
        ClassTypeField = MyBaseClass.ClassTypeEnum.A;            
    }
}
class MyClassB : MyBaseClass
{
    public override bool IsB() { return true; }
    public override ClassTypeEnum GetClassType() { return ClassTypeEnum.B; }
    public MyClassB() : base(MyBaseClass.ClassTypeEnum.B)
    {
        ClassType = MyBaseClass.ClassTypeEnum.B;
        ClassTypeField = MyBaseClass.ClassTypeEnum.B;
    }
}