When programming interfaces, I've found I'm doing a lot of casting or object type conversion.

Is there a difference between these two methods of conversion? If so, is there a cost difference or how does this affect my program?

public interface IMyInterface
{
    void AMethod();
}

public class MyClass : IMyInterface
{
    public void AMethod()
    {
       //Do work
    }

    // Other helper methods....
}

public class Implementation
{
    IMyInterface _MyObj;
    MyClass _myCls1;
    MyClass _myCls2;

    public Implementation()
    {
        _MyObj = new MyClass();

        // What is the difference here:
        _myCls1 = (MyClass)_MyObj;
        _myCls2 = (_MyObj as MyClass);
    }
}

Also, what is "in general" the preferred method?

Solution 1

The answer below the line was written in 2008.

C# 7 introduced pattern matching, which has largely replaced the as operator, as you can now write:

if (randomObject is TargetType tt)
{
    // Use tt here
}

Note that tt is still in scope after this, but not definitely assigned. (It is definitely assigned within the if body.) That's slightly annoying in some cases, so if you really care about introducing the smallest number of variables possible in every scope, you might still want to use is followed by a cast.


I don't think any of the answers so far (at the time of starting this answer!) have really explained where it's worth using which.

  • Don't do this:

    // Bad code - checks type twice for no reason
    if (randomObject is TargetType)
    {
        TargetType foo = (TargetType) randomObject;
        // Do something with foo
    }
    

    Not only is this checking twice, but it may be checking different things, if randomObject is a field rather than a local variable. It's possible for the "if" to pass but then the cast to fail, if another thread changes the value of randomObject between the two.

  • If randomObject really should be an instance of TargetType, i.e. if it's not, that means there's a bug, then casting is the right solution. That throws an exception immediately, which means that no more work is done under incorrect assumptions, and the exception correctly shows the type of bug.

    // This will throw an exception if randomObject is non-null and
    // refers to an object of an incompatible type. The cast is
    // the best code if that's the behaviour you want.
    TargetType convertedRandomObject = (TargetType) randomObject;
    
  • If randomObject might be an instance of TargetType and TargetType is a reference type, then use code like this:

    TargetType convertedRandomObject = randomObject as TargetType;
    if (convertedRandomObject != null)
    {
        // Do stuff with convertedRandomObject
    }
    
  • If randomObject might be an instance of TargetType and TargetType is a value type, then we can't use as with TargetType itself, but we can use a nullable type:

    TargetType? convertedRandomObject = randomObject as TargetType?;
    if (convertedRandomObject != null)
    {
        // Do stuff with convertedRandomObject.Value
    }
    

    (Note: currently this is actually slower than is + cast. I think it's more elegant and consistent, but there we go.)

  • If you really don't need the converted value, but you just need to know whether it is an instance of TargetType, then the is operator is your friend. In this case it doesn't matter whether TargetType is a reference type or a value type.

  • There may be other cases involving generics where is is useful (because you may not know whether T is a reference type or not, so you can't use as) but they're relatively obscure.

  • I've almost certainly used is for the value type case before now, not having thought of using a nullable type and as together :)


EDIT: Note that none of the above talks about performance, other than the value type case, where I've noted that unboxing to a nullable value type is actually slower - but consistent.

As per naasking's answer, is-and-cast or is-and-as are both as fast as as-and-null-check with modern JITs, as shown by the code below:

using System;
using System.Diagnostics;
using System.Linq;

class Test
{
    const int Size = 30000000;

    static void Main()
    {
        object[] values = new object[Size];
        for (int i = 0; i < Size - 2; i += 3)
        {
            values[i] = null;
            values[i + 1] = "x";
            values[i + 2] = new object();
        }
        FindLengthWithIsAndCast(values);
        FindLengthWithIsAndAs(values);
        FindLengthWithAsAndNullCheck(values);
    }

    static void FindLengthWithIsAndCast(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = (string) o;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and Cast: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithIsAndAs(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = o as string;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and As: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithAsAndNullCheck(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            string a = o as string;
            if (a != null)
            {
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("As and null check: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }
}

On my laptop, these all execute in about 60ms. Two things to note:

  • There's no significant difference between them. (In fact, there are situations in which the as-plus-null-check definitely is slower. The above code actually makes the type check easy because it's for a sealed class; if you're checking for an interface, the balance tips slightly in favour of as-plus-null-check.)
  • They're all insanely fast. This simply will not be the bottleneck in your code unless you really aren't going to do anything with the values afterwards.

So let's not worry about the performance. Let's worry about correctness and consistency.

I maintain that is-and-cast (or is-and-as) are both unsafe when dealing with variables, as the type of the value it refers to may change due to another thread between the test and the cast. That would be a pretty rare situation - but I'd rather have a convention which I can use consistently.

I also maintain that the as-then-null-check gives a better separation of concerns. We have one statement which attempts a conversion, and then one statement which uses the result. The is-and-cast or is-and-as performs a test and then another attempt to convert the value.

To put it another way, would anyone ever write:

int value;
if (int.TryParse(text, out value))
{
    value = int.Parse(text);
    // Use value
}

That's sort of what is-and-cast is doing - although obviously in a rather cheaper way.

Solution 2

"as" will return NULL if not possible to cast.

casting before will raise an exception.

For the performance, raising an exception is usually more costly in time.

Solution 3

Here's another answer, with some IL comparison. Consider the class:

public class MyClass
{
    public static void Main()
    {
        // Call the 2 methods
    }

    public void DirectCast(Object obj)
    {
        if ( obj is MyClass)
        { 
            MyClass myclass = (MyClass) obj; 
            Console.WriteLine(obj);
        } 
    } 


    public void UsesAs(object obj) 
    { 
        MyClass myclass = obj as MyClass; 
        if (myclass != null) 
        { 
            Console.WriteLine(obj);
        } 
    }
}

Now look at the IL each method produces. Even if the op codes mean nothing to you, you can see one major difference - isinst is being called followed by castclass in the DirectCast method. So two calls instead of one basically.

.method public hidebysig instance void  DirectCast(object obj) cil managed
{
  // Code size       22 (0x16)
  .maxstack  8
  IL_0000:  ldarg.1
  IL_0001:  isinst     MyClass
  IL_0006:  brfalse.s  IL_0015
  IL_0008:  ldarg.1
  IL_0009:  castclass  MyClass
  IL_000e:  pop
  IL_000f:  ldarg.1
  IL_0010:  call       void [mscorlib]System.Console::WriteLine(object)
  IL_0015:  ret
} // end of method MyClass::DirectCast

.method public hidebysig instance void  UsesAs(object obj) cil managed
{
  // Code size       17 (0x11)
  .maxstack  1
  .locals init (class MyClass V_0)
  IL_0000:  ldarg.1
  IL_0001:  isinst     MyClass
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  brfalse.s  IL_0010
  IL_000a:  ldarg.1
  IL_000b:  call       void [mscorlib]System.Console::WriteLine(object)
  IL_0010:  ret
} // end of method MyClass::UsesAs

The isinst keyword versus the castclass

This blog post has a decent comparison between the two ways of doing it. His summary is:

  • In a direct comparison, isinst is quicker than castclass (although only slightly)
  • When having to perform checks to ensure the conversion was successful, isinst was significantly quicker than castclass
  • A combination of isinst and castclass should not be used as this was far slower than the quickest "safe" conversion (over 12% slower)

I personally always use As, because it's easy to read and is recommended by the .NET development team (or Jeffrey Richter anyway)

Solution 4

One of the more subtle differences between the two is that the "as" keyword can not be used for casting when a cast operator is involved:

public class Foo
{
    public string Value;

    public static explicit operator string(Foo f)
    {
        return f.Value;
    }

}

public class Example
{
    public void Convert()
    {
        var f = new Foo();
        f.Value = "abc";

        string cast = (string)f;
        string tryCast = f as string;
    }
}

This will not compile (although I think it did in previous versions) on the last line since the "as" keywords do not take cast operators into account. The line string cast = (string)f; works just fine though.

Solution 5

as never throws an exception if it cannot perform the conversion returning null instead (as operates on reference types only). So using as is basically equivalent to

_myCls2 = _myObj is MyClass ? (MyClass)_myObj : null;

C-style casts, on the other hand, throw an exception when no conversion is possible.

Solution 6

Not really an answer to your question, but what I think is an important related point.

If you are programming to an interface you shouldn't be needing to cast. Hopefully these casts are very rare. If not you likely need to rethink some of your interfaces.

Solution 7

Please ignore Jon Skeet's advice, re:avoid test-and-cast pattern, ie.:

if (randomObject is TargetType)
{
    TargetType foo = randomObject as TargetType;
    // Do something with foo
}

The idea that this costs more than a cast and a null test is a MYTH:

TargetType convertedRandomObject = randomObject as TargetType;
if (convertedRandomObject != null)
{
    // Do stuff with convertedRandomObject
}

It's a micro-optimization that does not work. I ran some real tests, and test-and-cast is actually faster than cast-and-null-comparison, and it's safer too because you don't have the possibility of having a null reference in the scope outside the if should the cast fail.

If you want a reason why test-and-cast is faster, or at least not slower, there's a simple and complex reason.

Simple: even naive compilers will coalesce two similar operations, like test-and-cast, into a single test and branch. cast-and-null-test may force two tests and a branch, one for the type test and conversion to null on failure, one for the null check itself. At the very least, they will both optimize to a single test and branch, so test-and-cast would be neither slower nor faster than cast-and-null-test.

Complex: why test-and cast is faster: cast-and-null-test introduces another variable into the outer scope which the compiler must track for liveness, and it may not be able to optimize away that variable depending on how complex your control-flow is. Conversely, test-and-cast introduces a new variable only in a delimited scope so the compiler knows that the variable is dead after the scope exits, and so can optimize register allocation better.

So please, PLEASE let this "cast-and-null-test is better than test-and-cast" advice DIE. PLEASE. test-and-cast is both safer and faster.

Solution 8

If the cast fails, the 'as' keyword doesn't throw an exception; it sets the variable to null (or to its default value for value types) instead.

Solution 9

This is not an answer to the question but comment to the question's code example:

Usually you should not have to cast an Object from e.g. IMyInterface to MyClass. The great thing about interfaces is that if you take an object as input that implements an interface, than you don't have to care what kind of object you are getting.

If you cast IMyInterface to MyClass, than you already assume that you get an object of type MyClass and it makes no sense to use IMyInterface, because if you feed your code with other classes that implement IMyInterface, it would break your code...

Now, my advice: if your interfaces are well designed you can avoid a lot of typecasting.

Solution 10

The as operator can only be used on reference types, it cannot be overloaded, and it will return null if the operation fails. It will never throw an exception.

Casting can be used on any compatible types, it can be overloaded, and it will throw an exception if the operation fails.

The choice of which to use depends on the circumstances. Primarily, it's a matter of whether you want to throw an exception on a failed conversion.

Solution 11

My answer is only about speed in cases when we don't check the type and we don't check nulls after casting. I added two additional tests to Jon Skeet's code:

using System;
using System.Diagnostics;

class Test
{
    const int Size = 30000000;

    static void Main()
    {
        object[] values = new object[Size];

        for (int i = 0; i < Size; i++)
        {
            values[i] = "x";
        }
        FindLengthWithIsAndCast(values);
        FindLengthWithIsAndAs(values);
        FindLengthWithAsAndNullCheck(values);

        FindLengthWithCast(values);
        FindLengthWithAs(values);

        Console.ReadLine();
    }

    static void FindLengthWithIsAndCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = (string)o;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and Cast: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithIsAndAs(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = o as string;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and As: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithAsAndNullCheck(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            string a = o as string;
            if (a != null)
            {
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("As and null check: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }
    static void FindLengthWithCast(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            string a = (string)o;
            len += a.Length;
        }
        sw.Stop();
        Console.WriteLine("Cast: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithAs(object[] values)
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            string a = o as string;
            len += a.Length;
        }
        sw.Stop();
        Console.WriteLine("As: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }
}

Result:

Is and Cast: 30000000 : 88
Is and As: 30000000 : 93
As and null check: 30000000 : 56
Cast: 30000000 : 66
As: 30000000 : 46

Don't try to focus on speed (as I did) because all this is very very fast.

Solution 12

If you use the Office PIAs targeting the .NET Framework 4.X you should use the as keyword, otherwise it won't compile.

Microsoft.Office.Interop.Outlook.Application o = new Microsoft.Office.Interop.Outlook.Application();
Microsoft.Office.Interop.Outlook.MailItem m = o.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem) as Microsoft.Office.Interop.Outlook.MailItem;

Casting is OK when targeting .NET 2.0 though:

Microsoft.Office.Interop.Outlook.MailItem m = (Microsoft.Office.Interop.Outlook.MailItem)o.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);

When targeting .NET 4.X the errors are:

error CS0656: Missing compiler required member 'Microsoft.CSharp.RuntimeBinder.Binder.Convert'

error CS0656: Missing compiler required member 'Microsoft.CSharp.RuntimeBinder.CSharpArgumentInfo.Create'

Solution 13

What you choose strongly depends on what required. I prefer explicit casting

IMyInterface = (IMyInterface)someobj;

because if object should by of IMyInterface type and it is not - it is definitely problem. It is better to get error as early as possible because exact error will be fixed instead of fixing its side effect.

But if you deal with methods that accepts object as parameter then you need to check its exact type prior executing any code. In such case as would be useful so you can avoid InvalidCastException.

Solution 14

Besides all what was already exposed here, I just came across a practical difference I think is worth noting, between explicit casting

var x = (T) ...

versus using the as operator.

Here is the example:

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(GenericCaster<string>(12345));
        Console.WriteLine(GenericCaster<object>(new { a = 100, b = "string" }) ?? "null");
        Console.WriteLine(GenericCaster<double>(20.4));

        //prints:
        //12345
        //null
        //20.4

        Console.WriteLine(GenericCaster2<string>(12345));
        Console.WriteLine(GenericCaster2<object>(new { a = 100, b = "string" }) ?? "null");

        //will not compile -> 20.4 does not comply due to the type constraint "T : class"
        //Console.WriteLine(GenericCaster2<double>(20.4));
    }

    static T GenericCaster<T>(object value, T defaultValue = default(T))
    {
        T castedValue;
        try
        {
            castedValue = (T) Convert.ChangeType(value, typeof(T));
        }
        catch (Exception)
        {
            castedValue = defaultValue;
        }

        return castedValue;
    }

    static T GenericCaster2<T>(object value, T defaultValue = default(T)) where T : class
    {
        T castedValue;
        try
        {
            castedValue = Convert.ChangeType(value, typeof(T)) as T;
        }
        catch (Exception)
        {
            castedValue = defaultValue;
        }

        return castedValue;
    }
}

Bottom line: GenericCaster2 will not work with struct types. GenericCaster will.

Solution 15

The as keyword works the same as an explicit cast between compatible reference types with the major difference that it does not raise an exception if conversion fails. Rather, it yields a null value in the target variable. Since Exceptions are very expensive in terms of performance, it is considered a much better method of casting.

Solution 16

It depends, do you want to check for null after using "as" or would you prefer your app to throw an exception?

My rule of thumb is if I always expect the variable to be of the type I am expecting at the time I want I use a cast. If it is possible that the variable will not cast to what I want and I am prepared to handle nulls from using as, I will use as.

Solution 18

The OP's problem is limited to a specific casting situation. The title covers much more situations.
Here's an overview of all relevant casting situations that I currently can think of:

private class CBase
{
}

private class CInherited : CBase
{
}

private enum EnumTest
{
  zero,
  one,
  two
}

private static void Main (string[] args)
{
  //########## classes ##########
  // object creation, implicit cast to object
  object oBase = new CBase ();
  object oInherited = new CInherited ();

  CBase oBase2 = null;
  CInherited oInherited2 = null;
  bool bCanCast = false;

  // explicit cast using "()"
  oBase2 = (CBase)oBase;    // works
  oBase2 = (CBase)oInherited;    // works
  //oInherited2 = (CInherited)oBase;   System.InvalidCastException
  oInherited2 = (CInherited)oInherited;    // works

  // explicit cast using "as"
  oBase2 = oBase as CBase;
  oBase2 = oInherited as CBase;
  oInherited2 = oBase as CInherited;  // returns null, equals C++/CLI "dynamic_cast"
  oInherited2 = oInherited as CInherited;

  // testing with Type.IsAssignableFrom(), results (of course) equal the results of the cast operations
  bCanCast = typeof (CBase).IsAssignableFrom (oBase.GetType ());    // true
  bCanCast = typeof (CBase).IsAssignableFrom (oInherited.GetType ());    // true
  bCanCast = typeof (CInherited).IsAssignableFrom (oBase.GetType ());    // false
  bCanCast = typeof (CInherited).IsAssignableFrom (oInherited.GetType ());    // true

  //########## value types ##########
  int iValue = 2;
  double dValue = 1.1;
  EnumTest enValue = EnumTest.two;

  // implicit cast, explicit cast using "()"
  int iValue2 = iValue;   // no cast
  double dValue2 = iValue;  // implicit conversion
  EnumTest enValue2 = (EnumTest)iValue;  // conversion by explicit cast. underlying type of EnumTest is int, but explicit cast needed (error CS0266: Cannot implicitly convert type 'int' to 'test01.Program.EnumTest')

  iValue2 = (int)dValue;   // conversion by explicit cast. implicit cast not possible (error CS0266: Cannot implicitly convert type 'double' to 'int')
  dValue2 = dValue;
  enValue2 = (EnumTest)dValue;  // underlying type is int, so "1.1" beomces "1" and then "one"

  iValue2 = (int)enValue;
  dValue2 = (double)enValue;
  enValue2 = enValue;   // no cast

  // explicit cast using "as"
  // iValue2 = iValue as int;   error CS0077: The as operator must be used with a reference type or nullable type
}