Difference between DirectCast() and CType() in VB.NET

Question

I am an experienced C/C++/C# programmer who has just gotten into VB.NET. I generally use CType (and CInt, CBool, CStr) for casts because it is fewer characters and was the first way of casting which I was exposed to, but I am aware of DirectCast and TryCast as well.

Simply, are there any differences (effect of cast, performance, etc.) between DirectCast and CType? I understand the idea of TryCast.

Cacoepy

Exact duplicate of this Casting DataTypes with DirectCast, CType, TryCast #2704085 – Grave

Possible duplicate of Casting DataTypes with DirectCast, CType, TryCast – Swinson

Answer 1

The first thing to note is VB.NET does not have a direct analog to C#'s (type)instance casting mechanism. I bring this up because it's useful as a starting point and common reference in comparing the two VB.NET operators (and they are operators, not functions, even though they have function semantics).

DirectCast() is more strict than the C# casting operator. It only allows you to cast when the item being cast already is the type you are casting to. I believe it will still unbox value types, but otherwise it won't do any conversion. So, for example, you can't cast from short to int, like you could with a C# (int) cast. But you can cast from an IEnumerable to an array, if your underlying IEnumerable object variable really is an Array. And of course you can cast from Object to anything, assuming the type of your object instance really is somewhere below your cast type in the inheritance tree.

This is desirable because it's much faster. There's less conversion and type checking that needs to take place.

CType() is less strict than the C# casting operator. It will do things you just can't do with a simple (int)-style cast, like convert a string to an integer. It has as much power as calling Convert.To___() in C#, where the ___ is the target type of your cast.

This is desirable because it's very powerful. However, this power comes at the cost of performance; it's not as fast as DirectCast() or C#'s cast operator because it might need to do quite a lot of work to finish the cast. Generally you should prefer DirectCast() when you can.

Finally, you missed one casting operator: TryCast(), which is a direct analog to C#'s as operator.

Nonresistance

+1 I'd say the strictness of DirectCast is another advantage. If you make a mistake the compiler tells you immediately, but with CType a mistake just might cause occasional wrong behaviour at run-time - maybe on some user machine with different regional settings. – Grave

Great answer. So in order of complexity (small to large): DirectCast, TryCast, CType/Convert.ToXYZ(), C<xyz>() would be correct? – Dentifrice

@Dentifrice - close. The C<xyz>() "functions" should be moved higher on the list, as they really are operators rather than functions, even though they have function semantics. For those types that have them, they are very close to C#'s (type)casting, but will do just a little more work. – Nonresistance

@Grave +1 for your comment, but note DirectCast is only strict with classes, not interfaces (because you can have COM types -- and perhaps others -- that actually implement interfaces not defined by the .NET Type's .GetInterfaces list). – Alburnum

@JoelCoehoorn +1, But actually, TryCast() and as are not exactly the same. TryCast() only works with reference types, while as works with anything that can be null. So int? icast = myNum as int?; will work just fine, but Dim icast as Integer? = TryCast(myNum, Integer?) will give a compiler error. Just one more peculiar difference between the two languages. lol – Puissant

Also see Han Passant's answer to an earlier, similar, question, for an example of the risks of the non-strictness of CType. IMHO, such risks are more important than performance or a few extra characters to type - use DirectCast unless you really need CType. – Zelig

DirectCast is usually too pedantic because you can't even cast an Integer to a Double – Custombuilt

Answer 2

With CType you can write something like Ctype("string",Integer). But with DirectCast the above statement would give a compile time error.

 Dim a As Integer = DirectCast("1", Integer) 'Gives compiler error
 Dim b As Integer = CType("1", Integer) 'Will compile

Kalindi

Answer 3

DirectCast is more restrictive than CType.

For example, this will throw an error:

Sub Main()
    Dim newint As Integer = DirectCast(3345.34, Integer)
    Console.WriteLine(newint)
    Console.ReadLine()
End Sub

It will also be shown in the Visual Studio IDE.

This however, does not throw an error:

Sub Main()
    Dim newint As Integer = CType(3345.34, Integer)
    Console.WriteLine(newint)
    Console.ReadLine()
End Sub

Limen