Introduction
When should you use CType and when should you use DirectCast and which one performs better? The short answer is: DirectCast is twice
as fast for value types (integers, etc.), but identical for reference types.
Background
The first thing to understand is that CType and DirectCast are not the same thing. Only CType can convert the underlying object
to a new instance of an object of a different type. For example, you want to turn an integer into a string. Since an integer doesn't inherit a string, a new instance
of a String object must be created in order to store the number as a string. CType can do this, DirectCast cannot. Note: There are other ways
to do this too such as the Convert.ToString method or CStr().
Dim MyInt As Integer = 123
Dim MyString1 As String = CType(MyInt, String)
Dim MyString2 As String = DirectCast(MyInt, String)
What DirectCast and CType do have in common is their ability to convert an object to a new type based on inheritance or implementation.
For example, if you have a string but it is stored in a variable of type Object, you can use DirectCast or CType to treat that variable
as an object of type String because type String inherits type Object. In this case, the underlying data in memory is not actually changing,
nor is any processing happening on that data.
Dim MyObject As Object = "Hello World"
Dim MyString1 As String = CType(MyObject, String)
Dim MyString2 As String = DirectCast(MyObject, String)
The danger: You must know what type you are dealing with before using DirectCast. If you have a variable of type Object and you
use DirectCast to treat it as a String, you'd better be sure that variable actually contains a String (or Nothing).
If an integer somehow found its way into that variable, an exception will be thrown.
A way to check in code if DirectCast will work is by using the TypeOf operator:
If TypeOf MyObject Is String Then
So, assuming you are doing a conversion based on inheritance or implementation, you have a choice: DirectCast vs. CType. Which is better?
The Answer
DirectCast. According to the .NET documentation: DirectCast does not use the Visual Basic run-time helper routines for conversion, so it can provide somewhat better
performance than CType.
The included project runs a performance test on each scenario: DirectCast vs. CType for value and reference types. Remember, reference types are types like Forms,
Controls, Strings, and custom classes, whereas Value types are types like integers, doubles, and custom structures. Here are the results; the numbers are in milliseconds
for 1 million iterations:
DirectCast on a reference type:
8.7885
CType on a reference type:
11.718
DirectCast on a value type:
18.5535
CType on a value type:
39.06
The verdict: DirectCast out performs CType for value types almost 2 to 1.
DirectCast also out performs CType for reference types albeit by a much slimmer margin.
Note: Subsequent tests produced very similar results; try for yourself.
Another reason to use DirectCast is that it's not VB.NET specific and is therefore more portable to other languages. So C# and other programmers might have an easier
time reading their code if they encountered DirectCast.
