Introduction

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, if 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) ' This will not work

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) ' This will work

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