Casting DataTypes with DirectCast, CType, TryCast

Question

Ever since I moved from VB6 to VB.NET somewhere in 2005, I've been using CType to do casting from one data type to another. I do this because it is simply faster to type, used to exist in VB6 and I do not know why I have to be using DirectCast if there is apparently no difference between them.

I use TryCast once in a while because I understand that sometimes casting can fail. I however cannot get the difference between CType and DirectCast.

Can anyone tell me the difference in plain simple English what the difference the two (CType and DirectCast)? Adding examples of where to use what as well would be helpful.

Delayedaction

possible duplicate of Difference between DirectCast() and CType() in VB.Net – Ligon

@RMalke: My question was asked before the linked 'duplicate' question. This is corroborated by MarkJ's comment. Additionally, the accepted answer here explains CType casting in detail, something the accepted answer on the other question doesn't really go into. If anything, I believe it is the other one that needs to be closed or otherwise linked to this. – Delayedaction

Answer 1

TryCast and DirectCast are casting operators that directly map to the CLR's support for casting. They can quickly cast an object of a base type to a derived type or unbox a value of a value type. DirectCast throws an exception when the cast isn't possible, TryCast returns Nothing if it failed. You typically want to favor DirectCast to catch programming mistakes.

CType allows a superset of conversions, ones that the CLR frowns on. The best example I can think of is converting a string to a number or date. For example:

Dim obj As Object
obj = "4/1/2010"
Dim dt As DateTime = CType(obj, DateTime)

Which you'll have to use if Option Strict On is in effect. If it is Off then you can do it directly:

Option Strict Off
...
    Dim dt As DateTime = obj

Very convenient of course and part of VB.NET's legacy as a dynamically typed language. But not without problems, that date is Unicorn day at stackoverflow.com but will be a day in January when a Briton enters the string. Unexpected conversions is the reason the CLR doesn't permit these directly. The explicit, never a surprise conversion looks like this:

Dim dt As DateTime = DateTime.Parse(obj.ToString(), _
    System.Globalization.CultureInfo.GetCultureInfo("en-US").DateTimeFormat)

Whether you should buy into Try/DirectCast vs CType vs explicit conversions is rather a personal choice. If you now program with Option Strict On then you should definitely start using Try/DirectCast. If you favor the VB.NET language because you like the convenience of dynamic typing then don't hesitate to stay on CType.

Running

Answer 2

DirectCast is twice as fast for value types (integers...etc), but identical for reference types.

For more information see the "Conversion Functions, CType, DirectCast, and System.Convert" section on this MSDN page.

Bronze

Answer 3

This page explains it well.

Reading it, I think that when you use DirectCast, you are sure that conversion will work without narrowing or expansion (in this case, numeric data). Whereas, CType will try to convert to it,with developer being aware of narrowing/expansion.

Singh

It is that runtime type and specified type thing that I do not understand. If I declare a Double, I expect that explicitly casting it to an integer should work. How then is it that CType is able to do it perfectly and DirectCast can't. Am I missing something here? Plain English please... – Delayedaction

msdn.microsoft.com/en-us/library/7k6y2h6x%28VS.71%29.aspx not found – Landseer

Answer 4

By "conversion" mean converting one datatype to another (e.g. string to integer, decimal to integer, object to string etc).

By "cast" mean changing one type of object into another type that is related to it by one of the following rules.

http://www.devdescent.com/2012/09/directcast-vs-ctype.html

Counterglow