CODEWITHSUNDEEP
c#.netvb.netcastingtype-conversion
ElektroStudios
ElektroStudios

Reputation: 20464

How to implement the DirectCast operator in a type?

When implementing the CType operator in a custom Type like this one below, the Type is not casteable using DirectCast operator:

Public Structure ColorInfo
    ...
    Public Shared Widening Operator CType(ByVal colorInfo As ColorInfo) As Color

        Return Color.FromArgb(colorInfo.R, colorInfo.G, colorInfo.B)

    End Operator
    ...
End Structure

On the other hand it is directly assignable to a Color object, which is very confussing:

Dim obj As color = MyColorInfo

Then, I would like to implement the DirectCast operator firstly to obtain typing comfort in my environment (instead of using the CType) and secondly to obtain the beneffits, if any, of what explains the MSDN docs here:

DirectCast does not use the Visual Basic run-time helper routines for conversion, so it can provide somewhat better performance than CType when converting to and from data type Object.

How I can implement it in C# or Vb.Net?.

Upvotes: 1

Views: 1145

Answers (1)

MicroVirus
MicroVirus

Reputation: 5477

DirectCast is a 'compile-time' cast with added type-checking at runtime. It's meant for casting when type inheritance or interface implementation are in play. It will not consider user defined casts, such as you have here, so it's not applicable.

CType is appropriate and you should just use that, rather than trying to subvert the language. It will look for user defined conversions and apply them.

Performance-wise, the CType is as good as it gets here. Because the types are known at compile time in your example, you'll get the optimal code for it.

To address the comment: The principal thing is that you, as a programmer, do nothing for DirectCast, VB.NET does everything. In fact, you can't do anything to get a DirectCast other than starting your class with implementing a particular interface or inheriting from a base class.

If you want to provide any other conversions, then you use CType. This is a fundamental split between the two. DirectCast comes automatically and you can't modify its behaviour. CType will allow you to extend its behaviour by providing custom conversions.

So to summarise: either your class inherits from a type, in which case DirectCast of an instance to and from that base type works, or it doesn't, in which case you need to create your own CType override.

Upvotes: 2

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