Dynamic Keyword equivalent in VB.Net?

Question

Ok i seen another answer similar question to mine but it didn't go further to answer my real question so here goes. The "Dynamic" keyword in c# 4.0 is a great feature. It looks like there is no equivalent in vb.net. In the other thread people suggesting just turning option strict off and you would, in essence, be doing the same thing. I see two problems with this:

1) Most people are going to want to turn option strict off the the entire project just want to do it in a couple places
2) What i'm trying to do is return / accept anonymous types.

My question really is around point 2 above. For example you can return results from LINQ and not worry that the return could potentially have a different structure. I tested this with dynamic & c# and it worked great. This is probably what i will use dynamic keyword for the most in C#. Is there any way to do this in vb.net that i'm not seeing?

thanks,
Choclabs

Answer 1

There is not an equivalent VB keyword at this time.  Those who are advising you to juggle the Option settings may not fully understand all that the dynamic keyword does.  It allows to resolve problems such as generic covariance casting  exceptions.  For example.....

Public Class Class1

    Sub TestMethod()
        Dim list1 As List(Of IInterface) = New List(Of IInterface)() ' this works
        list1.Add(New BaseClass()) ' this works, you can add a BaseClass objects
        Dim list2 As List(Of IInterface) = New List(Of BaseClass)() ' this does not
        list2.Add(New BaseClass()) ' this does not, you should add a BaseClass objects

    End Sub

End Class

Interface IInterface

End Interface

Class BaseClass : Implements IInterface

End Class

Class Consumer : Inherits BaseClass

End Class

Rudy  =8^D

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Answer 2

Here's a clearer example of generic covariance.


    Sub TestMethod_02()
        Dim obj1 As BaseClass = New BaseClass()
        Dim obj2 As IInterface = CType(obj1, IInterface)
        '
        '
        Dim list1 As List(Of BaseClass) = New List(Of BaseClass)()
        Dim List2 As List(Of IInterface) = CType(list1, List(Of IInterface))
    End Sub


Those 2 sections of code are very similar. 
Casting the "obj#" objects is easy and straightforward.
Casting the "list#" objects is prohibited by the compiler.
This second scenario where the dynamic keyword is used to defer type resolution until run time.

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Answer 3

For #2, the "var" keyword is what you'd normally use in C# for this - are you saying that you have a situation where the compiler cannot detect the LINQ result type?  Wherever "var" works in C#, inferred typing will work in VB (omitting the type with Option Infer On).

If you definitely need "dynamic" in C#, then Option Strict Off in VB used in conjunction with the "Object" type should work in the same way.

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Answer 4

To see examples of what David was talking about ... I have several on my blog. I have both C# code that uses var and VB code that uses Dim without a data type.

http://msmvps.com/blogs/deborahk/archive/2009/10/11/lambda-expressions-syntax.aspx

Hope this helps.

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Answer 5

Check out 

CTypeDynamic()

and the

System.Dynamic()

object namespace in .NET 4. It should give you everything you need.

Answer 6

You can define the variable as Object.

Answer 7

You can define the variable as Object.

And you also need to throw type safety out the window by setting Option Strict Off.

Option Strict Off

 

Module Module2

 

    Public Sub DynamicObjectDemo()

        Dim obj As Object = 3

        obj = obj + 3  'Requires Option Strict Off

    End Sub

 

End Module

 

 

If that is your wish, then proceed with what best meets your needs.  The dynamic keyword allows developers to maintain type safety at compile time with the rest of their code.  To my knowledge that degree of selectivity is not currently implement in Visual Basic.

Rudy   =8^D

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