Casting in VB.NET

Question

I would like to be able to cast a value dynamically where the type is known only at runtime. Something like this:

myvalue = CType(value, "String, Integer or Boolean")

The string that contains the type value is passed as an argument and is also read from a database, and the value is stored as string in the database.

Is this possible?

Answer 1

Sure, but myvalue will have to be defined as of type Object, and you don't necessarily want that. Perhaps this is a case better served by generics.

What determines what type will be used?

Answer 2

 Dim bMyValue As Boolean
 Dim iMyValue As Integer
 Dim sMyValue As String 
 Dim t As Type = myValue.GetType


 Select Case t.Name
     Case "String"
        sMyValue = ctype(myValue, string)
     Case "Boolean"
        bMyValue = ctype(myValue, boolean)
     Case "Integer"
        iMyValue = ctype(myValue, Integer)
 End Select

It's a bit hacky but it works.

Answer 3

This is the shortest way to do it. I've tested it with multiple types.

Sub DoCast(ByVal something As Object)

    Dim newSomething = Convert.ChangeType(something, something.GetType())

End Sub

Answer 4

Well, how do you determine which type is required? As Joel said, this is probably a case for generics. The thing is: since you don't know the type at compile time, you can't treat the value returned anyway so casting doesn't really make sense here.

Answer 5

Maybe instead of dynamically casting something (which doesn't seem to work) you could use reflection instead. It is easy enough to get and invoke specific methods or properties.

Dim t As Type = testObject.GetType()
Dim prop As PropertyInfo = t.GetProperty("propertyName")
Dim gmi As MethodInfo = prop.GetGetMethod()
gmi.Invoke(testObject, Nothing)

It isn't pretty but you could do some of that in one line instead of so many.