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Runtime Type Casting .. need help

Posted on 2004-09-28
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Last Modified: 2010-04-23
I have some code that looks like this:

               Dim mytype As Type = Type.GetType(table.Columns(map.DataColumnName).DataType.ToString())
               datarow.Item(map.DataColumnName) = CType(row(IntegerIndex(map.TextColumnIndex)), mytype)

But that doesn't work. It says "mytype" is not defined. I think the problem has to do with the fact that the CType method only works with design time values.

I need to cast at runtime this object: row(IntegerIndex(map.TextColumnIndex))
to a type that I will only know at runtime. The type that I want to cast it to is the underlying datatype of the strongly typed datatable.

Can someone help me? This code without the CType is blowing up when

row(IntegerIndex(map.TextColumnIndex)) = "0"

and the datatable column type = "System.Boolean"

It seems this should be possible, and quite frankly, I am amazed that I am stuck on this.
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Question by:gregasm
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Expert Comment

by:wguerram
ID: 12174991
CType(row(IntegerIndex(map.TextColumnIndex)), mytype)

This is not the way to use CTYPE(), the second parameter is the name of the type, this garantee the compiler the type of object you are trying to convert can be converted.

If you are having problem with some types you have to use a "select case"

Although is weird you are getting an error with this:

row(IntegerIndex(map.TextColumnIndex)) = "0"

and the datatable column type = "System.Boolean"

"0" is false and non zero is true, now if for some reason you assign "s", you will get an error.

But the compiler try to make the conversion to the new type if posible. otherwise you will get an error.

I don't recommend you to leave the compiler make the conversions, since you can get unexpected results in some types.
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Assisted Solution

by:DotNetLover_Baan
DotNetLover_Baan earned 100 total points
ID: 12175298
Did you you try this ?
CType(row(IntegerIndex(map.TextColumnIndex)), GetType(mytype))

-Baan
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Author Comment

by:gregasm
ID: 12175318
Hi wguerram,

The select case approach is what I ahve been working with.

can you throw out a skeleton of the select case for me to have a look at?
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LVL 8

Author Comment

by:gregasm
ID: 12175810
Yes, I think I tried that. The compiler doesn't like that expression though because it doesn't know the value at design time. Therefore, it kind of defeats one of the purposes of strong typing.

CType(row(IntegerIndex(map.TextColumnIndex)), GetType(mytype))

What is mytype? It's not known until runtime...

i have a feeling tomorrow i'm gonan write something like this:

select case table.Columns(map.DataColumnName).DataType.ToString()

case "System.Boolean"

.....

case "System.String"

case "System.Double"

case "This.Approach.Sucks"

case else

end select
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LVL 10

Assisted Solution

by:123654789987
123654789987 earned 200 total points
ID: 12178047
U need not use Ctype for this. It is working well without it

Dim mytype As Type = Type.GetType(table.Columns(map.DataColumnName).DataType.ToString())
               datarow.Item(map.DataColumnName) =mytype


For the System.Boolean problem

Loop through the column and convert the 0, 1 values

u can use CBool to convert it to boolean
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LVL 8

Accepted Solution

by:
wguerram earned 200 total points
ID: 12179551
Here is a list of conversion functions

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vblr7/html/vagrptypeconversion.asp

----------------------------
You already have it, now just use the proper function to make the conversion.

select case table.Columns(map.DataColumnName).DataType.ToString()

case "System.Boolean"

datarow.Item(map.DataColumnName)  = CBoll(row(IntegerIndex(map.TextColumnIndex)))

case "System.String"

case "System.Double"

case "This.Approach.Sucks"
         'There is no an equivalent function for this...

case else
    datarow.Item(map.DataColumnName)  = row(IntegerIndex(map.TextColumnIndex))
end select
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Author Comment

by:gregasm
ID: 12181548
Oops. I meant to accept the answer from wguerram. But the points are distributed as I wanted them at least. Thanks guys.
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Expert Comment

by:wguerram
ID: 12181776
You can change it

http://www.experts-exchange.com/help.jsp#hi17

Post a question with zero points and add the link to the question.
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LVL 8

Author Comment

by:gregasm
ID: 12182159
Alrightie.
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