VB.Net - Can you access the expected data type within a function?

Question

I was wondering if there is any way to access the expected data type within a function similar to an event arg. I am doubtful that this is possible, though it would be an excellent feature.

I frequently work with (old and disorganized)Mysql databases creating interfaces through VB.Net. Often I will have an optional field which contains a NULL value in the database. I am frequently dealing with errors due to NULL and dbnull values in passing data to and from the database.

To complicate things, I often am dealing with unexpected datatypes. I might have an integer zero, a double zero, an empty string, or a string zero.

So I spend a fair amount of code checking that each entry is of the expected type and or converting NULLs to zeros or empty strings depending on the case. I have written a function ncc(null catch convert) to speed up this process.

Public Function ncc(obj As Object, tp As Type) As Object 'Null Catch Convert Function...

My function works great, but I have to manually set the type every time I call the function. It would be so much easier if it were possible to access the expected type of the expression. Here is an example of what I mean.

Dim table as datatable
adapter.fill(table)

dim strinfo as string
dim intinfo as long

strinfo = ncc(table.Rows(0).Item(0),gettype(String)) 'here a string is expected
intinfo = ncc(table.Rows(0).Item(0),gettype(Long)) 'here a long is expected

It would be so much more efficient if it were possible to access the expected type directly from the function.

Something like this would be great:

Public Function ncc(obj As Object, optional tp As Type = nothing) As Object

If tp Is Nothing Then tp = gettype(ncc.expectedtype)

That way I do not have to hard code the type on each line.

strinfo = ncc(table.Rows(0).Item(0))

Answer 1

You can make the ncc function generic to simplify calling it:

Public Function ncc(Of T)(obj As T) As T
    If DbNull.Value.Equals(obj) Then Return Nothing
    Return Obj
End Function

This kind of function will be able to in some cases infer the type, but if there's any possibility of null you'll still want to include a type name (because DBNull will be the inferred type for those values). The advantage is not needing to call gettype() and so gaining a small degree of type safety:

strinfo = ncc(Of String)(table.Rows(0).Item(0))

But I think this has a small chance to blow up at run time if your argument is not implicitly convertible to the desired type. What you should be doing is adding functions to accept a full row and return a composed type. These functions can exist as static/shared members of the target type:

 Shared Function FromDataRow(IDataRow row) As MyObject

And you call it for each row like this:

 Dim record As MyObject = MyObject.FromDataRow(table.Rows(i))

Answer 2

But, you problem still exists.

What happens if the column in the database row is null?

then you DO NOT get a data type!

Worse yet? Assume the data column is null, do you want to return null into that variable anyway?

Why not specify a value FOR WHEN its null.

You can use "gettype" on the passed value, but if the data base column is null, then you can't determine the type, and you right back to having to type out the type you want as the 2nd parameter.

You could however, adopt a nz() function (like in VBA/Access).

So, this might be better:

Public Function ncc(obj As Object, Optional nullv As Object = Nothing) As Object

    If obj Is Nothing OrElse IsDBNull(obj) Then
        Return nullv
    End If

    Return obj

End Function

So, I don't care if the database column is null, or a number, for such numbers, I want 0.

So

 dim MyInt    as integer
 Dim MyDouble As Double

 MyInt = ncc(rstData.Rows(0).Item("ContactID"), 0)
 MyDouble = ncc(rstData.Rows(0).Item("ContactID"), 0)

 dim strAddress as string = ""
 strAddress = ncc(rstData.Rows(0).Item("Address"), "") 

Since in NEAR ALL cases, you need to deal with the null from the DB, then above not only works for all data types, but also gets you on the fly conversion.

I mean, you CAN declare variables such as integer to allow null values.

eg:

  dim myIntValue as integer?

But, I not sure above would create more problems than it solves.

So,

Answer 3

You can't get exactly what you want, because a function never has knowledge of how it's going to be used. It's not guaranteed that it will be on the right-hand side of an assignment statement.

If you want to have knowledge of both sides, you either need to be assigning to a custom type (so that you can overload the assignment operator) or you need to use a Sub instead of an assignment.

You could do something like this (untested):

Public Sub Assign(Of T)(ByVal field As Object, ByRef destination As T,
    Optional ByVal nullDefault As T = Nothing)

    If TypeOf field Is DBNull Then
        destination = nullDefault
    Else
        destination = CType(field, T)
    End If
End Sub

I haven't tested this, so I'm not completely certain that the compiler would allow the conversion, but I think it would because field is type Object. Note that this would yield a runtime error if field is not convertible to T.

You could even consider putting on a constraint requiring T to be a value type, though I don't think that would be likely to work because you probably need to handling String which is a reference type (even though it basically acts like a value type).

Because the destination is an argument, you wouldn't ever need to specify the generic type argument, it would be inferred.