Interview Question

Question

I was recently asked a weird question in an interview here it is;

Private Function sorttable  (ByVal dt As DataTable, ByVal sorttype$, ByVal sort_direction$) As DataTable 
         Dim dv As DataView  
         Dim dt2 As DataTable  
         dt2 = dt.Clone  
         dt2.Merge(dt)  
         dv = dt2.DefaultView  
         dv.Sort = sorttype & " " & sort_direction  
         Return dv.ToTable()  
End Function

well this function works sorts correctly but there is a problem about it(well at least the interviewer claimed). Well I couldn't figure out what it was.Any ideas?

Answer 1

I've never used the DataView orDataTable, but the code doesn't dispose the objects.

Answer 2

Edit: Merge only copies scheme, not rows. That's why you need it.

Only two problems I see is the Merge and redundant variables (dt2). Since they are passed by value, there is no reason to make extra local copies and create more room for bugs. This is known as a code smell. Also, why merge when you just created a clone of the original data table that already contains the constraints and schema ? It's like doubling up.

Answer 3

You're creating DataTables for no reason.

You can use this code to provide the same result

Private Function sorttable  (ByVal dt As DataTable, ByVal sorttype$, ByVal sort_direction$) As DataTable  
         Dim dv As DataView   
         dv = dt.DefaultView   
         dv.Sort = sorttype & " " & sort_direction   
         Return dv.ToTable()   
End Function 

Answer 4

I was creaing such nice answer but my FF has crashed...

so in few words..

But what is going there

• DataTable - Represents one table of in-memory data.

  • Clone - Clones the structure of the DataTable, including all DataTable schemas and constraints.

  • Merge(DataTable dataTable) - This member is overloaded. For complete information about this member, including syntax, usage, and examples, click a name in the overload list.

  • DefaultView - Gets a customized view of the table that may include a filtered view, or a cursor position.

• DataView - Represents a databindable, customized view of a DataTable for sorting, filtering, searching, editing, and navigation.

  • Sort - Gets or sets the sort column or columns, and sort order for the DataView. Sort string structure - "A string that contains the column name followed by "ASC" (ascending) or "DESC" (descending). Columns are sorted ascending by default. Multiple columns can be separated by commas." The method Clone work almost the same as Copy, the only differance is that the new DataTable created by the Clone method does not contain any DataRows.

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The redundation that You have in middtime accepted as good answer, in some cases can be resonable for example we want only sorted copy of those data for other purpose. But there is no need to Clone and then merge, member Copy will do the same.

I'm not a expert but the property Sort of DataView sorts the DataView as we return the DataTable so we do not sorted anything.