Question

In a DataTable object, is there added overhead to looking up a column value by name thisRow("ColumnA") rather than by the column index thisRow(0)? In which scenarios might this be an issue.

I work on a team that has lots of experience writing VB6 code and I noticed that didn’t do column lookups by name for DataTable objects or data grids. Even in .NET code, we use a set of integer constants to reference column names in these types of objects. I asked our team lead why this was so, and he mentioned that in VB6, there was a lot of overhead in looking up data by column name rather than by index. Is this still true for .NET?

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Example code (in VB.NET, but same applies to C#):

Public Sub TestADOData()
Dim dt As New DataTable

'Set up the columns in the DataTable    '
dt.Columns.Add(New DataColumn("ID", GetType(Integer)))
dt.Columns.Add(New DataColumn("Name", GetType(String)))
dt.Columns.Add(New DataColumn("Description", GetType(String)))

'Add some data to the data table    '
dt.Rows.Add(1, "Fred", "Pitcher")
dt.Rows.Add(3, "Hank", "Center Field")

'Method 1: By Column Name   '
For Each r As DataRow In dt.Rows
  Console.WriteLine( _
   "{0,-2} {1,-10} {2,-30}", r("ID"), r("Name"), r("Description"))
Next

Console.WriteLine()

'Method 2: By Column Name   '
For Each r As DataRow In dt.Rows
  Console.WriteLine("{0,-2} {1,-10} {2,-30}", r(0), r(1), r(2))
Next

End Sub

Is there an case where method 2 provides a performance advantage over method 1?

Answer 1

Yes, there should be a slight overhead connected to looking up columns by name instead of by index. I wouldn’t worry about it, unless you keep looking up that same column in a loop, like in your code example. Because then the slight overhead might accumulate to a measurable overhead, depending on the table’s number of rows.

The fastest way to access a particular column’s value of some row is to lookup using the DataColumn object itself. For example:

Dim dt As DataTable = ...

Dim idColumn As DataColumn = dt.Columns("ID")
Dim nameColumn As DataColumn = dt.Columns("Name")
Dim descriptionColumn As DataColumn = dt.Columns("Description")

For Each r As DataRow In dt.Rows

    ' NB: lookup through a DataColumn object, not through a name, nor an index: '
    Dim id = r(idColumn)
    Dim name = r(nameColumn)
    Dim description = r(descriptionColumn)

    ...
Next

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One last piece of advice: I would strongly advise you against using numerical indices! It makes your code more fragile, and also more difficult to understand and maintain: As soon as the logical order of a column changes, you need to adapt your code accordingly, possibly in several places (and you might easily oversee one of them, leading to bugs). If you instead use column names or DataColumn objects themselves for the lookup, you can change your columns’ order without having to change the remaining code.