How to Select Rows in a Datatable? - vb.net

Question

How can I use the Variable Itmnmbr instead of hard-coding its value, 'i-2051'?
Dim fr() As DataRow
Dim Itmnmbr As string = "i-2051"
fr = dt.Select("item = 'i-2051'")

Answer 1

The most direct way is to use an Interpolated String, which are available from Visual Studio 2015, VB.Net 14:
Dim Itmnmbr As string = "i-2051"
fr = dt.Select($"item = '{Itmnmbr}'")
As a suggestion, let's change the names of Variables / Fields so it's easier to read them and also understand what these objects are used for. For example:
Dim dt as New DataTable()
'[...]
Dim itemNunmber As string = "i-2051"
Dim filteredRows As DataRow() = dt.Select(...)
itemNunmber is easier to read than Itmnmbr and filteredRows is more explicit than fr. There are some convetions that most are used to, as dt for DataTable, ds for DataSet etc., in this context. Better be sure that when you read your code after some time you don't get mad with yourself :)
Note that an Interpolated String is the same as a string formatted with String.Format(), so these two are actually the same thing:
Dim filteredRows As DataRow() = dt.Select($"item = '{itemNumber}'")
Dim filteredRows As DataRow() = dt.Select(String.Format("item = '{0}'", itemNumber))
Setting Option Infer On (should be On already), to make use of local type inference, you can write:
Dim filteredRows = dt.Select($"item = '{itemNumber}'")
and let the compiler infer the Type. In Visual Studio, if you move the mouse pointer over the variable, it will tell you what Type that is.
You have other options, if you need more dynamic selections.
The DataTableExtensions (which require a Project Reference to the System.Data.DataSetExtensions assembly - usually already linked along with System.Data), let you use the the AsEnumerable() method.
In LINQ to Objects style:
Here, using the default string Comparer
Dim filteredRows =
dt.AsEnumerable().Where(Function(dr) dr("item").ToString().Equals(itemNumber))
Or in LINQ to SQL style:
Here, using the InvariantCulture for the comparison.
Dim filteredRows =
From row In dt.AsEnumerable()
Where row.Field(Of String)("item").Equals(itemNumber, StringComparison.InvariantCulture)
Select row
See also: StringComparison and Best practices for comparing strings in .NET
These two last methods don't return an array of DataRow objects references, but a EnumerableRowCollection. The advantage is (when you can make use of it) that the collection is returned only when you actually use it (the execution is deferred).
When used correctly, it can improve the performance of your code. Try it out.
Instead of DataTable.Select(), you could also filter your DataTable, using its DefaultView.RowFilter property.
dt.DefaultView.RowFilter = $"item = '{itemNumber}'"
' You can save the filter to restore it later, if needed
Dim previousFilter = dt.DefaultView.RowFilter
When you present the Rows of your DataTable, only the Rows that meet the criteria defined by the Filter are shown (e.g., in a DataGrid of sort).
As mentioned, you're working with References here. The Collection of Rows returned by DataTable.Select() contain references of the Rows in the DataTable.
For example, if you consider the Collection and the filtered DataTable:
Dim filteredRows = dt.Select($"item = '{itemNumber}'")
dt.DefaultView.RowFilter = $"item = '{itemNumber}'"
Assume that filteredRows contains a single Row. Then you apply a Filter.
If you now change the value of filteredRows(0)("item"):
filteredRows(0)("item") = "Some other value"
when you present your DataTable in a UI, no Rows will be shown, since the Filter is active and now none of the Rows meet the filter's criteria: setting filteredRows(0)("item") has changed the value of the Row it refers to.
To remove a Filter, set it to string.Empty:
dt.DefaultView.RowFilter = Sting.Empty
To restore the previously saved filter:
dt.DefaultView.RowFilter = previousFilter