Populating a List(Of String) from a Datatable using LINQ in VB.NET

问题

So I have a DataTable that looks more or less like

 Column 0      |  Column 1 
 Something     |  Something Else 
 Another Thing |  Another Something Else

And I want to put everything in column 0 into a List(Of String)

I could do

Dim returnValue as List(Of String)    
For Each r As DataRow In dt.Rows
     returnValue.Add(r.Item(0).ToString)
Next

But that's old and busted. I want to do

returnValue = (From r In dt.Rows Select DirectCast(r, DataRow).Item(0)).ToList

But that gives me a list(of Object).

How can I directly create a list(of String)

(the DirectCast is there because I have Option Strict On)

回答1:

dt.Rows is from before the time of .NET generics, so it won't return an IEnumerable(Of DataRow). However, there is an extension method, DataTable.AsEnumerable, which does exactly what you need:

returnValue = (From r In dt.AsEnumerable() Select r.Field(Of String)(0)).ToList()

Note that my example also uses the DataRow.Field extension method, which allows type-safe access to DataRow fields without needing an explicit cast (even with Option Strict On).

回答2:

It is in an datarow collection so we need to cast it out.

Cast

The function in the Select asks which field do you want from the casted object.

returnValue = dt.Rows.Cast(Of DataRow).Select(Function(dr) dr(0).ToString).ToList

来源：https://stackoverflow.com/questions/30644853/populating-a-listof-string-from-a-datatable-using-linq-in-vb-net