Linq dynamic select casting issue

Question

I want to run a query to get a string array of distinct "logName" items for a logType. The following works great:

Dim stringArray() As String = (From item In dc.Vw_Logs 
    Where item.LogType = [Passed in logType] 
    Select item.LogName Distinct).ToArray()

However, this only works when a specific LogType is set. I would like the logType clause to be optional. To try and achieve this I have rewritten the query:

Dim q = From item In dc.Vw_Logs Distinct
If not logType is nothing Then 
    q = q.Where(Function(item) item.LogType = logType)
End If
q.Select(Function(item) item.LogName)
Dim stringArray() As String = q.ToArray()

With this I get the following error:

Value of type '1-dimensional array of Vw_Log' cannot be converted 
to '1-dimensional array of String'

What is the best way to get round this? I would like to avoid iterating each item and casting.

Thanks for any help

Answer 1

Do not reuse q. The first assignment is Vw_Log(), the second is String().

You can also combine the last two lines to avoid this.

Answer 2

The error is because q is not strings but the type contained in dc.Vw_Logs.

I don't do much programming in VB anymore but this should get you close.

Dim q = From item In dc 
        Where (logType Is Nothing Or item.LogType = logType) 
        Select item.LogName Distinct


logType Is Nothing Or item.LogType = logType

If logType has a value, you will get the items that match; otherwise, you will get all of the items.

Answer 3

The problem is that this line

q.Select(Function(item) item.LogName)

doesn't change q, it simply discards the result of the select so that when you do

Dim stringArray() As String = q.ToArray()

You're trying to convert the original q (which is of type Vw_Log) to an array of strings. Replace the last two lines with this:

Dim stringArray() As String = q.Select(Function(item) item.LogName).ToArray()