Question

Code block four (below) is giving me an error that I am at a loss with regard to fixing…

Here’s the XML Schema that I’m using:

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>

<!DOCTYPE ctqcfg SYSTEM "cache.dtd">

<cache version="1.0">
    <configuration name="Test">
        <cacheControl>
            <cache name="Customer" mode="off"/>
            <cache name="Vendor" mode="off"/>
            <cache name="Agency" mode="off"/>
            <cache name="Partner" mode="off"/>
        </cacheControl>
    </configuration>

    <configuration name="Production">
        <cacheControl>
            <cache name="Customer" mode="preload"/>
            <cache name="Vendor" mode="dynamic"/>
            <cache name="Agency" mode="dynamic"/>
            <cache name="Partner" mode="dynamic"/>
        </cacheControl>
    </configuration>
</cache>

The XML file is loaded

Private XElement As XElement = Nothing

Public Sub Load()
    XElement = XElement.Load(ConfigurationResource)
End Sub

When the user selects a configuration to edit, a reference to the root of the selected configuration element is held

Private ConfigurationRoot As System.Collections.Generic.IEnumerable(Of System.Xml.Linq.XElement)

Private ConfigurationName_ As String

Public Property ConfigurationName() As String
    Get
        Return ConfigurationName_
    End Get
    Set(ByVal Value As String)
        ConfigurationName_ = Value
        ConfigurationRoot = From Configuration In XElement.<configuration> Where Configuration.@name = Value
    End Set
End Property

Trying to retrieve the cache mode that corresponds to the cache name (in this case Customer)

Public Property CustomerCache() As String
    Get
        Try
            Return From Cache In ConfigurationRoot.<cacheControl>.<cache> Where Cache.@name = "Customer" Select Cache.@mode
        Catch Exception As Exception
            Return Nothing
        End Try
    End Get
    Set(ByVal Value As String)
        'ToDo
    End Set
End Property

I’m getting the following error

System.InvalidCastException was caught
Message=Unable to cast object of type 'WhereSelectEnumerableIterator`2[System.Xml.Linq.XElement,System.String]' to type 'System.String'.

This is my first day working with LINQ and I seem to have a basic misunderstanding regarding how to access the attribute – it seems that the query is returning a collection and I know that there is only ever one possible value that can be found…

Answer 1

You need .First() or .Single():

Return (From Cache In ConfigurationRoot.<cacheControl>.<cache> Where Cache.@name = "Customer" Select Cache.@mode).First()