Question

I am trying to write extension methods in VB.NET

Imports System.Runtime.CompilerServices

Module ExtensionMethods

    <Extension()> _
    Public Function FindByText(ByVal collection As ListItemCollection, text As String, comparisonType As StringComparison) As ListItem
        Dim result As ListItem = collection.OfType(Of ListItem)().FirstOrDefault(Function(s) s.Text.Equals(text, comparisonType))
        Return result
    End Function

    <Extension()> _
    Public Function FindByValue(ByVal collection As ListItemCollection, text As String, comparisonType As StringComparison) As ListItem
        Dim result As ListItem = collection.OfType(Of ListItem)().FirstOrDefault(Function(s) s.Value.Equals(text, comparisonType))
        Return result
    End Function

End Module

But I am getting this error.

Class ‘System.Web.UI.WebControls.ListItem’ cannot be indexed because
it has no default property

What could be wrong?

I am calling the code like this.

ddlSalesmanager.Items.FindByText(survey, StringComparison.CurrentCultureIgnoreCase)

P.S: I ported this wonderful code from C# to VB

Answer 1

1. Your code works, so the exception must be raised somewhere else(what is survey?).
2. Use collection.Cast(Of ListItem)() instead of collection.OfType(Of ListItem)() since all objects in a ListItemCollection are of type ListItem by nature.

Tested with

<asp:DropDownList ID="DdlFoo" runat="server" AutoPostBack="true" OnSelectedIndexChanged="FooSelected" >
    <asp:ListItem Text="Foo1" Value="1"></asp:ListItem>
    <asp:ListItem Text="Foo2" Value="2"></asp:ListItem>
    <asp:ListItem Text="Foo3" Value="3"></asp:ListItem>
</asp:DropDownList>

And in the SelectedIndexChanged event-handler:

Dim foo2 = DirectCast(sender, DropDownList).Items.FindByText("FOO2", StringComparison.CurrentCultureIgnoreCase)
If Not foo2 Is Nothing Then
    ' your overloaded extension is called successfully
End If