Check if a list of strings contains a value

Question

I have:

Public lsAuthors As List(Of String)

I want to add values to this list, but before adding I need to check if the exact value is alre

Answer 1

You can use List.Contains:

If Not lsAuthors.Contains(newAuthor) Then
    lsAuthors.Add(newAuthor)
End If

or with LINQs Enumerable.Any:

Dim authors = From author In lsAuthors Where author = newAuthor
If Not authors.Any() Then
    lsAuthors.Add(newAuthor)
End If

You could also use an efficient HashSet(Of String) instead of the list which doesn't allow duplicates and returns False in HashSet.Add if the string was already in the set.

 Dim isNew As Boolean = lsAuthors.Add(newAuthor)  ' presuming lsAuthors is a HashSet(Of String)

Answer 2

The generic List has a method called Contains that returns true if the default comparer for the choosen type finds an element matching the searching criteria.

For a List(Of String) this is the normal string comparison, so your code could be

Dim newAuthor = "Edgar Allan Poe"
if Not lsAuthors.Contains(newAuthor) Then
    lsAuthors.Add(newAuthor)
End If 

As a side note, the default comparison for strings considers two strings different if they don't have the same case. So if your try to add an author named "edgar allan poe" and you already have added one with the name "Edgar Allan Poe" the barebone Contains fails to notice that they are the same.
If you have to manage this situation then you need

....
if Not lsAuthors.Contains(newAuthor, StringComparer.CurrentCultureIgnoreCase) Then
    .....

Answer 3

To check whether an element is present in a list you can use the list.Contains() method. If you are using a button click to populate the list of strings then see the code:

Public lsAuthors As List(Of String) = New List(Of String) ' Declaration of an empty list of strings

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click ' A button click populates the list
    If Not lsAuthors.Contains(TextBox2.Text) Then ' Check whether the list contains the item that to be inserted
        lsAuthors.Add(TextBox2.Text) ' If not then add the item to the list
    Else
        MsgBox("The item Already exist in the list") ' Else alert the user that item already exist
    End If
End Sub

Note: Line by line explanation is given as comments

Answer 4

You can get a list of matching items of your condition like this:

Dim lsAuthors As List(Of String)

Dim ResultData As String = lsAuthors.FirstOrDefault(Function(name) name.ToUpper().Contains(SearchFor.ToUpper()))
If ResultData <> String.Empty Then
    ' Item found
Else
    ' Item Not found
End If