`RemoveAll`

Suppose a List contains one or more elements that are not needed—we can remove these elements with RemoveAll. Only one function call is needed.

By removing many elements with a function call, we can avoid confusing loops. We must specify a lambda expression when calling RemoveAll, but the syntax in VB.NET is easy to understand.

Example

To test the RemoveAll method on the List generic type, we first need a List to modify. Here we create a List and remove elements from it.

Step 1 We invoke RemoveAll() and pass a lambda Function to it, which returns true for each element greater than or equal to 4.

Step 2 We display the contents of the List after all elements greater than or equal to 4 were removed.

Step 3 Here we remove all elements with values not equal to 1—this leaves the List with just a single element (with value 1) in it.

Step 4 In this final For Each loop, we again print the elements contained within the List collection.

Module Module1
    Sub Main()
        Dim list = New List(Of Integer)({ 1, 2, 3, 4, 5, 6 })

        ' Step 1: call RemoveAll with a predicate lambda expression.
        list.RemoveAll(Function (item)
            Return item >= 4
        End Function)

        ' Step 2: display results.
        For Each value in list
            Console.WriteLine($"REMOVEALL: {value}")
        Next

        ' Step 3: remove all values not equal to 1.
        list.RemoveAll(Function (item)
            Return item <> 1
        End Function)

        ' Step 4: display results again.
        For Each value in list
            Console.WriteLine($"REMOVEALL AGAIN: {value}")
        Next
    End Sub
End ModuleREMOVEALL: 1
REMOVEALL: 2
REMOVEALL: 3
REMOVEALL AGAIN: 1

Return value

The RemoveAll Function returns an Integer indicating how many elements were removed. Here we remove 2 strings with value "bird" and the value returned is 2.

Module Module1
    Sub Main()

        Dim list = New List(Of String)({ "bird", "bird", "dog" })
        ' Remove all "bird" elements from list, and print count of items removed.
        Dim removed = list.RemoveAll(Function (item)
            Return item = "bird"
        End Function)
        Console.WriteLine($"{removed} birds were removed!")

    End Sub
End Module
2 birds were removed!

Summary

Removing multiple elements from a List is a complex task when For-loops are used. But RemoveAll, a Function on the List type, helps and simplifies this task.