Question

I have an inline lambda expression that I would like to use throughout my application. I just can’t seem to find a reference on how to do this with more parameters than the element being tested. Here is a quick example of what I currently have.

Private Sub Test()
    Dim List As New List(Of String) From {"Joe", "Ken", "Bob", "John"}
    Dim Search As String = "*Jo*"
    Dim Result = List.Where(Function(Name) Name Like Search)
End Sub

I know the IEnumerable.Where accepts a method with the element type as a parameter and a returning boolean.

Private Sub Test()
    Dim List As New List(Of String) From {"Joe", "Ken", "Bob", "John"}
    Dim Search As String = "*Jo*"
    Dim Result = List.Where(AddressOf FindName)
End Sub

Private Function FindName(Name As String) As Boolean
    Return Name Like "*Jo*"
End Function

I would like to pass the Search variable into FindName as well. I just can’t seem to get the syntax figured accomplish this. The only working solution I’ve come up with is passing everything to a function to perform the original statement.

Private Sub Test()
    Dim List As New List(Of String) From {"Joe", "Ken", "Bob", "John"}
    Dim Search As String = "*Jo*"
    Dim Result = FindName(List, Search)
End Sub

Private Function FindName(List As IEnumerable(Of String), Search As String) As IEnumerable(Of String)
    Return List.Where(Function(Name) Name Like Search)
End Function

Don’t feel obligated to answer in VB.

Answer 1

The nice thing with lambda expressions is that they allow closures to automatically capture variables that are local to your scope. For example (pardon the C#):

List.Where(i => FindName(i, Search));

In the above code, the Search variable is actually being encapsulated in a closure. If you want to just pass a method, you’ll have to simulate what closures do with an actual class structure:

public class NameFinder
{
    private string _search;
    public NameFinder(string search) {
        _search = search;
    }
    public bool Match(string item) {
        // C# equivalent of "return item LIKE _search"
    }
}

// Usage
var nameFinder = new NameFinder(Search);
List.Where(nameFinder.Match);

However, this strategy is only useful in a small subset of cases. I find it’s usually best to just use a lambda expression that passes the appropriate variables into the function that does all the work.