Question

I am currently using the function below to return the nearest date from a list of dates to a date(today). My problem is, the function returns the closest date regardless of it being the past or the future of todays date. How can I change this code so there is an option to return the nearest date AFTER today and the nearest date BEFORE today? It’s confusing the hell out of me.

Thanks a lot for your input.

Function GetNearestDate(ByVal source As IEnumerable(Of DateTime), ByVal target As DateTime) As DateTime
    Dim result As DateTime = Nothing
    Dim lowestDifference = TimeSpan.MaxValue

    For Each _date As DateTime In source

        If _date >= target Then
            Continue For
        End If

        Dim difference = target - _date

        If difference < lowestDifference Then
            lowestDifference = difference
            result = _date
        End If
    Next

    Return result
End Function

Answer 1

Seems like this is what you are looking for. you simply need to be able to pass something in to the function so it knows what you want. I chose an enum for explicitness. I also updated it to pass back a nullable date. This should correct your time issues, but you’ll need to account for null returned values.

Public Enum DateCompare
    LessThanEqualTo
    GreaterThanEqualTo
End Enum

Public Function GetNearestDate(ByVal source As IEnumerable(Of DateTime), _
                               ByVal target As DateTime, _
                               ByVal dt As DateCompare) As Nullable(Of DateTime)
    Dim result As Nullable(Of DateTime) = Nothing
    Dim lowestDifference As TimeSpan = TimeSpan.MaxValue
    Dim difference As TimeSpan

    For Each _date As DateTime In source
        If dt = DateCompare.LessThanEqualTo And _date > target Then
            Continue For
        ElseIf dt = DateCompare.GreaterThanEqualTo And _date < target Then
            Continue For
        End If

        If target > _date Then
            difference = target - _date
        Else
            difference = _date - target
        End If

        If difference < lowestDifference Then
            lowestDifference = difference
            result = _date
        End If
    Next

    Return result
End Function