Question

I am working on an ASP.NET MVC project. The data access in through a repository that has a simple caching functionality. It contains several functions like the follwing two:

Public Function SelectAllCurrencies() As List(Of store_Currency) 
    Dim AllCurrencies As List(Of store_Currency)
    If UseCaching Then
        AllCurrencies = HttpContext.Current.Cache("AllCurrencies")
        If AllCurrencies Is Nothing Then
            AllCurrencies = (From Currencies In db.Currencies Order By Currencies.Title Ascending).ToList
            Cache.AddToCache("AllCurrencies", AllCurrencies)
        End If
    Else
        AllCurrencies = (From Currencies In db.Currencies Order By Currencies.Title Ascending).ToList
    End If
    Return AllCurrencies
End Function

Public Function SelectAllCountries() As List(Of store_Country) 
    Dim AllCountries As List(Of store_Country)
    If UseCaching Then
        AllCountries = HttpContext.Current.Cache("AllCountries")
        If AllCountries Is Nothing Then
            AllCountries = (From Countries In db.Countries Order By Countries.Title Ascending).ToList
            Cache.AddToCache("AllCountries", AllCountries)
        End If
    Else
        AllCountries = (From Countries In db.Countries Order By Countries.Title Ascending).ToList
    End If
    Return AllCountries
End Function

As you can see, they use the same workflow, over and over again. I want to remove this redundancy. I think that generics should provide a solution, but I can’t for the life of me figure out how to deal with the LINQ statements in a generic SelectAllEntities(Of T) function. Is there a way to ‘generalize’ the queries, maybe with dynamic LINQ?

Answer 1

My VB is a little rusty but I think you want to write your function like this.

Public SelectAll(Of T, TOrder)(
        IEnumerable(Of T) source,
        Func(Of T, TOrder) keySelector,
        cacheKey As String = Nothing) As IList(Of T)

    Dim all As List(Of T)
    If me.UseCaching Then
        all = HttpContext.Current.Cache(cacheKey)
        If all Is Nothing Then
            all = source.OrderBy(keySelector).ToList()
            Cache.AddToCache(cacheKey, all)
        End If
    Else
        all = source.OrderBy(keySelector).ToList()
    End If
    Return all
End Function

Which you could use like this

Dim allCountries = SelectAll(db.Countries, Function(c) c.Title, "AllCountries")