Question

I’m trying to DRY out some lambda expressions for security rights. Is it possible to take a lamda expression and apply it to a single entity for true?

Like lets say I have a Person and a DocumentFolder

    Expression<Func<Person, bool>> CanSeePerson()
    {
        return c =>  !c.IsPrivate;
    }

And one for the folder

    Expression<Func<DocumentFolder, bool>> CanSeeFolder()
    {
        return c => !c.IsPrivate && c.Owner.CanSeePerson(); // <- ???
    }

How the heck can I use that CanSeePerson() function on a single type to return true and maintain an expression that can be used in linq queries like such

Entities.DocumentFolder.Where(CanSeeFolder());

I know how to use the where on an iqueryable but I can’t see how to apply the expression tree to a single value.

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This throws the error: Unable to create a constant value of type ‘Person’. Only primitive types (‘such as Int32, String, and Guid’) are supported in this context.

Expression<Func<DocumentFolder, bool>> CanSeeFolder()
    {
        return c => !c.IsPrivate &&_entities.Persons.Where(x => x.Id == c.Owner.Id).Any(CanSeePerson());
}

---

The difference seems to be based on putting the IQueryable Directly in the statement. This also does NOT work

Expression<Func<DocumentFolder, bool>> CanSeeFolder()
    {
        return c => !c.IsPrivate &&_entities.Persons.Where(CanSeePerson()).Contains(c.User);
    }

but this DOES work

Expression<Func<DocumentFolder, bool>> CanSeeFolder()
        {
            var canSeePersons = _entities.Persons.Where(CanSeePerson());
            return c => !c.IsPrivate && canSeePersons.Contains(c.User);
        }

p.s. I know I suck @ using this stackoverflow formatting thing lol

---

The reason why this works is because the CanSeePerson() function cannot be converted and used in an expression. When you put the canSeePersons variable in the function instead you are placing in an iQueryable type which can be used in an expression. Using “var” convolutes it a bit.

Answer 1

You defined a Expression<Func<Person, bool>> that solves your problem. The trick is to simply transform your list of objects to the list you want to filter. It is not always possible, but many times it is. You just have to be a bit creative 🙂

When using extension methods you can easily come up with a solution that allows you to do this:

var visibleFolders = Entities.DocumentFolder.WhereCanSeeFolder();

Here is the (completely DRY) solution:

public static class SecurityExtensions
{
    public static IQueryable<DocumentFolder> WhereCanSeeFolder(
        this IQueryable<DocumentFolder> folders)
    {
        var visibleOwners = folders.Select(f => f.Owner)
            .Where(CanSeePerson);

        return
            from folder in folders.Where(CanSeeFolder)
            where visibleOwners.Contains(folder.Owner)
            select folder;
    }

    private static readonly Expression<Func<DocumentFolder, bool>>
        CanSeeFolder = folder => !folder.IsPrivate;

    private static readonly Expression<Func<Person, bool>>
        CanSeePerson = person => !person.IsPrivate;
}

I hope this helps.