CODEWITHSUNDEEP
c#lambdaexpressionpredicate
nev.angelova
nev.angelova

Reputation: 94

Lambda expression alternative to several if statements

I'm trying to accomplish an expression function alternative

private static Expression<Func<UserProfile, bool>> CompareFilter(FilterViewModel f)
{
       ...
}

on this one:

private static bool CompareFilter(UserProfile profile, FilterViewModel filter)
{
        if (filter.FirstName != null)
        {
            if (profile.FirstName != null)
            {
                if (profile.FirstName.CompareTo(filter.FirstName) != 0)
                {
                    return false;
                }
            }
            else
            {
                return false;
            }
        }

        if (filter.TownId != null)
        {
            if (profile.TownId != filter.TownId)
            {
                return false;
            }
        }

        // true if at least one of the filter interests match
        if (filter.InterestsIds != null)
        {
            var firstInterestFound = profile.Interests
                 .Where(i => filter.InterestsIds.Contains(i.Id))
                 .FirstOrDefault();

            if (firstInterestFound == null)
            {
                return false;
            }
        }
        ...

        return true;
}

Is there a way to fit this many nested if statements to a lambda expression or something else that will work for the expression function? The idea is the verification to go trough all the if statements and to return true only if all of them are not returning false.

Thanks in advance!

Upvotes: 1

Views: 1676

Answers (1)

JLRishe
JLRishe

Reputation: 101662

First of all, your current method is so long because you are not making good use of boolean logic. It can be simplified to this:

private static bool CompareFilter(UserProfile profile, FilterViewModel filter)
{
    if (filter.FirstName != null && filter.FirstName != profile.FirstName)
    {
        return false;
    }

    if (filter.TownId != null && filter.TownId != profile.TownId)
    {
        return false;
    }

    // true if at least one of the filter interests match
    if (filter.InterestsIds != null &&
        !profile.Interests.Any(i => filter.InterestsIds.Contains(i.Id)))
    {
        return false;
    }
    ...

    return true;
}

You can turn this into a big hulking expression by inverting all the logic1:

private static bool CompareFilter(UserProfile profile, FilterViewModel filter)
{
    return (filter.FirstName == null || filter.FirstName == profile.FirstName) &&

           (filter.TownId == null || filter.TownId == profile.TownId) &&

           (filter.InterestsIds == null ||
            profile.Interests.Any(i => filter.Interests.Contains(i.Id)));

    // etc. etc.
}

And once you have this, it's a piece of cake to turn it into a lambda and get your Expression<T>:

private static Expression<Func<UserProfile, bool>> CompareFilter(FilterViewModel f)
{
    return profile => 
           (filter.FirstName == null || filter.FirstName == profile.FirstName) &&

           (filter.TownId == null || filter.TownId == profile.TownId) &&

           (filter.InterestsIds == null ||
            profile.Interests.Any(i => filter.Interests.Contains(i.Id)));
}
  1. Technically, you can just || together all the conditions from the first method and put a big !( ) around it: return !( (...) || (...) || (...));, but inverting everything and joining the conditions with && is a lot nicer.

Upvotes: 5

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