Question

Let’s say I have an entity that looks like this:

public class Album()
{
    public DateTime LastUpdated { get; set; }
    public List<Picture> Pictures { get; set; }
}

What I want to do is create a LastActivity property that will return the latest date of the activity. This is easy enough for the Pictures collection:

public DateTime LastActivity
{
    get { return Pictures.Max(x => x.LastUpdated); }
}

However, I also want to consider the LastUpdated property on the Album entity as well. I could use this code:

public DateTime LastActivity
{
    get { return Pictures.Max(x => x.LastUpdated) > this.LastUpdated
              ? Pictures.Max(x => x.LastUpdated)
              : this.LastUpdated) };
}

But this is bad because it will do the Max() transformation twice. Is there a better way of writing this code?

Answer

This is the solution I came up with, based on the accepted answer:

public virtual DateTime LastActivity
{
    get
    {
        var max = Pictures.Any() ? Pictures.Max(x => x.LastUpdated) : DateTime.MinValue;
        return max > this.LastUpdated ? max : this.LastUpdated;
    }
}

The thing to watch out for is that if you do a Max() on an empty collection, you’ll get an exception, so you have to check to see if there’s anything in the collection first.

Answer 1

Just store the max in a variable rather that performing the calculation twice.

public DateTime LastActivity
{
    get 
    { 
        var max = Pictures.Max(x => x.LastUpdated);
        return max > this.LastUpdated
          ? max
          : this.LastUpdated 
    };
}