Counter.cs

public class Counter
{
    public static int CountLines(string path)
    {
        return 
            File.ReadLines(path)
                .Count(Predicate);
    }

    private static bool Predicate(string line)
    {
        // do not match empty lines
        // do not match comments
        // do not match braces
        // ... do all of this without violating the open/closed principle

        return true;
    }
}

Parser.cs

public class Parser
{
    public static IEnumerable<string> Parse(string path)
    {
        var document = XDocument.Load(path);

        return
            document.Descendants()
                    .Where(descendant => descendant.Name.LocalName == "Compile")
                    .Select(descendant => descendant.FirstAttribute.Value);
    }
}

Processor.cs

public static class Processor
{
    public static int Process(string path)
    {
        var paths = Parser.Parse(path);

        var directoryPath = Path.GetDirectoryName(path);
        // convert relative path to an absoloute path
        paths = paths.Select(p => Path.Combine(directoryPath, p)); 

        return paths.Sum(p => Counter.CountLines(p));
    }
}