C# LINQ : Summarise specific inner class properties from outer class collection

Drawing on Loop Through An Objects Properties In C# and Using LINQ to loop through inner class properties in outer class collection

Where you have objects (Phase) in a collection (PhaseRepo), I believe it is possible to specify propertiesOfInterest in the objects (Phase) and create a Dictionary to summarise the properties.

Please find below my attempt in LinqPad. Please assist with the syntax or advise an alternate approach.

Thank you

enum Dir {Up, Dn}

struct BmkKey
{
    public Dir Direction;
    public string DetailType;
}

class Phase
{
    public Dir Direction { get; set; }
    public double StartToTerminationBars { get; set; }
    public double StartToTerminationPriceChange { get; set; }
    public double StartToTerminationGa { get; set; }
}

class PhaseRepo
{
    public List<Phase> Phases { get; private set; }

    public List<Phase> GetPhases()
    {
        return new List<Phase>()
        { 
            new Phase() { Direction = Dir.Up, StartToTerminationBars = 3.0, StartToTerminationPriceChange = 4.0, StartToTerminationGa = 4.0},
            new Phase() { Direction = Dir.Up, StartToTerminationBars = 6.0, StartToTerminationPriceChange = 8.0, StartToTerminationGa = 4.0},
            new Phase() { Direction = Dir.Dn, StartToTerminationBars = 3.0, StartToTerminationPriceChange = -4.0, StartToTerminationGa = -4.0},
            new Phase() { Direction = Dir.Dn, StartToTerminationBars = 6.0, StartToTerminationPriceChange = -8.0, StartToTerminationGa = -4.0},
        };
    }
}

void Main()
{
    var phaseRepo = new PhaseRepo();
    var phases = phaseRepo.GetPhases();
    //phases.Dump();

    var propertiesOfInterest = typeof (Phase).GetProperties(BindingFlags.Public | BindingFlags.Instance)
    .Where(prop => prop.Name == "StartToTerminationBars" 
        || prop.Name == "StartToTerminationPriceChange" 
        || prop.Name == "StartToTerminationGa")
    .ToList();
    //propertiesOfInterest.Dump();

    // Please Help...
    var test = propertiesOfInterest
        .SelectMany(propertyInfo => phases
            .Select(phase => phase)
            .Select(keyValuePair => new
            {
                phase.Direction,
                keyValuePair.Key,
                keyValuePair.Value
            })
            .Select(arg => new
            {
                Key = new BmkKey
                {
                    Direction,
                    DetailType = propertyInfo.Name
                },
                Value = (double)propertyInfo.GetValue(arg.Value, null)
            }))
        .GroupBy(grp => grp.Key)
        .ToDictionary(grp => grp.Key, grp => x => x.ToList());

    test.Dump();

}