Create generic Linq extension Method to "sew" data by Key

I have a collection of courses (Id, name, status, instructor, etc) and another collection of Pricing (CourseId, Price).

I am trying to write a linq extension method to merge these two together, but my reflection is a bit rusty. What am I missing?

This is how I want to call it:

courses.SewData(pricing, p => p.Id, c => c.CourseId);

This is the method that is being called. Im having issues with the ToDictionary Line. Its not compiling. How can I create a dictionary with my existing expression

public static class DataExtension {
    public static IEnumerable<TParent> SewData<TParent, TChild>(this IList<TParent> parentCollection, IList<TChild> childCollection, Expression<Func<TParent, object>> parentProperty, Expression<Func<TChild, object>> childProperty) {

        var parentPropertyInfo = ReflectionHelper.GetProperty(parentProperty);
        var childPropertyInfo = ReflectionHelper.GetProperty(childProperty);

        var parentDict = parentCollection.ToDictionary(parentProperty, x => x); //need help here
        foreach (var child in childCollection) {
            var childId = childPropertyInfo.GetValue(child);
            var parentItem = parentDict[childId];
            
            //use reflection here to map properties by name
            

            
            yield return parentItem;
        }          
    }
}