Question

I’m new to C# and I have the following 3 methods. These methods allow the caller to retrieve differents properties of the table by specifying a lambda expression on the given method. However, I have the feeling an expert would combine them even further into a single generic method. If this is possible, please let me know how.

private KeyValuePair<T, int> GetHeaderProperty<T>(Func<Header, T> Property, 
                                                  Source ds)
{
    Func<Source, Header> GetValue =
        a => Books.Where(str => str.BookId == a.DiscardBookId)
                 .First().Header;
    return new KeyValuePair<T,int>(Property(GetValue(ds)), 0);
} 

private KeyValuePair<T, int> GetBookProperty<T>(Func<Book, T> Property,
                                               Source ds)
{
    Func<Source, Book> GetValue = 
        a => Books.Where(str => str.BookId == a.DiscardBookId).First();

    return new KeyValuePair<T, int>(Property(GetValue(ds)), 0);
} 

private KeyValuePair<T, int> GetFleetProperty<T>(Func<Fleet, T> Property,
                                                 Source ds)
{
    Func<Source,Fleet> GetValue =
        a => Books.Where(str => str.BookId == a.DiscardBookId).First()
                 .Header.Fleet;

    return new KeyValuePair<T,int>(Property(GetValue(ds)), 0);
} 

Answer 1

I think the following will be equivalent to calling all three methods in a row and adding the results to a list:

private IEnumerable<KeyValuePair<T, int>> GetFleetProperty<T>(
                                              Func<Book, T> PropertyBook,
                                              Func<Header, T> PropertyHeader,
                                              Func<Fleet, T> PropertyFleet,
                                              Source ds)
{
    Func<Source,Fleet> GetValue =
        a => Books.Where(str => str.BookId == a.DiscardBookId).First();

    var book = GetValue(ds);
    var result = new List<KeyValuePair<T, int>>();
    result.Add(new KeyValuePair<T, int>(PropertyBook(book), 0);
    result.Add(new KeyValuePair<T, int>(PropertyHeader(book.Header), 0);
    result.Add(new KeyValuePair<T, int>(PropertyFleet(book.Header.Fleet), 0);
    return result;
} 

UPDATE:
You could also create a method like this:

private KeyValuePair<T, int> GetProperty<T, TProperty>(
                                 Func<TProperty, T> Property, 
                                 Func<Book, TProperty> GetProperty, 
                                 Source ds)
{
    Func<Source, Header> GetValue =
        a => Books.Where(str => str.BookId == a.DiscardBookId)
                 .First();
    var book = GetValue(ds);
    return new KeyValuePair<T,int>(Property(GetProperty(book)), 0);
} 

You would call it like this for Header:

GetProperty(xyz, b => b.Header, ds);

You would call it like this for Book:

GetProperty(xyz, b => b, ds);

You would call it like this for Fleet:

GetProperty(xyz, b => b.Header.Fleet, ds);