Linq查询以使用列表列表来过滤列表

Question

I'm still learning LINQ, and I'm attempting to use an object from one list as an identifier to find objects in another list.

I have two lists of objects, similar to my mock-up code below. Using LINQ, I want to list all Colors.Hex of a specific Country's flag color, using the variable selectedCountry.

So if selectedCountry = "USA", I would like to be able to console.write:

USA
RED FF0000
Blue 0000FF
White FFFFFF

Preferably in query syntax, for readability.

    public class Countries
{
    public string Name;
    public List<string> FlagColors = new List<string>();
}

public class Colors
{
    public string Name;
    public string Hex;
}

public partial class Form1 : Form
{
    public static List<Countries> country = new List<Countries>();
    public static List<Colors> color = new List<Colors>();

    public void foo()
    {
        color.Add(new Colors { Name= "Red", Hex = "FF0000"});
        color.Add(new Colors { Name= "Blue", Hex = "0000FF" });
        color.Add(new Colors { Name= "White", Hex = "FFFFFF" });
        color.Add(new Colors { Name= "Yellow", Hex = "FFFF00" });

        Countries newCountry = new Countries();
        newCountry.Name = "USA";
        newCountry.FlagColors.Add("Red");
        newCountry.FlagColors.Add("White");
        newCountry.FlagColors.Add("Blue");
        country.Add(newCountry);

        Countries newCountry2 = new Countries();
        newCountry2.Name = "Sweden";
        newCountry2.FlagColors.Add("Blue");
        newCountry2.FlagColors.Add("Yellow");
        country.Add(newCountry2);

        string selectedCountry = "USA";

        // Linq query here
    }
}

Thanks in advance

Answer 1

You can do this like so:

Country selectedCountry = country.SingleOrDefault(x => x.Name == selectedCountry);
if (selectedCountry != null) {
    Console.WriteLine(selectedCountry.Name);
    foreach (string flagColor in selectedCountry.FlagColors) {
        Colors color = color.SingleOrDefault(x => x.Name == flagColor);
        if (color != null) {
            Console.WriteLine(color.Name + " " + color.Hex);
        }
    }
}

As you can see the LiNQ queries are pretty simple, you basically want to return the first element that matches the condition predicate (in this case, where the Name is equal to the selectedCountry or the flagColor .

Answer 2

Something like this, perhaps:

    var q = from c1 in country
        from c2 in c1.FlagColors
        from c3 in color
        where c3.Name == c2 && c1.Name == selectedCountry
        select c3.Hex;

Or alternatively:

    var q = from c1 in country
        from c2 in c1.FlagColors
        join c3 in color on c2 equals c3.Name
        where c1.Name == selectedCountry
        select c3.Hex;