Need help in a grouping using LINQ in C#

Question

I have these two classes in my code and a List of Class2. I want to group the list elements by date and ID using LINQ.

public class Class1
{
    public string ID;
    public int y;
    public int z;
}

public class Class2 
{
    public List<Class1> a;
    public int p, q;
    public string s;
    public DateTime date;
}

My list is similar to this:

List<Class2> object1 = new List<Class2>
{
    new Class2 {p = 5, q = 6, date = DateTime.Now, a = new List<Class1> { new Class1 { ID = "3643746HDJ", y = 0, z = 9 }, new Class1 { ID = "746327846HDJ", y = 0, z = 9 } } },
    new Class2 {p = 5, q = 6, date = DateTime.Now.AddDays(1), a = new List<Class1> { new Class1 { ID = "3643746HDJ", y = 0, z = 9 }, new Class1 { ID = "746327846HDJ", y = 0, z = 9 } } },
    new Class2 {p = 5, q = 6, date = DateTime.Now.AddDays(2), a = new List<Class1> { new Class1 { ID = "3643746HDJ", y = 0, z = 9 }, new Class1 { ID = "746327846HDJ", y = 0, z = 9 } } },
    new Class2 {p = 5, q = 6, date = DateTime.Now.AddDays(3), a = new List<Class1> { new Class1 { ID = "3643746HDJ", y = 0, z = 9 }, new Class1 { ID = "746327846HDJ", y = 0, z = 9 } } },
};

Sorry if this is a stupid question, but I am a beginner in C# programming and LINQ and I am not sure if this is even possible.

This is what I have tried based on examples I saw on the web but its not correct

var newList = from item in object1
              group new
              {
                  item.s,
                  item.a[0].x, //I think its wrong because of this
                  item.a[0].y  //and this
              }
              by new
              {
                  item.date,
                  item.a[0].ID  //and this
              }
              into temp
              select temp;

Since I have hardcoded index 0 in the grouping I am missing a lot of elements in my final list. How can I do this for all elements of list a in Class2?

Expected output is similar to this:

Key: {date: 19-09-2017 ID: 3643746HDJ}, Element: {{s: "abc", y = 1, z = 2}, {s: "pqr", y = 2, z = 4}, {s: "abc", y = 1, z = 2}}
Key: {date: 20-09-2017 ID: 3643746HDJ}, Element: {{s: "pop", y = 1, z = 2}, {s: "dfr", y = 2, z = 4}, {s: "abc", y = 1, z = 2}}
Key: {date: 19-09-2017 ID: 746327846HDJ}, Element: {{s: "abc", y = 7, z = 8}, {s: "asar", y = 2, z = 111}, {s: "abc", y = 1, z = 2}}
Key: {date: 20-09-2017 ID: 746327846HDJ}, Element: {{s: "abc", y = 7, z = 8}, {s: "asar", y = 2, z = 111}, {s: "abc", y = 1, z = 2}}

Answer 1

Your question states:

Since I have hardcoded index 0 in the grouping I am missing a lot of elements in my final list. How can I do this for all elements of list a in Class2?

As you want all the items from the nested collections you need to flatten the nested collections before grouping. In query syntax do so using another from:

var result = from item in object1
             from nested in item.a
             group new { item.s, nested.y, nested.z } by new { item.date, nested.ID } into g
             select g;

As each nested object does not have a p and q properties but your code shows accessing the nested items I used y and z instead

This can also be done using method syntax. To flatten the nested collections use SelectMany. For the GroupBy you can use the overload where you specify both key and element selector. General idea for that:

var result = object1.SelectMany(item => item.a.Select(nested => new {item, nested }))
                    .GroupBy(key => new { key.item.Date, key.nested.Id },
                             val => new { val.item.s, val.nested.x, val.nestd.y });

---

Even though it seems it is only for question purposes only I recommend giving meaningful names for properties and variables and to have a look at:

• MSDN Naming Conventions
• Dofactory