使用Linq为另一个对象列表中的每个项目创建新的对象列表

Question

I have an object with

public class test
 { 
     public object obj1;
     public List<string> items;
 }

I have a List<test> . For each of the item si want to create a new object with properties from obj1 and an item . The items list can be null or empty. Is there a way to do in Linq ?

Answer 1

List<test> tests = new List<test>();
var ob1=new test{ obj1 = "obj1" };
var ob2=new test{ obj1 = "obj2" };
var ob3=new test{ obj1 = "obj3" };
var ob4=new test{ obj1 = null };
tests.Add(ob1);
tests.Add(ob2);
tests.Add(ob3);
tests.Add(ob4);

var result = tests.Select(e => new NewType
{
     name = e.obj1 != null ? e.obj1.ToString() : null
});

foreach (var item in result)
{
    Console.WriteLine(item.name);
}

Is this what you are looking for?

Answer 2

Say Test.Obj1 has two properties you want to include in each new item, 'Name' & 'ID', along with an item from the list - try a query like this.

Will generate a list of new (anonymous) objects with (for example) the first element of each Test.Obj1.List

IEnumerable<dynamic> results = tests
    .Select(o => new { Name = o.obj1.Name, 
                       ID = o.obj1.Id, 
                       Item = o.items.First() ?? null} )
    .ToList();

Console.WriteLine(results.ElementAt(0));
Console.WriteLine(results.ElementAt(1));
Console.WriteLine(results.ElementAt(2));

//  { Name = Foo, ID = 1, Item = a }
//  { Name = Bar, ID = 2, Item = d }
//  { Name = Goo, ID = 3, Item = g }


// Example types below...
public class Obj1
{
    public string Name {get; set;}
    public int Id {get; set;}

    public Obj1(string name, int id)
    {
        Name = name;
        Id = id;
    }
}

// Create some instances
Obj1 objA = new MyObject("Foo", 1);
Obj1 objB = new MyObject("Bar", 2);
Obj1 objC = new MyObject("Goo", 3);

// Your test class that contains the custom object, and a list
public class Test
{ 
    public Obj1 obj1;
    public List<string> items;

    public Test(Obj1 myobject, List<string> myItems)
    {
        obj1 = myobject;
        items = myItems;
    }
}

// Make a list of test objects
List<Test> tests = new List<Test>{
    new Test(objA, new List<string>{"a", "b", "c"}),
    new Test(objB, new List<string>{"d", "e", "f"}),
    new Test(objC, new List<string>{"g", "h", "i"})};

For new{}, you can replace with a named type T and constructor as needed and update the results list to be

IEnumerable<T>

Also, you may have a specific criteria for which item in the list you want (or all) so you could add a more interesting query in the constructor rather than .First()

Or to just grab the whole test item adjust the constructor as

new { Obj = o.obj1, Items = o.items}

Hope this helps - there are some great links in this collection: https://docs.microsoft.com/en-us/dotnet/csharp/linq/perform-a-subquery-on-a-grouping-operation