I am wanting to find the cartesian product from several lists:

var list1 = new List<int> { 1, 2, 3};
var list2 = new List<string> { "A" };
var list3 = new List<int>();

The end result I am trying to get is a list of objects with every possible combination:

{ list1Value: 1, list2Value: "A", list3Value: null },
{ list1Value: 2, list2Value: "A", list3Value: null },
{ list1Value: 3, list2Value: "A", list3Value: null }

To accomplish this I am using linq query syntax:

var combinations = from value1 in list1
    from value2 in list2
    from value3 in list3
    select new Combination
    {
        List1Value: value1,
        List2Value: value2,
        List3Value: value3
    }

However, because list3 is empty, combinations also is empty. If I give list3 at least 1 item then everything works correctly. How can I handle the empty lists in the query syntax so that it still selects from the other lists?

CodePudding user response：

Here you go:

var list1 = new List<int> { 1, 2, 3 };
var list2 = new List<string> { "A" };
var list3 = new List<int>();

var combinations =
    from value1 in list1
    from value2 in list2
    from value3 in
        list3
            .Select(x => (int?)x)
            .DefaultIfEmpty()
    select new
    {
        List1Value = value1,
        List2Value = value2,
        List3Value = value3
    };

The key change is to use .Select(x => (int?)x) which turns an int into a int? that then allows .DefaultIfEmpty() to return null, thus making the output clear when the value was missing.

The output I get is:

If I make this change:

var list3 = new List<int>() { 0, 4 };

...then the output becomes:

CodePudding user response：

Seems you are trying to bind null or empty list on another list as you have shown above. But in that case you should need a little modification of your existing list defination which is like below:

Your Given List:

var list1 = new List<int> { 1, 2, 3};
var list2 = new List<string> { "A" };
var list3 = new List<int>();

Required Modification:

 var list3 = new List<int?>();

Why The Modification Required :

As you wanted to bind your final list with nullable value which output should be like below:

Note: So you have to tell the compiler that you are going to bind list with nullable value.

Final Code Snippet :

 var list1 = new List<int> { 1, 2, 3 };
                var list2 = new List<string> { "A" };
                var list3 = new List<int?>();


                var combinations = from value1 in list1
                                   from value2 in list2
                                   from value3 in list3.DefaultIfEmpty()
                                   select new
                                   {
                                       list1 = value1,
                                       list2 = value2,
                                       list3 = value3 == 0 ? "" : null

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

Note: If you stikely want to print output as null. then replace the ternary operator like this: list3 = value3 == 0 ? null : "null". It would generate that as well.

If you want to master on linq I would suggest you to check our official document here. Hope it would help you accordingly.