Add List Element Based on Conditional with Linq Expression

Question

I have this very simple Linq query (the .Dump() function is being used with LinqPad in order to view the results):

void Main()
{
    var myList = new List<string>
    {
        "One", "Two", "Three"
    };


    var result = from o in myList
    select o;

    result.Dump();
}

How can I inject an if statement into the Linq query that populates result so that it checks to see if "Four" is in the list--and if it is not, then add it? I realize that there are ways to do this by looking at result after Linq has populated result, but I would like to add this logic into a single Linq query.

Thank you.

EDIT/CLARIFICATION: The "if" logic needs to be inside the Linq query. Here is the skeletal code:

 var result = from o in myList
    select o
    [now add "four" if and only if it is not in the result]
    [now end the query and ensure `result` contains "one", "two", "three", and "four"];

SECOND EDIT: I am trying to query and add to the list in a single operation. By the way, I made this very simple example instead of giving the much more complex scenario I'm trying to handle, by the way, so it's not as if I am struggling with how to add a new element to a List object. :)

So here is what I was going to do:

// query the list to see if "four" is in the list (a Linq query goes here to check for "four")

// if the result of the above is false then add "four" to the myArray List (add the new array element here)

Instead, I would like to do the following:

// query the list for its contents and also add "four" if it is not in the list. // all this needs to be done in a single "select" using Linq

Is this not possible?

Thank you.

What makes you think that the list will be queried twice? LINQ expressions are declarative not execution logic. — Tom Blodget
– Tom Blodget, Commented Nov 8, 2016 at 1:57
Revised after your comment. Actually, while making this easy example, I think I am creating confusion. My "more complex" real Linq issue would iterate through the list twice if it has nested selects. I don't want to go into that complexity in this question, however, so I revised my OP. Thanks. — Jazimov
– Jazimov, Commented Nov 8, 2016 at 1:59

D Stanley · Accepted Answer · 2016-11-08 01:34:39Z

3

You can use Union if you switch to method syntax:

var result = myList.Union(new [] {"Four"});

There's not really a clean way to to it using just query syntax. Query syntax only supports a small set of Linq operators, of which union is not one.

answered Nov 8, 2016 at 1:34

D Stanley

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9 Comments

D Stanley Over a year ago

@Jazimov Union does that by definition. It's the "union" set of the two collections, which by definition does not include items in both lists twice.

D Stanley Over a year ago

There's not an if operator if that's what you're looking for. I suppose you could do .Concat(new [] {"four"}).Where(s => !myList.Contains(s)) but that's effectively the same as Union.

D Stanley Over a year ago

Union will not query either list twice. It will keep track of what items have already bee found so it doesn't return the same element twice.

D Stanley Over a year ago

Note that Where(!Contains) will enumerate myList again, so if that is your goal then Where is not the right answer.

Tom Blodget Over a year ago

Note: Union removes all duplicates, which has not been discussed.

|

Will Ray · Accepted Answer · 2016-11-08 00:44:35Z

3

You could create a second list of items that you want to conditionally add, then combine the Except and Concat methods:

var myList = new List<string>
{
    "One", "Two", "Three"
};

var others = new List<string> {
    "Four"
};

var result = myList.Concat(others.Except(myList));

result.Dump();

answered Nov 8, 2016 at 0:44

Will Ray

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5 Comments

Jazimov Over a year ago

Please see clarification in question. I need the conditional logic to be inside a single Linq construct. Thanks.

Will Ray Over a year ago

@Jazimov Do you mean that you want to use the query syntax? Or use only one method call?

Jazimov Over a year ago

I gave a skeletal example. I suppose I would answer that I want to use the query syntax vs one method call. Thanks.

Jazimov Over a year ago

I might have missed your .Except() above. If so, apologies. I think, however, that Union might be a more direct answer (see answer below this one). But thanks for illustrating .Except, it will be useful to me.

Tom Blodget Over a year ago

You could also flip it: myList.Except(others).Concat(others) but this wouldn't preserve the order of the original list.

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