Can I use a TryParse inside Linq Comparable?

Question

A sort of:

Documenti = Documenti
    .OrderBy(o => string.IsNullOrEmpty(o.Note))
    .ThenBy(o => Int32.TryParse(o.Note))
    .ToList();

That will "ignore" (not order, putting at the end) if o.Note is "" or not an int.

How can I do it?

Answer 1

Everyone who uses C#7 or newer scroll to the bottom, everyone else can read the original answer:

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Yes, you can, if you pass the correct parameters to int.TryParse. Both overloads take the int as out-parameter and initialize it inside with the parsed value. So like this:

int note;
Documenti = Documenti
    .OrderBy(o => string.IsNullOrEmpty(o.Note))
    .ThenBy(o => Int32.TryParse(o.Note, out note)) 
    .ToList();

The clean approach is using a method that parses to int and returns int? if unparseable:

public static int? TryGetInt(this string item)
{
    int i;
    bool success = int.TryParse(item, out i);
    return success ? (int?)i : (int?)null;
}

Now you can use this query(OrderByDescending because true is "greater" than false):

Documenti = Documenti.OrderByDescending(d => d.Note.TryGetInt().HasValue).ToList();

It's cleaner than using a local variable that is used in int.TryParse as out parameter.

Eric Lippert commented another answer of me where he gives an example when it might hurt:

C# LINQ: How is string("[1, 2, 3]") parsed as an array?

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Update, this has changed with C#7. Now you can declare the variable directly where you use out parameters:

Documenti = Documenti
.OrderBy(o => string.IsNullOrEmpty(o.Note))
.ThenBy(o => Int32.TryParse(o.Note, out int note)) 
.ToList();

Answer 2

Documenti = Documenti.OrderBy(o =>
        int.TryParse(o.Note, out int val)
            ? val
            : int.MaxValue /* or int.MinValue */
    ).ToList();

Note: Toggling between int.MaxValue and int.MinValue will either put the empty values at the front or the end of the list.

EDIT: 2020-02-07 Using an inline out variable which was introduced in C# 7

Answer 3

You can actually put much more complex logic in the lambda expression:

List<Doc> Documenti = new List<Doc>() {
        new Doc(""),
        new Doc("1"),
        new Doc("-4"),
        new Doc(null) };

Documenti = Documenti.OrderBy(o => string.IsNullOrEmpty(o.Note)).ThenBy(o => 
{
    int result;
    if (Int32.TryParse(o.Note, out result))
    {
        return result;
    } else {
        return Int32.MaxValue;
    }
}).ToList();

foreach (var item in Documenti)
{
    Console.WriteLine(item.Note ?? "null");
    // Order returned: -4, 1, <empty string>, null
}

Remember, o => Int32.TryParse(...) is just a shorthand for creating a delegate that just takes in o as a parameter and returns Int32.TryParse(...). You can make it do whatever you want as long as it still is a syntacticly correct method with the correct signature (ex, all code paths return an int)

Answer 4

That won't produce the expected results b/c TryParse returns a bool rather than int. The easiest thing to do is create a function that returns an int.

private int parseNote(string note) 
{   
  int num;   
  if (!Int32.TryParse(note, out num)) 
  {
    num = int.MaxValue; // or int.MinValue - however it should show up in sort   
  }

  return num; 
}

call that function from your sort

Documenti = Documenti
    .OrderBy(o => parseNote(o.Note))
    .ToList();

you could do it inline too, but, i think a separate method makes the code more readable. i'm sure the compiler will inline it, if it's an optimization.

Answer 5

C# 7 has some new features that make this even easier

var ints = from a in str.Split(',').Select(s=> new { valid = int.TryParse(s, out int i), result = i })
           where  a.valid
           select a.result;

or as you are asking specifically about sorting

var ints = from a in str.Split(',')
           orderby (int.TryParse(s, out int i) ? i : 0 )
           select a.result;