'Failed to compare two elements in the array' when Order is done

Question

I am getting an output from a DataTable for certain columns that have been defined in a string array: string[] columnsToBeUnique;

var ordered = dataTable1
                .AsEnumerable()
                .Select(column => columnsToBeUnique.Select(name => column[name]))
                .Order()
                .ToArray()
                ;

Without the .Order() I am getting the desired output. The the output however is not ordered so I want to order the output. When I add the .Order(), an error Failed to compare two elements in the array. is thrown.

I also tried:

var ordered = dataTable1
                .AsEnumerable()
                .Select(column => columnsToBeUnique.Select(name => column[name]))
                .OrderBy(x=>x)
                .ToList();

What am I doing wrong?

When you use AsEnumerable(), the item in the Select() call is a row, not a column. Additionally, you should probably also skip the ToList()/ToArray() call, and just keep the result as an Enumerable until you really need the array or list. — Joel Coehoorn
– Joel Coehoorn, Commented Apr 3 at 14:56
@JoelCoehoorn, if I skip whatever comes after the select. then I still have an unsorted output... — Rico Strydom
– Rico Strydom, Commented Apr 3 at 15:00
How do you want the results ordered? What column(s) decide this? — Joel Coehoorn
– Joel Coehoorn, Commented Apr 3 at 15:07
@JoelCoehoorn, ideally I want the sorting done on the columns in my columnsToBeUnique array. If I have 3 columns defined in my array, then first, second and then third... — Rico Strydom
– Rico Strydom, Commented Apr 3 at 15:13

AztecCodes · Accepted Answer · 2025-04-03 15:03:15Z

3

The error message you're getting is because the object you're trying to order is an IEnumerable<object> (or similar) and .NET doesn't know how to compare two sequences by default.

You are selecting a sequence of values per row (via columnsToBeUnique.Select(...)), which results in an IEnumerable<object> per row. But IEnumerable<object> doesn't implement IComparable so .Order() or .OrderBy(x => x) doesn't know how to sort it.

Solve this by using a similar type:

Join the values into a String:

var ordered = dataTable1
    .AsEnumerable()
    .Select(row => string.Join("|", columnsToBeUnique.Select(col => row[col]?.ToString())))
    .OrderBy(x => x)
    .ToList();

Use ValueTuple if you know how many columns:

var ordered = dataTable1
    .AsEnumerable()
    .Select(row => (
        columnsToBeUnique.Length > 0 ? row[columnsToBeUnique[0]] : null,
        columnsToBeUnique.Length > 1 ? row[columnsToBeUnique[1]] : null,
        columnsToBeUnique.Length > 2 ? row[columnsToBeUnique[2]] : null
    ))
    .OrderBy(x => x)
    .ToList();

The second solution would only work if columnsToBeUnique has a fixed number of columns.

answered Apr 3 at 15:03

AztecCodes

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2 Comments

Rico Strydom Apr 3 at 15:11

That did the trick! Thanks. (The number of columns are not know so the first solved the issue)

AztecCodes Apr 3 at 15:12

@RicoStrydom Glad to hear! You are welcome.

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