c# - 'Failed to compare two elements in the array' when Order is done - Stack Overflow

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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?

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?

• c#
• linq
• lambda
• datatable

Share Improve this question Follow edited 20 hours ago jonrsharpe 122k3030 gold badges268268 silver badges475475 bronze badges asked 20 hours ago Rico StrydomRico Strydom 61111 gold badge99 silver badges2828 bronze badges 4

• 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 Commented 20 hours ago
• @JoelCoehoorn, if I skip whatever comes after the select. then I still have an unsorted output... – Rico Strydom Commented 20 hours ago
• How do you want the results ordered? What column(s) decide this? – Joel Coehoorn Commented 20 hours ago
• @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 Commented 20 hours ago

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1 Answer 1

Sorted by: Reset to default 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.

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