Case Sensitivity Stuff.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication1
{
    internal static class Program
    {
        private static void Main()
        {
            string[] expectedColumns =
            {
                "COLUMN1",
                "COLUMN2",
                "COLUMN3"
            };

            string[] actualColumns =
            {
                "COLUMN1",
                "coluMN2",
                "EXTRA1"
            };

            // here's where your logic starts.
            Dictionary<string, string> expectedMapping = expectedColumns.ToDictionary(x => x, StringComparer.OrdinalIgnoreCase);
            foreach (string actualColumn in actualColumns)
            {
                string expectedColumn;
                if (!expectedMapping.TryGetValue(actualColumn, out expectedColumn))
                {
                    Console.WriteLine($"WARNING: extra unexpected column ({actualColumn} wasn't expected.)");
                    continue;
                }

                // remove it to enable the "missing expected columns" loop
                expectedMapping.Remove(actualColumn);

                if (!expectedColumn.Equals(actualColumn, StringComparison.Ordinal))
                {
                    Console.WriteLine($"WARNING: case-insensitive match (expected {expectedColumn}, got {actualColumn})");
                }
            }

            // TODO: you'd actually have a "column name and importance enum" in this loop.
            foreach (string missingColumn in expectedMapping.Values)
            {
                Console.WriteLine($"CRITICAL: missing column ({missingColumn} wasn't found)");
            }
        }
    }
}