Midoliy.Office.Interop.Excel.Extensions.cs

using System;

static void Test(string columnName) => Console.WriteLine("{0,-3} => {1,4} => {2,-3}", columnName, columnName.ToColumnNumber(), columnName.ToColumnNumber().ToColumnName());

Test("A");
Test("Z");
Test("AA");
Test("ZZ");
Test("AAA");
Test("BBB");

public static class StringEx {
    static int ToNumber(char ch) =>
        'A' <= ch && ch <= 'Z' ? ch - 'A' + 1 :
        'a' <= ch && ch <= 'z' ? ch - 'a' + 1 :
        throw new ArgumentOutOfRangeException("The string must contain only A to Z.");
    public static int ToColumnNumber(this string @this) =>
        @this.Length switch {
            1 => ToNumber(@this[0]),
            2 => ToNumber(@this[0]) * 26 + ToNumber(@this[1]),
            3 => ToNumber(@this[0]) * 26 * 26 + ToNumber(@this[1]) * 26 + ToNumber(@this[2]),
            _ => throw new ArgumentOutOfRangeException("The string length must be 1 to 3."),
        };
}

public static class IntegerEx {
    const string _1digits = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const string _2digits = "AAABACADAEAFAGAHAIAJAKALAMANAOAPAQARASATAUAVAWAXAYAZBABBBCBDBEBFBGBHBIBJBKBLBMBNBOBPBQBRBSBTBUBVBWBXBYBZCACBCCCDCECFCGCHCICJCKCLCMCNCOCPCQCRCSCTCUCVCWCXCYCZDADBDCDDDEDFDGDHDIDJDKDLDMDNDODPDQDRDSDTDUDVDWDXDYDZEAEBECEDEEEFEGEHEIEJEKELEMENEOEPEQERESETEUEVEWEXEYEZFAFBFCFDFEFFFGFHFIFJFKFLFMFNFOFPFQFRFSFTFUFVFWFXFYFZGAGBGCGDGEGFGGGHGIGJGKGLGMGNGOGPGQGRGSGTGUGVGWGXGYGZHAHBHCHDHEHFHGHHHIHJHKHLHMHNHOHPHQHRHSHTHUHVHWHXHYHZIAIBICIDIEIFIGIHIIIJIKILIMINIOIPIQIRISITIUIVIWIXIYIZJAJBJCJDJEJFJGJHJIJJJKJLJMJNJOJPJQJRJSJTJUJVJWJXJYJZKAKBKCKDKEKFKGKHKIKJKKKLKMKNKOKPKQKRKSKTKUKVKWKXKYKZLALBLCLDLELFLGLHLILJLKLLLMLNLOLPLQLRLSLTLULVLWLXLYLZMAMBMCMDMEMFMGMHMIMJMKMLMMMNMOMPMQMRMSMTMUMVMWMXMYMZNANBNCNDNENFNGNHNINJNKNLNMNNNONPNQNRNSNTNUNVNWNXNYNZOAOBOCODOEOFOGOHOIOJOKOLOMONOOOPOQOROSOTOUOVOWOXOYOZPAPBPCPDPEPFPGPHPIPJPKPLPMPNPOPPPQPRPSPTPUPVPWPXPYPZQAQBQCQDQEQFQGQHQIQJQKQLQMQNQOQPQQQRQSQTQUQVQWQXQYQZRARBRCRDRERFRGRHRIRJRKRLRMRNRORPRQRRRSRTRURVRWRXRYRZSASBSCSDSESFSGSHSISJSKSLSMSNSOSPSQSRSSSTSUSVSWSXSYSZTATBTCTDTETFTGTHTITJTKTLTMTNTOTPTQTRTSTTTUTVTWTXTYTZUAUBUCUDUEUFUGUHUIUJUKULUMUNUOUPUQURUSUTUUUVUWUXUYUZVAVBVCVDVEVFVGVHVIVJVKVLVMVNVOVPVQVRVSVTVUVVVWVXVYVZWAWBWCWDWEWFWGWHWIWJWKWLWMWNWOWPWQWRWSWTWUWVWWWXWYWZXAXBXCXDXEXFXGXHXIXJXKXLXMXNXOXPXQXRXSXTXUXVXWXXXYXZYAYBYCYDYEYFYGYHYIYJYKYLYMYNYOYPYQYRYSYTYUYVYWYXYYYZZAZBZCZDZEZFZGZHZIZJZKZLZMZNZOZPZQZRZSZTZUZVZWZXZYZZ";
    public static string ToColumnName(this int @this) =>
        @this <= 0 ? throw new ArgumentOutOfRangeException("Should be a value greater than 0.") :
        @this <= 26 ? _1digits.Substring(@this - 1, 1) :
        @this <= 27 * 26 ? _2digits.Substring((@this - 27) * 2, 2) :
        string.Concat((char)('A' + @this / 26 / 26 - 1), (char)('A' + @this / 26 % 26 - 1), (char)('A' + @this % 26 - 1));
}