C# string.MaskStart(), .MaskEnd(), .Chop() .ToWikiWords(), .PascalCaseToWords() .WithWhiteSpaceRemoved() StringJoin.WhereNotNull() StringJoin.WhereNotNullOrEmpty()()

StringJoinWhereNotNull.cs

/// <summary>
/// Extensions of <see cref="string.Join"/>
/// </summary>
public static partial class StringJoin
{
    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{string})"/>
    /// </returns>
    public static string WhereNotNull(string separator, IEnumerable<string?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }
    
    

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{object})"/>
    /// </returns>
    public static string WhereNotNull(string separator, IEnumerable<object?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(string,string[])"/>
    /// </returns>
    public static string WhereNotNull(string separator, params string?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null elements.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(string,object[])"/>
    /// </returns>
    public static string WhereNotNull(string separator, params object?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }
    
    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(char,IEnumerable{string})"/>
    /// </returns>
    public static string WhereNotNull(char separator, IEnumerable<string?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(char,IEnumerable{object})"/>
    /// </returns>
    public static string WhereNotNull(char separator, IEnumerable<object?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(char,string[])"/>
    /// </returns>
    public static string WhereNotNull(char separator, params string?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null values.
    /// Otherwise returns <see cref="string.Join(char,object[])"/>
    /// </returns>
    public static string WhereNotNull(char separator, params object?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s != null));
    }    
}

StringJoinWhereNotNullOrEmpty.cs

/// <summary>
/// Extensions of <see cref="string.Join"/>
/// </summary>
public static partial class StringJoin
{
    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{string})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(string separator, IEnumerable<string?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));
    }
    
    

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,string[])"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(string separator, params string?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member.
    /// omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(char,IEnumerable{string})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(char separator, IEnumerable<string?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(char,string[])"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(char separator, params string?[] values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));
    }

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{T})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(char separator,params object[] values)
        => WhereNotNullOrEmpty(separator, (IEnumerable<object?>)values);

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{T})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(char separator,IEnumerable<object?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s is not null && !string.IsNullOrEmpty(s.ToString())));
    }
    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{T})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(string separator,params object[] values)
        => WhereNotNullOrEmpty(separator, (IEnumerable<object?>)values);

    /// <summary>
    /// Concatenates the elements of a specified array or collection,
    /// using the specified separator between each element or member,
    /// but omitting null or empty strings.
    /// </summary>
    /// <param name="separator">is included in the returned string only if
    /// <paramref name="values"/> has more than one element</param>
    /// <returns>
    /// Empty string if values is null or empty or contains only null or empty values.
    /// Otherwise returns <see cref="string.Join(string,IEnumerable{T})"/>
    /// </returns>
    public static string WhereNotNullOrEmpty(string separator,IEnumerable<object?> values)
    {
        if (values == null) return string.Empty;
        return string.Join(separator, values.Where(s => s is not null && !string.IsNullOrEmpty(s.ToString())));
    }
}

Strings.cs

using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Text.RegularExpressions;

/// <summary>
/// String extensions including
/// <list type="bullet">
/// <item>Mask</item> <item>Chop</item> <item>Trim(str)</item>
/// <item>PascalCaseToWords</item> <item>RegexReplace</item> <item>Matches</item>
/// <item>WithHtmlLineBreaks</item> <item>IsNullOrEmpty</item> <item></item>
/// <item>WithWhiteSpaceRemoved</item>
/// </list>
/// </summary>
public static class Strings
{
    /// <summary>Trim one occurrence of <paramref name="terminator"/> string from
    /// the end of <paramref name="str"/>, if there is one. 
    /// </summary>
    /// <param name="str"></param>
    /// <param name="terminator"></param>
    /// <returns>
    /// <c>str.Substring(0, str.Length - terminator.Length)</c> if <paramref name="str"/>
    /// ends with <paramref name="terminator"/>, or else returns <paramref name="str"/> if not.
    /// </returns>
    [return: NotNullIfNotNull("str")]public static string? TrimEnd(this string? str, string terminator)
    {
        if (terminator is null || str is null 
                               || str.Length == 0 || terminator.Length == 0 
                               || terminator.Length > str.Length)  return str;

        if (str.EndsWith(terminator)) return str.Substring(0, str.Length - terminator.Length);
        return str;
    }
        
    /// <summary>Trim one occurrence of <paramref name="terminator"/> string from
    /// the start of <paramref name="str"/>, if there is one. 
    /// </summary>
    /// <param name="str"></param>
    /// <param name="terminator"></param>
    /// <returns>
    /// <c>str.Substring(terminator.Length, str.Length - terminator.Length)</c>
    /// if <paramref name="str"/> starts with <paramref name="terminator"/>,
    /// or else returns <paramref name="str"/> if not.
    /// </returns>
    [return: NotNullIfNotNull("str")]public static string? TrimStart(this string str, string terminator)
    {
        if (terminator is null || str is null 
                               || str.Length == 0 || terminator.Length == 0 
                               || terminator.Length > str.Length)  return str;

        if (str.StartsWith(terminator)) 
            return str.Substring(terminator.Length, str.Length - terminator.Length);
        return str;
    }
        
    [return: NotNullIfNotNull("str")]public static string? Trim(this string? str, string terminator) 
        => str?.TrimEnd(terminator)?.TrimStart(terminator);

       
    /// <summary>Chop the string to maximum length of <paramref name="maxlength"/></summary>
    /// <param name="str"></param>
    /// <param name="maxlength"></param>
    /// <returns>the chopped string. If the string is shorter than <paramref name="maxlength"/>
    /// then the whole string is returned.</returns>
    [return: NotNullIfNotNull("str")]public static string? Chop(this string? str, int maxlength)
    {
        if (str is null) return null;
        if (str.Length <= maxlength) return str;
        return str[..maxlength];
    }
       
    /// <summary>
    /// A synonym for <see cref="Chop"/>.
    /// Chop the string to maximum length of <paramref name="maxlength"/>
    /// </summary>
    /// <param name="str"></param>
    /// <param name="maxlength"></param>
    /// <returns>the chopped string. If the string is shorter than <paramref name="maxlength"/>
    /// then the whole string is returned.</returns>
    [return: NotNullIfNotNull("str")]public static string? Truncate(this string? str, int maxlength)
        => Chop(str,maxlength);
    
    
    /// <summary>Replaces newline characters — <c>\n</c> or <see cref="Environment.NewLine"/> — 
    /// with <c>&gt;br/></c></summary>
    /// <param name="str"></param>
    /// <returns>the altered string</returns>
    public static string WithHtmlLineBreaks(this string str)
        => str.Replace("\n", "<br/>").Replace(Environment.NewLine,"<br/>");

    /// <summary>Abbreviation for <c>str.LastIndexOf(sought, StringComparison.Ordinal)</c></summary>
    public static int LastIndexOfOrdinal(this string str, string sought) 
        => str.LastIndexOf(sought, StringComparison.Ordinal);
        
    /// <summary>Abbreviation for <see cref="string.IsNullOrEmpty"/></summary>
    public static bool IsNullOrEmpty(this string? str) => string.IsNullOrEmpty(str);

    static readonly Regex RegexlowerUpper = new Regex(@"(\p{Ll}|\d)(\p{Lu})");
    static readonly Regex RegexWord_Word  = new Regex(@"(\S)_(\S)");
    static readonly Regex Regex_Word      = new Regex(@"_(\S)");
    static readonly Regex RegexWord_      = new Regex(@"(\S)_");

    
    /// <summary>
    /// Abbreviation for <see cref="TextInfo.ToTitleCase"/>
    /// </summary>
    /// <param name="str"></param>
    /// <param name="cultureInfo"></param>
    /// <param name="keepAllCaps">If true, preserves the behaviour of <see cref="TextInfo.ToTitleCase"/> which
    /// does not title-case an all-caps word (imagining it to be an acronym). If false, then also title-case
    /// allcaps words</param>
    /// <returns>
    /// <c>cultureInfo.TextInfo.ToTitleCase( keepAllCaps ? str : str.ToLower(cultureInfo))</c>
    /// if <paramref name="str"/> is not null. Otherwise <c>null</c>
    /// </returns>
    [return: NotNullIfNotNull("str")]
    public static string? ToTitleCase(this string? str, CultureInfo? cultureInfo = null, bool keepAllCaps = true)
    {
        if (str is null) return str;
        cultureInfo ??= CultureInfo.CurrentCulture;
        return cultureInfo.TextInfo.ToTitleCase( keepAllCaps ? str : str.ToLower(cultureInfo));
    }

    /// <summary>
    /// Convert PascalCase string to words, e.g. :
    /// <list type="bullet">
    /// <item>PascalCase->"Pascal Case"</item>
    /// <item>Pascal1Case->"Pascal1 Case"</item>
    /// <item>PascalCaseB->"Pascal Case B"</item>
    /// </list>
    /// </summary>
    /// <param name="wikiWordString"></param>
    /// <returns>The transformed string</returns>
    [return:NotNullIfNotNull("wikiWordString")]public static string? PascalCaseToWords(this string? wikiWordString) 
        => wikiWordString == null ? null :RegexlowerUpper.Replace(wikiWordString, "$1 $2");

    /// <summary>Convert Snake_Cased_Strings to Words - with - Hyphens
    /// <list type="bullet">
    /// <item>Snake_Cased_Strings->"Snake - Cased - Strings"</item>
    /// <item>Snake _ Cased->"Snake - Cased"</item>
    /// <item>Snake_Cased_ ->"Snake - Cased -"</item>
    /// </list>
    /// </summary>
    /// <param name="snakeCasedString"></param>
    /// <returns>The transformed string</returns>
    [return:NotNullIfNotNull("snakeCasedString")]public static string? UnderscoreToHyphenedWords(this string? snakeCasedString)
        => snakeCasedString?.RegexReplace(RegexWord_Word, "$1 - $2")
            .RegexReplace(Regex_Word, "- $1")
            .RegexReplace(RegexWord_, "$1 -")
            .Replace("_","-");

    /// <summary>
    /// Combines both <see cref="PascalCaseToWords"/> and <see cref="UnderscoreToHyphenedWords"/>
    /// <list type="bullet">
    /// <item>PascalCased_Snake->"Pascal Cased - Snake"</item>
    /// <item>PascalCased _ Snake->"Pascal Cased - Snake"</item>
    /// <item>PascalCased_->"Pascal Cased -"</item>
    /// </list>
    /// </summary>
    /// <param name="str"></param>
    /// <returns>the transformed string</returns>
    public static string PascalSnakeToHyphenedWords(this string str)
        => str.UnderscoreToHyphenedWords().PascalCaseToWords();
        
    /// <summary>
    /// Combines both <see cref="PascalCaseToWords"/> and <see cref="UnderscoreToHyphenedWords"/>
    /// to an enum's ToString() value
    /// <list type="bullet">
    /// <item>PascalCased_Snake->"Pascal Cased - Snake"</item>
    /// <item>PascalCased _ Snake->"Pascal Cased - Snake"</item>
    /// <item>PascalCased_->"Pascal Cased -"</item>
    /// </list>
    /// </summary>
    /// <param name="value">an enum value of type <typeparamref name="T"/></param>
    /// <returns>the transformed string</returns>
    public static string PascalSnakeToHyphenedWords<T>(this T value) where T : struct,Enum
        => value.ToString().UnderscoreToHyphenedWords().PascalCaseToWords();
        
    /// <summary>Abbreviation for <c>regex.Replace(str, replace)</c></summary>
    [return: NotNullIfNotNull("str")]public static string? RegexReplace(this string? str, Regex regex, string replace)
        => str is null ? null : regex.Replace(str, replace);
        
    /// <summary>Abbreviation for <c>regex.Replace(str, replace)</c></summary>
    [return: NotNullIfNotNull("str")]public static string? RegexReplace(this string? str, string pattern, string replace)
        => str is null ? null : Regex.Replace(str, pattern,replace);
    
    /// <summary>Removes spaces, newlines (<c>\n</c>)and tabs (<c>\t</c>) from <paramref name="str"/> 
    /// and returns the results</summary>
    /// <param name="str"></param>
    /// <returns></returns>
    [return: NotNullIfNotNull("str")]public static string? WithWhiteSpaceRemoved(this string str) 
        => str?.Replace(" ", "").Replace("\n", "").Replace("\r", "").Replace("\t", "");

    /// <summary>Abbreviation for <c>Regex.IsMatch(@this, pattern, options)</c></summary>
    /// <param name="str">The input string</param>
    /// <param name="pattern">the regex pattern to match against</param>
    /// <param name="options">the <see cref="RegexOptions"/> to use</param>
    /// <returns><c>true</c> if <paramref name="str"/> matches the pattern with the options</returns>
    public static bool Matches(this string str, string pattern, RegexOptions options = RegexOptions.None) 
        => Regex.IsMatch(str, pattern, options);

    /// <summary>Masks – that is, overwrites with the mask character – the beginning of a string.
    /// </summary>
    /// <example>
    /// <c>"hello there".MaskToFirst('l')</c> => <c>"***lo there"</c>
    /// </example>
    /// <param name="str"></param>
    /// <param name="marker">The phrase to mask as far as.</param>
    /// <param name="maskChar"></param>
    /// <param name="inclusive">If this is 0, then the marker phrase itself is not masked.
    /// If it is 1, then the first character of marker is masked.
    /// Any other value (positive or negative) is an offset from the marker for how far to mask
    /// from the first character of marker.</param>
    /// <seealso cref="MaskEnd"/>
    /// <seealso cref="MaskStart"/>
    /// <returns>The masked string.</returns>
    public static string MaskToFirst(this string str, string marker, char maskChar = '*', int inclusive=1)
        => MaskStart(str, str.LastIndexOfOrdinal(marker)+inclusive, maskChar);

    /// <summary>Masks – that is, overwrites with the mask character – the beginning of a string.
    /// </summary>
    /// <example>
    /// <c>"hello there".MaskToFirst('l')</c> => <c>"***lo there"</c>
    /// </example>
    /// <param name="str"></param>
    /// <param name="marker">The character to mask as far as.</param>
    /// <param name="maskChar"></param>
    /// <param name="offset">If this is 0, then the marker phrase itself is masked.
    /// If it is 1, then the first character of the marker phrase is not masked.
    /// Any other value (positive or negative) is an offset from the first character of marker
    /// for how far to mask.</param>
    /// <seealso cref="MaskEnd"/>
    /// <seealso cref="MaskStart"/>
    /// <returns>The masked string.</returns>
    public static string MaskFromLast(this string str, string marker, char maskChar = '*', int offset=0)
    {
        var index = str.LastIndexOfOrdinal(marker);
        return index == -1 
            ? MaskEnd(str, offset, maskChar) 
            : MaskEnd(str, str.Length-index + offset, maskChar);
    }

    /// <summary>Masks – that is, overwrites with the mask character – the beginning of a string.
    /// </summary>
    /// <example>
    /// <c>"hello there".MaskStart(3)</c> => <c>"***lo there"</c>
    /// <c>"hello there".MaskStart(-3)</c> => <c>"********ere"</c>
    /// </example>
    /// <param name="str"></param>
    /// <param name="masked">How many characters to mask. If masked is negative it is interpreted
    /// as “How many characters to leave unmasked”</param>
    /// <param name="maskChar"></param>
    /// <seealso cref="MaskEnd"/>
    /// <returns>The masked string.
    /// If <paramref name="str"/> is shorter than <paramref name="masked"/> then only a
    /// string of <paramref name="maskChar"/> of length <paramref name="masked"/> is returned
    /// If <paramref name="masked"/> is longer than <c>me.Length</c> then only a
    /// string of <paramref name="maskChar"/> of length <c>me.Length</c> is returned.
    /// If <paramref name="masked"/>==0 then the original string is returned.
    /// </returns>
    public static string MaskStart(this string str, int masked=4, char maskChar = '*')
    {
        if (string.IsNullOrEmpty(str)) return str;
        if (masked==0) return str;
        var length = str.Length;
        if (masked >= length) return new string(maskChar,length);
        int unmasked;
        if (masked < 0)
        {
            unmasked = -masked;
            if (unmasked > length) unmasked = length;
            masked = length - unmasked;
        }
        else
        {
            unmasked = length - masked;
        }
        return str.Substring(masked, unmasked).PadLeft(str.Length, maskChar);
    }
        
    /// <summary>Masks – that is, overwrites with the mask character – the end of a string.
    /// </summary>
    /// <example>
    /// <c>"hello there".MaskEnd(3)</c> => <c>"Hello th***"</c>
    /// <c>"hello there".MaskEnd(-3)</c> => <c>"Hel********"</c>
    /// </example>
    /// <param name="str"></param>
    /// <param name="masked">How many characters to mask. If masked is negative it is interpreted
    /// as “How many characters to leave unmasked”</param>
    /// <param name="maskChar"></param>
    /// <seealso cref="MaskStart"/>
    /// <returns>The masked string.
    /// If <paramref name="str"/> is shorter than <paramref name="masked"/> then only a
    /// string of <paramref name="maskChar"/> of length <paramref name="masked"/> is returned
    /// If <paramref name="masked"/> is longer than <c>me.Length</c> then only a
    /// string of <paramref name="maskChar"/> of length <c>me.Length</c> is returned.
    /// If <paramref name="masked"/>==0 then the original string is returned.
    /// </returns>
    public static string MaskEnd(this string str, int masked=4, char maskChar = '*')
    {
        if (string.IsNullOrEmpty(str)) return str;
        if (masked==0) return str;
        var length = str.Length;
        if (masked >= length) return new string(maskChar,length);
        int unmasked;
        if (masked < 0)
        {
            unmasked = -masked;
            if (unmasked > length) unmasked = length;
        }
        else
        {
            unmasked = length - masked;
        }
        return str.Substring(0, unmasked).PadRight(str.Length, maskChar);
    }
}