Replace all occurrences of a string (in array) with a single value

Instead of using the Replace over and over you could just write your own. This might even be a performance gain since you mentioned

But my array may get a lot bigger.

public string Replace(string original, char replacement, params char[] replaceables)
{
    StringBuilder builder = new StringBuilder(original.Length);
    HashSet<char> replaceable = new HashSet<char>(replaceables);
    foreach(Char character in original)
    {
        if (replaceable.Contains(character))
            builder.Append(replacement);
        else
            builder.Append(character);
    }
    return builder.ToString();
}

public string Replace(string original, char replacement, string replaceables)
{
    return Replace(original, replacement, replaceables.ToCharArray());
}

Can be called like this:

Debug.WriteLine(Replace("a/ new string, with some@ values&", '_', '/', '@', '&'));
Debug.WriteLine(Replace("a/ new string, with some@ values&", '_', new[] { '/', '@', '&' }));
Debug.WriteLine(Replace("a/ new string, with some@ values&", '_', existingArray));
Debug.WriteLine(Replace("a/ new string, with some@ values&", '_',"/@&"));

Output:

a_ new string, with some_ values_
a_ new string, with some_ values_
a_ new string, with some_ values_
a_ new string, with some_ values_

As @Sebi pointed out, this would also work as an extension method:

public static class StringExtensions
{
    public static string Replace(this string original, char replacement, params char[] replaceables)
    {
        StringBuilder builder = new StringBuilder(original.Length);
        HashSet<Char> replaceable = new HashSet<char>(replaceables);
        foreach (Char character in original)
        {
            if (replaceable.Contains(character))
                builder.Append(replacement);
            else
                builder.Append(character);
        }
        return builder.ToString();
    }

    public static string Replace(this string original, char replacement, string replaceables)
    {
        return Replace(original, replacement, replaceables.ToCharArray());
    }
}

Usage:

"a/ new string, with some@ values&".Replace('_', '/', '@', '&');
existingString.Replace('_', new[] { '/', '@', '&' });
// etc.

This is how i'd do it building a regex clause from the list of delimiters and replacing them with an underscore

string[] delimiters = { "/", "@", "&" };
string clause = $"[{string.Join("]|[", delimiters)}]";
string strValue = "a/ new string, with some@ values&";
Regex chrsToReplace = new Regex(clause);
string output = chrsToReplace.Replace(strValue, "_");

You'll probably want to encapsulate within if(delimiters.Any()), else it will crash if the array is empty

Sure. Here's one approach:

var newString = arr2.Aggregate(strValue, (net, curr) => net.Replace(curr, "_"));

If you're only substituting individual characters and have large enough input sizes to need optimization, you can create a set from which to substitute:

var substitutions = new HashSet<char>() { '/', '@', '&' };
var strValue = "a/ new string, with some@ values&";
var newString = new string(strValue.Select(c => substitutions.Contains(c) ? '_' : c).ToArray());

Maybe not the fastest but the easiest would be a Select with a Contains.

Something like this : source.Select(c => blacklist.Contains(c) ? letter : c)

Demo on .NetFiddle.

using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var strValue = "a/ new string, with some@ values&";

        Console.WriteLine(strValue.Replace("/@&", '_'));
    }
}

public static class Extensions {
    public static string Replace(this string source, string blacklist, char letter) => 
        new string(source.Select(c => blacklist.Contains(c) ? letter : c).ToArray());
}

You can split your string with your list of string []:

string[] arr2 = { "/", "@", "&" };
string strValue = "a/ new string, with some@ values&";
string Output = null;
string[] split = strValue.Split(arr2, StringSplitOptions.RemoveEmptyEntries);
foreach (var item in split)
{
    Output += item + "_";
}
Console.WriteLine(Output);
//-> a_ new string, with some_ values_

string[] arr2 = { "/", "@", "&" };
string strValue = "a/ new string, with some@ values&";
string[] split = strValue.Split(arr2, StringSplitOptions.RemoveEmptyEntries);

StringBuilder Output = new StringBuilder();
foreach (var item in split)
{
    Output.Append(item + "_");
}
Console.WriteLine(Output);
//-> a_ new string, with some_ values_

You could use a foreach in a single line to achieve what you want:

arr2.ToList().ForEach(x => strValue = strValue.Replace(x, "_"));