Convert Char, String. A char can be converted to a one-character string. There are several ways to do this. The clearest method is the ToString method.

Notes, performance. Other implementations may have better performance. For example, we can use the string constructor. We test these methods.

First example. Here we have a character "x" and we convert it into a string with 2 different method calls. We use ToString, and the string constructor.

using System;

char value = 'x';

// Perform conversions.
string result = value.ToString();
Console.WriteLine("RESULT  LENGTH: {0}", result.Length);

string result2 = new string(value, 1);
Console.WriteLine("RESULT2 LENGTH: {0}", result2.Length);
RESULT  LENGTH: 1
RESULT2 LENGTH: 1

Benchmark. In the Main entry point, both methods return an equivalent string from the char argument. Their results are equal. We test their runtime speed.

using System;
using System.Diagnostics;

class Program
{
    static string Method1(char value)
    {
        return new string(value, 1);
    }

    static string Method2(char value)
    {
        return value.ToString();
    }

    const int _max = 100000000;
    static void Main()
    {
        Console.WriteLine(Method1('c'));
        Console.WriteLine(Method2('c'));

        var s1 = Stopwatch.StartNew();
        // Version 1: use string constructor.
        for (int i = 0; i < _max; i++)
        {
            Method1('c');
        }
        s1.Stop();
        // Version 2: use ToString.
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            Method2('c');
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }
}
c
c
10.37 ns
10.48 ns

Timing results. On my system, the results consistently indicated that the string constructor was slightly faster. And this held true regardless of what order the methods were tested.

Lookup tables. We could implement a lookup table to get cached 1-char strings. The char could be used as an index into a precomputed array of strings.

A summary. It is simplest to just use the ToString method call. We can sometimes use Substring to take a one-character string from an input string, avoiding the char type.