As an alternative, and perhaps an overkill, here's a StrTran which uses Regex

// test
using System;

using GregoryAdam.Base.ExtensionMethods;

namespace BaseTest
{
	class test3
	{

		//______________________________________________________________________
		static void Main()
		{
			string[,] test = 
			{
				{ "hello", "o", "123", "hell123"},
				{ "+1+2", "+", "-", "-1-2" },
				{ "$1$2", "$", "-", "-1-2" },
				{ "{1+{2", "{", "-", "-1+-2" },
				{ "{1+{2", "{", "$", "$1+$2" },
				{ "hello", "O", "$$", "hell$$"},
				{ "hello", "H", @"$1\$2", @"$1\$2ello"},
				{ "hello", "H", null, @"ello"}
			};

			for (int i = 0; i <= test.GetUpperBound(0); i++)
			{
				string input = test[i, 0];
				string from = test[i, 1];
				string to = test[i, 2];
				string expect = test[i, 3];

				string got = input.Replace(from, to, true);

				Console.WriteLine("input: {0}, expect: {1}, got: {2}, result: {3}", input, expect, got, expect == got);
			}
			Console.ReadLine();
		}

	}
}

using System;
using System.Text;
using System.Text.RegularExpressions;

using GregoryAdam.Base.Constants;

namespace GregoryAdam.Base.ExtensionMethods
{
	public static partial class ExtensionMethods_String
	{
		//______________________________________________________________________
		/// <summary>
		/// Replaces every occurrence of oldValue with newValue using Regex
		/// </summary>
		/// <param name="s"></param>
		/// <param name="oldValue">The string to be replaced</param>
		/// <param name="newValue">The string that replaces all occurrences of oldValue
		/// <para>May be null or the empty string</para></param>
		/// </param>
		/// <param name="ignoreCase"></param>
		/// <returns></returns>
		public static string Replace(
				this string s,
				string oldValue,
				string newValue,
				bool ignoreCase
			)
		{
			if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(oldValue))
				return s;

			// oldValuePattern
			string oldValuePattern = Regex.Replace(
									oldValue, 
									RegexConstants.SpecialPatternCharsPattern, 
									RegexConstants.SpecialPatternCharsPatternReplace
									);

			string newValuePattern;
			if (string.IsNullOrEmpty(newValue))
				newValuePattern = "";
			else
				newValuePattern = Regex.Replace(
									newValue,
									RegexConstants.SpecialReplaceCharsPattern,
									RegexConstants.SpecialReplaceCharsPatternReplace
									);


			RegexOptions regexOptions = !ignoreCase?  
						RegexOptions.None 
					: RegexOptions.CultureInvariant | RegexOptions.IgnoreCase;

			return Regex.Replace(s, oldValuePattern, newValuePattern, regexOptions);

		}
		//______________________________________________________________________
	}
}

using System.Text;

namespace GregoryAdam.Base.Constants
{
	public static class RegexConstants
	{
		//______________________________________________________________________
		/// <summary>
		/// Characters that need to be escaped in a pattern
		/// </summary>
		public static readonly char[] SpecialPatternChars =
		{	'\\', '^', '$', '.', 
			'[', ']',
			'(', ')', '|', 
			'{', '}',
			'*', '+', '?'
		};
		//______________________________________________________________________
		public static readonly string SpecialPatternCharsPattern =
				Build_SpecialPatternCharsPattern(SpecialPatternChars);
		//______________________________________________________________________
		public static string SpecialPatternCharsPatternReplace = @"\$1";
		//______________________________________________________________________
		/// <summary>
		/// chars that have to be escaped with $ in the replacement pattern
		/// </summary>
		public static readonly char[] SpecialReplaceChars = 
			{	'$' };
		//______________________________________________________________________
		public static readonly string SpecialReplaceCharsPattern =
			Build_SpecialPatternCharsPattern(SpecialReplaceChars);
		//______________________________________________________________________
		public static readonly string SpecialReplaceCharsPatternReplace = @"$$$1";
		//______________________________________________________________________
		private static string Build_SpecialPatternCharsPattern(char[] chars)
		{
			// a b c   becomes
			// (\a|\b|\c)
			// length * 3 + 1 + 1 - 1
			StringBuilder sb = new StringBuilder(chars.Length * 3 + 1);
			sb.Append('(');

			foreach (char c in chars)
			{
				sb.AppendFormat(@"\{0}|", c);
			}

			sb[sb.Length-1] = ')';

			return sb.ToString();
		}
		//______________________________________________________________________
	}
}