An extension to StringBuilder for ReplaceAt functionality. Given an index, and optional length, and a string to overwrite, this method overwrites an area of characters.

StringBuilderExtensions.cs

namespace System.Text
{
  internal static class StringBuilderExtensions
	{
#if EXTENSIONS_SUPPORTED
		public static StringBuilder ReplaceAt(this StringBuilder sb, string value, int index, int count)
		{
			return StringBuilderExtensions.ReplaceAt(sb, value, index, count);
		}
#endif

		public static StringBuilder ReplaceAt(StringBuilder sb, int index, int length, string value)
		{
			if (string.IsNullOrEmpty(value))
			{
				sb.Remove(index, length);
			}
			else
			{
				if (index + length > sb.Length) 
					throw new ArgumentException(string.Format("Given the index of '{0}' and length of '{1}', the replacement area extends beyond the length of the string.", index, length));
				
				if (value.Length > length)
				{
					sb.Insert(index, new String('#',value.Length-length));
				}

				if (value.Length < length)
				{
					sb.Remove(index + value.Length, length - value.Length);
				}

				for (int i = index, j = 0; j < value.Length; i++, j++)
				{
					sb[i] = value[j];
				}
			}

			return sb;
		}
	}
}