Why doesn't StringBuilder have IndexOf method?

Question

I understand that I can call ToString().IndexOf(...), but I don\'t want to create an extra string. I understand that I can write a search routine manually. I just wonder why

Answer 1

Unfortunately, many of the methods implemented for String could have been implemented for StringBuilder but that was not done. Consider using extension methods to add what you care about.

Answer 2

Calling ToString() on a StringBuilder doesn't create an extra object, confusingly. Internally, StringBuilder stores a String object, for performance; calling ToString() simply returns that object.

Answer 3

I know this is an old question, however I have written a extension method that performs an IndexOf on a StringBuilder. It is below. I hope it helps anyone that finds this question, either from a Google search or searching StackOverflow.

/// <summary>
/// Returns the index of the start of the contents in a StringBuilder
/// </summary>        
/// <param name="value">The string to find</param>
/// <param name="startIndex">The starting index.</param>
/// <param name="ignoreCase">if set to <c>true</c> it will ignore case</param>
/// <returns></returns>
public static int IndexOf(this StringBuilder sb, string value, int startIndex, bool ignoreCase)
{            
    int index;
    int length = value.Length;
    int maxSearchLength = (sb.Length - length) + 1;

    if (ignoreCase)
    {
        for (int i = startIndex; i < maxSearchLength; ++i)
        {
            if (Char.ToLower(sb[i]) == Char.ToLower(value[0]))
            {
                index = 1;
                while ((index < length) && (Char.ToLower(sb[i + index]) == Char.ToLower(value[index])))
                    ++index;

                if (index == length)
                    return i;
            }
        }

        return -1;
    }

    for (int i = startIndex; i < maxSearchLength; ++i)
    {
        if (sb[i] == value[0])
        {
            index = 1;
            while ((index < length) && (sb[i + index] == value[index]))
                ++index;

            if (index == length)
                return i;
        }
    }

    return -1;
}

Answer 4

Dennis, great solution. Thanks. I suggest to optimize it a little:

public static int IndexOf(
    this StringBuilder sb,
    string value,
    int startIndex,
    bool ignoreCase)
{
    int len = value.Length;
    int max = (sb.Length - len) + 1;
    var v1 = (ignoreCase)
        ? value.ToLower() : value;
    var func1 = (ignoreCase)
        ? new Func<char, char, bool>((x, y) => char.ToLower(x) == y)
        : new Func<char, char, bool>((x, y) => x == y);
    for (int i1 = startIndex; i1 < max; ++i1)
        if (func1(sb[i1], v1[0]))
        {
            int i2 = 1;
            while ((i2 < len) && func1(sb[i1 + i2], v1[i2]))
                ++i2;
            if (i2 == len)
                return i1;
        }
    return -1;
}