Question

In my .NET program I allow a user to define “fields” which are values calculated by the business logic. These fields have a position and length, so that they can all be inserted into a single output string at a given index. I also allow a user to specify default content of this output string. If no field is defined to replace a given position, the default character is output instead

My question is, how can I do this efficiently? The StringBuilder class has an Insert(int index, string value) method, but this lengthens the output string each time rather than overwriting it. Am I going to have to set each char one at a time using the StringBuilder[int index] indexer, and is this inefficient? Since I am going to be doing this a lot of times I would like it to be as fast as possible.

Thanks.

Answer 1

Doing it one character at a time is likely your best bet. I say this because calling Insert and Remove on a StringBuilder results in characters being shifted right/left, just as the analogous methods would in any mutable indexed collection such as a List<char>.

That said, this is an excellent candidate for an extension method to make your life a bit easier.

public static StringBuilder ReplaceSubstring(this StringBuilder stringBuilder, int index, string replacement)
{
    if (index + replacement.Length > stringBuilder.Length)
    {
        // You could throw an exception here, or you could just
        // append to the end of the StringBuilder -- up to you.
        throw new ArgumentOutOfRangeException();
    }

    for (int i = 0; i < replacement.Length; ++i)
    {
        stringBuilder[index + i] = replacement[i];
    }

    return stringBuilder;
}

Usage example:

var builder = new StringBuilder("My name is Dan.");
builder.ReplaceSubstring(11, "Bob");

Console.WriteLine(builder.ToString());

Output:

My name is Bob.