In C#, perform "static" copy into a substring of a StringBuilder object

To build a sparsely populated fixed width开发者_开发知识库 record, I would like to copy a string field into a StringBuilder object, starting at a given position. A nice syntax for this would have been

StringBuilder sb = new StringBuilder(' ', 100);
string fieldValue = "12345";
int startPos = 16; 
int endPos = startPos + fieldValue.Length - 1;
sb[startPos..endPos] = fieldValue; // no such syntax

I could obviously do this C style, one character at a time:

for (int ii = 0; ii++; ii < fieldValue.Length)
    sb[startPos + ii] = fieldValue[ii];

But this seems way too cumbersome for c#, plus it uses a loop where the resulting machine code could more efficiently use a bulk copy, which can make a difference if the strings involved were long. Any ideas for a better way?

Your original algorithm can be supported in the following way

var builder = new StringBuilder(new string(' ', 100));
string toInsert = "HELLO WORLD";
int atIndex = 10;
builder.Remove(atIndex, toInsert.Length);
builder.Insert(atIndex, toInsert);

Debug.Assert(builder.Length == 100);
Debug.Assert(builder.ToString().IndexOf(toInsert) == 10);

You can write your own specialized string builder class that uses the efficient machinery of char[] and string underneath the hood, in particular String.CopyTo:

public class FixedStringBuilder
{
    char[] buffer;

    public FixedStringBuilder(int length)
    {
        buffer = new string(' ', length).ToCharArray();
    }

    public FixedStringBuilder Replace(int index, string value)
    {
        value.CopyTo(0, buffer, index, value.Length);
        return this;
    }

    public override string ToString()
    {
        return new string(buffer);
    }
}

class Program
{
    static void Main(string[] args)
    {
        FixedStringBuilder sb = new FixedStringBuilder(100);
        string fieldValue = "12345";
        int startPos = 16; 
        sb.Replace(startPos, fieldValue);
        string buffer = sb.ToString();
    }
}

The closest solution to your goal is to convert the source string in a char array, then substitute the cells. Any char array can be converted back to a string.

why are you pre-allocating memory in the string builder (it is only support performance).

i would append the known prefix, then the actual value and then the postfix to the string.

something like:

StringBuilder sb = new StringBuilder();
sb.Append(prefix).Append(value).Append(postfix);