In C#, perform "static" copy into a substring of a StringBuilder object
<p>To build a sparsely populated fixed width 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</p>

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

<p>I could obviously do this C style, one character at a time:</p>

<pre><code>for (int ii = 0; ii++; ii &lt; fieldValue.Length)
    sb[startPos + ii] = fieldValue[ii];
</code></pre>

<p>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?</p>
