Splitting a string into chunks of a certain size

In a combination of dove+Konstatin's answers...

static IEnumerable<string> WholeChunks(string str, int chunkSize) {
    for (int i = 0; i < str.Length; i += chunkSize) 
        yield return str.Substring(i, chunkSize);
}

This will work for all strings that can be split into a whole number of chunks, and will throw an exception otherwise.

If you want to support strings of any length you could use the following code:

static IEnumerable<string> ChunksUpto(string str, int maxChunkSize) {
    for (int i = 0; i < str.Length; i += maxChunkSize) 
        yield return str.Substring(i, Math.Min(maxChunkSize, str.Length-i));
}

However, the the OP explicitly stated he does not need this; it's somewhat longer and harder to read, slightly slower. In the spirit of KISS and YAGNI, I'd go with the first option: it's probably the most efficient implementation possible, and it's very short, readable, and, importantly, throws an exception for nonconforming input.

static IEnumerable<string> Split(string str, int chunkSize)
{
    return Enumerable.Range(0, str.Length / chunkSize)
        .Select(i => str.Substring(i * chunkSize, chunkSize));
}

Please note that additional code might be required to gracefully handle edge cases (null or empty input string, chunkSize == 0, input string length not divisible by chunkSize, etc.). The original question doesn't specify any requirements for these edge cases and in real life the requirements might vary so they are out of scope of this answer.