Split String into smaller Strings by length variable

You need to use a loop:

public static IEnumerable<string> SplitByLength(this string str, int maxLength) {
    for (int index = 0; index < str.Length; index += maxLength) {
        yield return str.Substring(index, Math.Min(maxLength, str.Length - index));
    }
}

Alternative:

public static IEnumerable<string> SplitByLength(this string str, int maxLength) {
    int index = 0;
    while(true) {
        if (index + maxLength >= str.Length) {
            yield return str.Substring(index);
            yield break;
        }
        yield return str.Substring(index, maxLength);
        index += maxLength;
    }
}

2^nd alternative: (For those who can't stand while(true))

public static IEnumerable<string> SplitByLength(this string str, int maxLength) {
    int index = 0;
    while(index + maxLength < str.Length) {
        yield return str.Substring(index, maxLength);
        index += maxLength;
    }

    yield return str.Substring(index);
}

Easy to understand version:

string x = "AAABBBCC";
List<string> a = new List<string>();
for (int i = 0; i < x.Length; i += 3)
{
    if((i + 3) < x.Length)
        a.Add(x.Substring(i, 3));
    else
        a.Add(x.Substring(i));
}

Though preferably the 3 should be a nice const.