Question

What are alternatives to this method

 tmp = c[0];
c[0] = c[1];
c[1] = c[2];
c[2] = c[3];
c[3] = tmp;

to left rotate a char array with 4 elements

Answer 1

Using generics and rotating in place (thanks Jon Skeet for the suggestion):

static void Rotate<T>(T[] source)
{
    var temp = source[0];
    Array.Copy(source, 1, source, 0, source.Length - 1);
    source[source.Length - 1] = temp;
}

These should work for any array of at least 2 length, and on any array.

If performance is critical and the arrays are always small, use this:

static void Rotate<T>(T[] source)
{
    var temp = source[0];
    for (int i = 0; i < source.Length - 1; i++)
        source[i] = source[i + 1];
    source[source.Length - 1] = temp;
}

The first method is the fastest with large arrays, but for 4 items, this one’s almost as fast as your example method.