private static void Main(string[] args)
{
    string source = "span like types are awesome!";
    // source.ToMemory() converts source from string to ReadOnlyMemory<char>,
    // and MemoryMarshal.AsMemory converts ReadOnlyMemory<char> to Memory<char>
    // so you can modify the elements.
    TitleCase(MemoryMarshal.AsMemory(source.AsMemory()));
    // You get "Span like types are awesome!";
    Console.WriteLine(source);
}

private static void TitleCase(Memory<char> memory)
{
    if (memory.IsEmpty)
    {
        return;
    }

    ref char first = ref memory.Span[0];
    if (first >= 'a' && first <= 'z')
    {
        first = (char)(first - 32);
    }
}