Quickest way to enumerate the alphabet

(Assumes ASCII, etc)

for (char c="A"; c <= 'Z'; c++)
{
    //do something with letter 
} 

Alternatively, you could split it out to a provider and use an iterator (if you’re planning on supporting internationalisation):

public class EnglishAlphabetProvider : IAlphabetProvider
{
    public IEnumerable<char> GetAlphabet()
    {
        for (char c="A"; c <= 'Z'; c++)
        {
            yield return c;
        } 
    }
}

IAlphabetProvider provider = new EnglishAlphabetProvider();

foreach (char c in provider.GetAlphabet())
{
    //do something with letter 
}