How do I return the first string variable that isn't null or empty

Define an extension method:

static string Or(this string value, string alternative) {
    return string.IsNullOrEmpty(value) ? alternative : value;
}

Now you can say the following:

string result = str1.Or(str2).Or(str3).Or(str4) …

var found = new[]{first, second, third, fourth, fifth}.FirstOrDefault(x =>!String.IsNullOrEmpty(x));