var countryAG = from elements in countryList
                where elements[0] >= 'A' && elements[0] <= 'H'
                select elements;

字符实际上只是数字，因此可以将它们作为数字进行比较

我现在没法测试但我会试试

countryList.Where((s) => s[0] <= 'A' && s[0] >= 'G');

您可以使用前缀列表，然后使用前缀列表进行比较-这样，您可以根据您感兴趣的范围轻松使用不同的前缀列表：

List<string> prefixList = new List<string>() { "A", "B", "C", "D", "E", "F", "G" };
 var countryAG = countryList.Where( x=> prefixList.Any( p => x.StartsWith(p)));

尝试

char[] startingLetters = new char[] {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'};
var countryAG = 
    from elements in countryList 
    where elements.IndexOfAny(startingLetters, 0, 1) == 0 
    select elements;

有关IndexOfAny的信息，请参见here。

尝试使用此代码：

var start = "a";
var end = "g";
var regex = new Regex(string.Format("^[{0}-{1}]", start, end));
var result = list.Where(x => regex.Match(x.ToLowerInvariant()).Success);

例如，“start”和“end”是静态的。

我有两个扩展函数：

public static IEnumerable<char> Range(char start, char end)
{
  return Enumerable.Range((int)start, (int)end - (int)start + 1).Select(i => (char)i);
}

它创建了一系列字符，并且

public static bool In(this string source, IEnumerable<string> collection)
{
  return collection.Contains(source);
}

它只是Contains的反数，主要是为了可读性。
我们可以一起做：
where elements[0].In(Range('a', 'f')))

List<string> mainList = new List<string>()
        {
           "A","B","DD","EE","F","G","EE","CC","DD","Q","R","CC"
        };

        List<string> searchList = new List<string>() { "DD", "EE", "CC" };

        var finalList = mainList.Where(x => searchList.Any(p => p == x)).ToList();

两种根据字符串首字母过滤列表的方法：

1. list = list.Where(x => x.WordText.ToUpper().StartsWith(Character));
2. list = list.Where(x => Regex.IsMatch(x.WordText.ToUpper().Substring(0, 1), Character));