目前实现的一个版本

public class CharComparer : IEqualityComparer<char>
{
    public static CharComparer IgnoreCase { get; } = new CharComparer();

    public bool Equals(char x, char y)
    {
        return char.IsLetter(x) && char.IsLetter(y) ?
            char.ToLowerInvariant(x) == char.ToLowerInvariant(y)
            : x.Equals(y);
    }

    public int GetHashCode([DisallowNull] char obj)
    {
        return char.IsLetter(obj) ?
            char.ToLowerInvariant(obj).GetHashCode() :
            obj.GetHashCode();
    }
}

测试代码

[Fact]
public void CharComparer_test()
{
    var dict = new Dictionary<char, TrieNode>(CharComparer.IgnoreCase);
    dict.Add('a', new TrieNode('c'));
    Assert.True(dict.ContainsKey('A'));
    Assert.False(dict.ContainsKey('b'));
}

测试通过

Passed!  - Failed:     0, Passed:     1