如果您使用GetFileNameWithoutExtension，将不会有扩展名，因此，按'_'进行拆分即可。然后，您可以使用.Last()完成最后一部分。

string[] filenames_partial_distinct = Array.ConvertAll(
        filenames,file => Path.GetFileNameWithoutExtension(file).Split('_').Last()
    )
    .Distinct()
    .ToArray();

使用输入

string[] filenames = { "aaa_ab12345.txt","bbb_ab12345.txt","aaa_ac12345.txt","bbb_ac12345","aaa_bbb_ab12345.txt" };

您得到的结果

{ "ab12345","ac12345" }

仅当文件名以StringSplitOptions.RemoveEmptyEntries结尾（扩展名之前）时，才需要_。

您似乎正在寻找这样的东西：

string[] arr = filenames.Select(n => n.Substring(n.IndexOf("_") + 1,7)).Distinct().ToArray();

我通常将这样的问题推迟到regex。他们非常强大。这种方法还使您有机会发现意外情况并进行适当处理。

这里是一个粗略的例子，假设我理解您的要求：

using System;
using System.Linq;
using System.Text.RegularExpressions;

public class Program
{
    public static void Main()       
    {
        string MyMatcher(string filename)
        {
           // this pattern may need work depending on what you need - it says
           // extract that pattern between the "()" which is 2 characters and
           // 4 digits,exactly; and can be found in `Groups[1]`.
           Regex r = new Regex(@".*_(\w{2}\d{4}).*",RegexOptions.IgnoreCase);
           Match m = r.Match(filename);
           return m.Success
               ? m.Groups[1].ToString()
               : null; // what should happen here? 
        }

        string[] filenames = 
        {
            "aaa_ab12345.txt","aaa_bbb_ab12345.txt","ae12345.txt" // MyMatcher() return null for this - what should you do if this happens?
        };

        var results = filenames
            .Select(MyMatcher)
            .Distinct();

        foreach (var result in results)
        {
            Console.WriteLine(result);
        }
    }
}

礼物：

ab1234
ac1234

这可以进一步完善，例如预编译的regex模式，封装在类中等等。