How to LINQ-ify the following loop?

I have a method in a C# program. It enumerates all the .cs files in a certain folder and then runs through the list. For each file, I read all the lines using File.ReadAllLines on it. I only want to process a file if it contains a class, whether conventional, static, or abstract, whose name begins with a certain phrase and does not end with the word Tests. Moreover, I wish to find the line index in the line of lines containing the declaration of the class --- i.e., the public static class Foo part.

Given that I take the result of File.ReadAllLines and call ToList() on it to create a List<string>, I wish to use the FindIndex method to find the index of the line matching my criteria (if it exists) using a Predicate.

My question is: What is a good way to write such a predicate?

I realize I could probably use more sophisticated methods, but I am just putting this code into a quick-and-dirty LINQPad script. So, I don't have to get super fancy.

Let me show you what I have so far (assume that the outermost namespace and class are already suitably declared):

void Main()
{
    var files = Directory
        .EnumerateDirectories(
            Path.Combine(
                Environment.GetFolderPath(
                    Environment.SpecialFolder.UserProfile
                ), @"source\repos\astrohart\MFR"
            ), "*", SearchOption.TopDirectoryOnly
        ).SelectMany(
            x => Directory.EnumerateFiles(
                x, "FileSystemEntry*.cs", SearchOption.AllDirectories
            )
        )
        .Where(x => !"FileSystemEntry.cs".Equals(Path.GetFileName(x)))
        .ToList();
    if (files == null || !files.Any()) return;

    foreach (var file in files)
    {
        var contents = string.Empty;

        try
        {
            contents = File.ReadAllText(file);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"ERROR: {ex.Message}");

            contents = string.Empty;
        }

        if (string.IsNullOrWhiteSpace(contents)) continue;
        if (contents.Contains("[TestFixture]")) continue;
        if (contents.Contains("[Log(AttributeExclude = true)]")) continue;

        file.Dump();

        var lines = new List<string>();
        lines.TrimExcess();

        try
        {
            lines = File.ReadAllLines(file).ToList();
        }
        catch (Exception ex)
        {
            Console.WriteLine($"ERROR: {ex.Message}");

            lines = new List<string>();
            lines.TrimExcess();
        }

        if (lines == null || !lines.Any()) continue;

        var index = -1;

        for (var i = 0; i < lines.Count; i++)
        {
            var currentLine = lines[i].Trim();
            if (currentLine.EndsWith("Tests")) continue;

            if (currentLine.StartsWith("public static class FileSystemEntry"))
            {
                index = i;
                break;
            }
            if (currentLine.StartsWith("public class FileSystemEntry"))
            {
                index = i;
                break;
            }
            if (currentLine.StartsWith("public abstract class FileSystemEntry"))
            {
                index = i;
                break;
            }
        }

        if (index < 0) continue;
     
        /*...*/
    }
}

How do I translate the for loop in:

var index = -1;

for (var i = 0; i < lines.Count; i++)
{
    var currentLine = lines[i].Trim();
    if (currentLine.EndsWith("Tests")) continue;

    if (currentLine.StartsWith("public static class FileSystemEntry"))
    {
        index = i;
        break;
    }
    if (currentLine.StartsWith("public class FileSystemEntry"))
    {
        index = i;
        break;
    }
    if (currentLine.StartsWith("public abstract class FileSystemEntry"))
    {
        index = i;
        break;
    }
}

if (index < 0) continue;

into a call thus:

var index = lines.FindIndex(currentLine => /*...*/);

I need help with how to derive the proper body of the lambda expression that matches what the for loop does.

Thanks in advance!

EDIT 1

I squinted my eyes at my loop just a little more. I am looking for a predicate to use specifically with the FindIndex method. I thought a little harder and I figured out maybe I can get away with:

var index = lines.FindIndex(currentLine => !currentLine.Trim.EndsWith("Tests") && currentLine.Trim().StartsWith("public static class FileSystemEntry") || currentLine.Trim().StartsWith("public class FileSystemEntry") || currentLine.Trim().StartsWith("public abstract class FileSystemEntry"));

Perhaps I can implement an extension method

public static bool StartsWithAnyOf(this string value, params string[] testStrings)
{
    var result = false;

    try
    {
        if (string.IsNullOrWhiteSpace(value.Trim())) return result;
        if (testStrings == null || !testStrings.Any()) return result;

        foreach(var element in testStrings)
            if (value.Trim().StartsWith(element))
            {
                result = true;
                break;
            }
    }
    catch
    {
        result = false;
    }

    return result;
}

Then I'd declare another method:

public static bool KeepLine(string currentLine)
{
     if (string.IsNullOrWhiteSpace(currentLine.Trim())) return false;
     if (currentLine.Trim().EndsWith("Tests")) return false;
     
     return currentLine.StartsWithAnyOf(
         "public static class FileSystemEntry",
         "public class FileSystemEntry",
         "public abstract FileSystemEntry"
     );
}

Then use it thus:

var index = lines.FindIndex(KeepLine);

Would that work?