How to validate multi part compressed (i.e zip) files have all parts or not in C#?

I am not sure if you will be able to see the exact error as shown in your snapshot. But I have a code which may help you to find if the multipart file is readble.

I have used nuget Package CombinationStream.

The ZipArchive constructor throws ArgumentException or InvalidDataException if the stream is not readable.

Below is the code:

public static bool IsZipValid()
{
    try
    {
        string basePath = @"C:\multi-part-zip\";
        List<string> files = new List<string> {
                                basePath + "somefile.zip.001",
                                basePath + "somefile.zip.002",
                                basePath + "somefile.zip.003",
                                basePath + "somefile.zip.004",
                                basePath + "somefile.zip.005",
                                basePath + "somefile.zip.006",
                                basePath + "somefile.zip.007",
                                basePath + "somefile.zip.008"
                            };

        using (var zipFile = new ZipArchive(new CombinationStream(files.Select(x => new FileStream(x, FileMode.Open) as Stream).ToList()), ZipArchiveMode.Read))
        {
            // Do whatever you want
        }
    }
    catch(InvalidDataException ex)
    {
        return false;
    }

    return true;
}

I am not sure if this is what you are looking for or you need more details in the error. But hope this helps you to come to solution of your issue.