(C#) How to Tell if a File is .p7m or .p7s

This example explains how to detect if a file is a .p7m or .p7s. It provides as solution to the following problem:

Sometimes people submit P7S/P7M with wrong extensions. Can I check if file is P7S or P7M with Chilkat?

Also see What is a P7M or P7S File?

Chilkat .NET Downloads Chilkat .NET Framework Chilkat for .NET Core

// A .p7m or .p7s file is binary and contains PKCS7.
// PKCS7 is ASN.1, and always begins with a SEQUENCE tag (a byte equal to 0x30)
// followed by an encoded length.
// If the file is larger than 128 bytes (and it SHOULD be), the encoded length will begin with either 0x80, 0x81, 0x82, or 0x83.
// It can also begin with 0x84, but only for very large files (where the length is too large for 3 bytes).
// See How ASN.1 Encodes Lengths

// Therefore, if we have a file that might be .p7m or .p7s, we can check by
// examining the 1st 2 bytes of the file.

Chilkat.BinData bd = new Chilkat.BinData();

bool success = bd.LoadFile("qa_data/p7m/a.p7m");
if (success == false) {
    Debug.WriteLine("Failed to load the file.");
    return;
}

Chilkat.StringBuilder sb = new Chilkat.StringBuilder();
sb.Append(bd.GetEncodedChunk(0,2,"hex"));

bool caseSensitive = true;
if (sb.ContentsEqual("3080",caseSensitive) || sb.ContentsEqual("3081",caseSensitive) || sb.ContentsEqual("3082",caseSensitive) || sb.ContentsEqual("3083",caseSensitive)) {
    Debug.WriteLine("This is a .p7m or .p7s file.");
}
else {
    Debug.WriteLine("This is not a .p7m or .p7s file.");
}