当被比较的两个字符串相同时，为什么会出现 md5 哈希不匹配？

Question

I am working on a C# program that uses md5 hashing. A file is created using an older version of the program and the hash is saved in a field in the file. Here is a code snippet showing the saving of the hash and the file:

using (MD5 md5Hash = MD5.Create())
{
     this.Hash = string.Empty;
     s = JsonConvert.SerializeObject(this, Formatting.Indented);
     string hash = MD5hash.GetMd5Hash(md5Hash, s);
     this.Hash = hash;
     s = JsonConvert.SerializeObject(this, Formatting.Indented);
 }

The string "s" is saved to a file in JSON format and "this" is the object that is serialized.

In a new version of the program I am attempting to load the above file but the md5 hashes don't match even though I compare the files and they are identical. Here is a snippet:

using (MD5 md5Hash = MD5.Create())
{
     hash = MD5hash.GetMd5Hash(md5Hash, s);
     if (fhash == hash)
     {
         return f;
     }
     else
     {
         throw new Exception("HashMismatch");
     }
 }

"fhash" is loaded from the file and is the hash that was saved in the older version. Since "s" in the the new version in the above code matches "s" in the older version "hash" and "fhash" should match but they don't. Any suggestions?

Thanks in advance for your help!

Answer 1

In the first block of code you generate a hash from the object that has a hash of string.empty but in the second block you are attempting to generate a hash from the file which was serialized with the Hash property set to the generated hash.

So when you generate the hash for the loaded file it will generate a different value because the Hash property is no longer the same value that was used when generated the original hash ie string.empty .

When you load the file in the second block of code you need to set the Hash property to string.empty to get the hashes to match.