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Posted 25 May 2018


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IEqualityComparer<FileInfo> using MD5 Hash

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28 May 2018CPOL1 min read
The presented code snippet compares two given files using the IEqualityComparer.


The presented code snippet compares two given files using the IEqualityComparer.

The comparison does the following:

- Use the FileInfo's equality operator (==) to try to determine whether these are the same instances of the FileInfo class
- If one of the objects is null, they can't represent the same files, thus false is returned
- If the file path is for both objects the same, true is returned since the file must be the same
- If the file sizes differ, the files can't be the same either thus false is returned
- And at the end we resort to comparing the MD5 hash of both files.

Please keep in mind that MD5 hashing is an expensive operation, which is not suitable for comparing a lot of files or large files. The code presented here was initally intended to be used within an integration test. If you need to compare a lot of files (or very large ones) you may resort to your own implementation - You may want to start by reading this stackoverflow discussion, though.


Please keep in mind that this implementation reads the file contents into the memory to create the MD5 for each file. If you're trying to compare very large files, this may slow down your application considerably.

The code

/// <summary>
/// An <see cref="IEqualityComparer{T}"/> for files using <see cref="FileInfo"/>
/// </summary>
public class FileMd5EqualityComparer : IEqualityComparer<FileInfo>
    /// <summary>
    /// See <see cref="IEqualityComparer{T}.Equals(T, T)"/>
    /// </summary>
    public bool Equals(FileInfo x, FileInfo y)
        // Use basic comparison
        if(x == y)
            return true;

        // if one of both parameters is null, they can't be
        // the same - Except both are null, but this case is
        // handled above.
        if(x == null || y == null)
            return false;

        // If both file paths are the same, the
        // files must be the same.
        if(x.FullName == y.FullName)
            return true;

        // The files can't be equal if they don't
        // have the same size
        if(x.Length != y.Length)
            return false;

        // At last, compare the MD5 of the files.
        var md5X = GetMd5(x.FullName);
        var md5Y = GetMd5(y.FullName);

        return md5X == md5Y;

    /// <summary>
    /// See <see cref="IEqualityComparer{T}.Equals(T, T)"/>
    /// </summary>
    public int GetHashCode(FileInfo obj)
        return obj.GetHashCode();

    /// <summary>
    /// Returns the MD5 of the file at <paramref name="filePath"/>
    /// as string
    /// </summary>
    private string GetMd5(string filePath)
        using (var md5 = MD5.Create())
            using (var stream = File.OpenRead(filePath))
                return Encoding.Default.GetString(md5.ComputeHash(stream));


2018-05-25 Initial version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Marco Bertschi
Software Developer
Switzerland Switzerland
I'm a young Software Engineer. Whilst I may accept a 'status quo' as given, it's my responsibility to improve it as soon as the opportunity arises.
Very familiar with the .Net framework since it has been the underlying foundation for most of my professional work so far. I'm that kind of guy who gets stuff done, while I may also improve the way things work whilst I'm at it. Generally can get along well with most people I meet as a professional.
Fervent volunteer firefigter, captain-ranked air force militia spec officer.
Great friend and generally nice to have me around - For a talk, a beer, or a bonfire (I might as well do all of those things combined). Oh, and I really love music - Rock & country that is, mostly.

Comments and Discussions

QuestionGetHashCode for object equality comparison Pin
Member 95112430-May-18 4:23
MemberMember 95112430-May-18 4:23 
Hi Marco,

According to Microsoft
Two objects that are equal return hash codes that are equal. However, the reverse is not true: equal hash codes do not imply object equality, because different (unequal) objects can have identical hash codes

Kind regards & Hop Suisse! Hopp Schwiiz! Forza Svizzera!

QuestionWhy determine hash... Pin
User 1106097925-May-18 12:43
MemberUser 1106097925-May-18 12:43 
AnswerRe: Why determine hash... Pin
Marco Bertschi28-May-18 4:19
professionalMarco Bertschi28-May-18 4:19 
Suggestionmd5 is expanciv and slow in this special case ! Pin
Peter BCKR25-May-18 2:18
MemberPeter BCKR25-May-18 2:18 
GeneralRe: md5 is expanciv and slow in this special case ! Pin
Marco Bertschi28-May-18 4:18
professionalMarco Bertschi28-May-18 4:18 

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