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How to: Query for Duplicate Files in a Directory Tree (LINQ)

Sometimes files that have the same name may be located in more than one folder. For example, under the Visual Studio installation folder, several folders have a readme.htm file. This example shows how to query for such duplicate file names under a specified root folder. The second example shows how to query for files whose size and creation times also match.

class QueryDuplicateFileNames
    static void Main(string[] args)
        // Uncomment QueryDuplicates2 to run that query.
        // QueryDuplicates2(); 

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");

    static void QueryDuplicates()
        // Change the root drive or folder if necessary 
        string startFolder = @"c:\program files\Microsoft Visual Studio 9.0\";

        // Take a snapshot of the file system.
        System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(startFolder);

        // This method assumes that the application has discovery permissions 
        // for all folders under the specified path.
        IEnumerable<System.IO.FileInfo> fileList = dir.GetFiles("*.*", System.IO.SearchOption.AllDirectories);

        // used in WriteLine to keep the lines shorter 
        int charsToSkip = startFolder.Length;

        // var can be used for convenience with groups. 
        var queryDupNames =
            from file in fileList
            group file.FullName.Substring(charsToSkip) by file.Name into fileGroup
            where fileGroup.Count() > 1
            select fileGroup;

        // Pass the query to a method that will 
        // output one page at a time.
        PageOutput<string, string>(queryDupNames);

    // A Group key that can be passed to a separate method. 
    // Override Equals and GetHashCode to define equality for the key. 
    // Override ToString to provide a friendly name for Key.ToString() 
    class PortableKey
        public string Name { get; set; }
        public DateTime CreationTime { get; set; }
        public long Length { get; set; }

        public override bool Equals(object obj)
            PortableKey other = (PortableKey)obj;
            return other.CreationTime == this.CreationTime &&
                   other.Length == this.Length &&
                   other.Name == this.Name;

        public override int GetHashCode()
            string str = String.Format("{0}{1}{2}", this.CreationTime, this.Length, this.Name);
            return str.GetHashCode();
        public override string ToString()
            return String.Format("{0} {1} {2}", this.Name, this.Length, this.CreationTime);
    static void QueryDuplicates2()
        // Change the root drive or folder if necessary. 
        string startFolder = @"c:\program files\Microsoft Visual Studio 9.0\Common7";

        // Make the the lines shorter for the console display 
        int charsToSkip = startFolder.Length;

        // Take a snapshot of the file system.
        System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(startFolder);
        IEnumerable<System.IO.FileInfo> fileList = dir.GetFiles("*.*", System.IO.SearchOption.AllDirectories);

        // Note the use of a compound key. Files that match 
        // all three properties belong to the same group. 
        // A named type is used to enable the query to be 
        // passed to another method. Anonymous types can also be used 
        // for composite keys but cannot be passed across method boundaries 
        var queryDupFiles =
            from file in fileList
            group file.FullName.Substring(charsToSkip) by 
                new PortableKey { Name = file.Name, CreationTime = file.CreationTime, Length = file.Length } into fileGroup
            where fileGroup.Count() > 1
            select fileGroup;

        var list = queryDupFiles.ToList();

        int i = queryDupFiles.Count();

        PageOutput<PortableKey, string>(queryDupFiles);

    // A generic method to page the output of the QueryDuplications methods 
    // Here the type of the group must be specified explicitly. "var" cannot
    // be used in method signatures. This method does not display more than one 
    // group per page. 
    private static void PageOutput<K, V>(IEnumerable<System.Linq.IGrouping<K, V>> groupByExtList)
        // Flag to break out of paging loop. 
        bool goAgain = true;

        // "3" = 1 line for extension + 1 for "Press any key" + 1 for input cursor.
        int numLines = Console.WindowHeight - 3;

        // Iterate through the outer collection of groups. 
        foreach (var filegroup in groupByExtList)
            // Start a new extension at the top of a page. 
            int currentLine = 0;

            // Output only as many lines of the current group as will fit in the window. 
                Console.WriteLine("Filename = {0}", filegroup.Key.ToString() == String.Empty ? "[none]" : filegroup.Key.ToString());

                // Get 'numLines' number of items starting at number 'currentLine'. 
                var resultPage = filegroup.Skip(currentLine).Take(numLines);

                //Execute the resultPage query 
                foreach (var fileName in resultPage)
                    Console.WriteLine("\t{0}", fileName);

                // Increment the line counter.
                currentLine += numLines;

                // Give the user a chance to escape.
                Console.WriteLine("Press any key to continue or the 'End' key to break...");
                ConsoleKey key = Console.ReadKey().Key;
                if (key == ConsoleKey.End)
                    goAgain = false;
            } while (currentLine < filegroup.Count());

            if (goAgain == false)

The first query uses a simple key to determine a match; this finds files that have the same name but whose contents might be different. The second query uses a compound key to match against three properties of the FileInfo object. This query is much more likely to find files that have the same name and similar or identical content.

  • Create a Visual Studio project that targets the .NET Framework version 3.5. By default, the project has a reference to System.Core.dll and a using directive (C#) or Imported namespace (Visual Basic) for the System.Linq namespace. In C# projects, add a using directive for the System.IO namespace.

  • Copy this code into your project.

  • Press F5 to compile and run the program.

  • Press any key to exit the console window.

For intensive query operations over the contents of multiple types of documents and files, consider using the Windows Desktop Search engine.

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