How to compare the contents of two folders (LINQ) (C#)

This example demonstrates three ways to compare two file listings:

  • By querying for a Boolean value that specifies whether the two file lists are identical.

  • By querying for the intersection to retrieve the files that are in both folders.

  • By querying for the set difference to retrieve the files that are in one folder but not the other.


    The techniques shown here can be adapted to compare sequences of objects of any type.

The FileComparer class shown here demonstrates how to use a custom comparer class together with the Standard Query Operators. The class is not intended for use in real-world scenarios. It just uses the name and length in bytes of each file to determine whether the contents of each folder are identical or not. In a real-world scenario, you should modify this comparer to perform a more rigorous equality check.


namespace QueryCompareTwoDirs  
    class CompareDirs  
        static void Main(string[] args)  
            // Create two identical or different temporary folders
            // on a local drive and change these file paths.  
            string pathA = @"C:\TestDir";  
            string pathB = @"C:\TestDir2";  
            System.IO.DirectoryInfo dir1 = new System.IO.DirectoryInfo(pathA);  
            System.IO.DirectoryInfo dir2 = new System.IO.DirectoryInfo(pathB);  
            // Take a snapshot of the file system.  
            IEnumerable<System.IO.FileInfo> list1 = dir1.GetFiles("*.*", System.IO.SearchOption.AllDirectories);  
            IEnumerable<System.IO.FileInfo> list2 = dir2.GetFiles("*.*", System.IO.SearchOption.AllDirectories);  
            //A custom file comparer defined below  
            FileCompare myFileCompare = new FileCompare();  
            // This query determines whether the two folders contain  
            // identical file lists, based on the custom file comparer  
            // that is defined in the FileCompare class.  
            // The query executes immediately because it returns a bool.  
            bool areIdentical = list1.SequenceEqual(list2, myFileCompare);  
            if (areIdentical == true)  
                Console.WriteLine("the two folders are the same");  
                Console.WriteLine("The two folders are not the same");  
            // Find the common files. It produces a sequence and doesn't
            // execute until the foreach statement.  
            var queryCommonFiles = list1.Intersect(list2, myFileCompare);  
            if (queryCommonFiles.Any())  
                Console.WriteLine("The following files are in both folders:");  
                foreach (var v in queryCommonFiles)  
                    Console.WriteLine(v.FullName); //shows which items end up in result list  
                Console.WriteLine("There are no common files in the two folders.");  
            // Find the set difference between the two folders.  
            // For this example we only check one way.  
            var queryList1Only = (from file in list1  
                                  select file).Except(list2, myFileCompare);  
            Console.WriteLine("The following files are in list1 but not list2:");  
            foreach (var v in queryList1Only)  
            // Keep the console window open in debug mode.  
            Console.WriteLine("Press any key to exit.");  
    // This implementation defines a very simple comparison  
    // between two FileInfo objects. It only compares the name  
    // of the files being compared and their length in bytes.  
    class FileCompare : System.Collections.Generic.IEqualityComparer<System.IO.FileInfo>  
        public FileCompare() { }  
        public bool Equals(System.IO.FileInfo f1, System.IO.FileInfo f2)  
            return (f1.Name == f2.Name &&  
                    f1.Length == f2.Length);  
        // Return a hash that reflects the comparison criteria. According to the
        // rules for IEqualityComparer<T>, if Equals is true, then the hash codes must  
        // also be equal. Because equality as defined here is a simple value equality, not  
        // reference identity, it is possible that two or more objects will produce the same  
        // hash code.  
        public int GetHashCode(System.IO.FileInfo fi)  
            string s = $"{fi.Name}{fi.Length}";
            return s.GetHashCode();  

Compiling the Code

Create a C# console application project, with using directives for the System.Linq and System.IO namespaces.

See also