This documentation is archived and is not being maintained.

How to: Compare the Contents of Two Folders (LINQ)

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.

    NoteNote:

    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(pathA);

            // Take a snapshot of the file system.
            IEnumerable<System.IO.FileInfo> list1 = dir1.GetFiles("*.*", System.IO.SearchOption.AllDirectories);
            IEnumerable<System.IO.FileInfo> list2 = dir1.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");
            }
            else
            {
                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.Count() > 0)
            {
                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
                }
            }
            else
            {
                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)
            {
                Console.WriteLine(v.FullName);
            }

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

    // 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 = String.Format("{0}{1}", fi.Name, fi.Length);
            return s.GetHashCode();
        }
    }
}

  • 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 Imports statement (Visual Basic) for the System.Linq namespace. In C# projects, add a using directive for the System.IO namespace.

  • Copy the 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.

Show: