This C# LINQ example shows how to join data from two comma-delimited files that share a common value that is used as a matching key. This technique can be useful if you have to combine data from two spreadsheets, or from a spreadsheet and from a file that has another format, into a new file. You can modify the example to work with any kind of structured text.

// ----------------------------------------------------------------------------
// Join Content from Dissimilar Files (LINQ) (C#)
// <https://msdn.microsoft.com/en-us/library/mt693031.aspx>
//
// This example shows how to join data from two comma-delimited files that share
// a common value that is used as a matching key. This technique can be useful
// if you have to combine data from two spreadsheets, or from a spreadsheet and
// from a file that has another format, into a new file. You can modify the
// example to work with any kind of structured text.
//
// To create the data files
//
// 1. Copy the following lines into a file that is named scores.csv and save it
//    to your project folder. The file represents spreadsheet data. Column 1 is
//    the student's ID, and columns 2 through 5 are test scores.
//
//        111, 97, 92, 81, 60
//        112, 75, 84, 91, 39
//        113, 88, 94, 65, 91
//        114, 97, 89, 85, 82
//        115, 35, 72, 91, 70
//        116, 99, 86, 90, 94
//        117, 93, 92, 80, 87
//        118, 92, 90, 83, 78
//        119, 68, 79, 88, 92
//        120, 99, 82, 81, 79
//        121, 96, 85, 91, 60
//        122, 94, 92, 91, 91
//
// 2. Copy the following lines into a file that is named names.csv and save it
//    to your project folder. The file represents a spreadsheet that contains
//    the student's last name, first name, and student ID.
//
//        Omelchenko,Svetlana,111
//        O'Donnell,Claire,112
//        Mortensen,Sven,113
//        Garcia,Cesar,114
//        Garcia,Debra,115
//        Fakhouri,Fadi,116
//        Feng,Hanying,117
//        Garcia,Hugo,118
//        Tucker,Lance,119
//        Adams,Terry,120
//        Zabokritski,Eugene,121
//        Tucker,Michael,122
// ----------------------------------------------------------------------------

using System.Linq;
using System;

class JoinStrings
{
    static void Main()
    {
        // Join content from dissimilar files that contain
        // related information. File names.csv contains the student
        // name plus an ID number. File scores.csv contains the ID
        // and a set of four test scores. The following query joins
        // the scores to the student names by using ID as a
        // matching key.

        string[] names = System.IO.File.ReadAllLines(@"../../../names.csv");
        string[] scores = System.IO.File.ReadAllLines(@"../../../scores.csv");


        // Name:    Last[0],       First[1],  ID[2]
        //          Omelchenko,    Svetlana,  11
        // Score:   StudentID[0],  Exam1[1]   Exam2[2],  Exam3[3],  Exam4[4]
        //          111,           97,        92,        81,        60

        // This query joins two dissimilar spreadsheets based on common ID value.
        // Multiple from clauses are used instead of a join clause
        // in order to store results of id.Split.
        IEnumerable<string> scoreQuery1 =
            from name in names
            let nameFields = name.Split(',')
            from id in scores
            let scoreFields = id.Split(',')
            where nameFields[2] == scoreFields[0]
            select nameFields[0] + "," + scoreFields[1] + "," + scoreFields[2]
                   + "," + scoreFields[3] + "," + scoreFields[4];

        // Pass a query variable to a method and execute it
        // in the method. The query itself is unchanged.
        OutputQueryResults(scoreQuery1, "Merge two spreadsheets:");

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

    static void OutputQueryResults(IEnumerable<string> query, string message)
    {
        Console.WriteLine(System.Environment.NewLine + message);
        foreach (string item in query)
        {
            Console.WriteLine(item);
        }
        Console.WriteLine("{0} total names in list", query.Count());
    }
}

/* Output:
Merge two spreadsheets:
Adams, 99, 82, 81, 79
Fakhouri, 99, 86, 90, 94
Feng, 93, 92, 80, 87
Garcia, 97, 89, 85, 82
Garcia, 35, 72, 91, 70
Garcia, 92, 90, 83, 78
Mortensen, 88, 94, 65, 91
O'Donnell, 75, 84, 91, 39
Omelchenko, 97, 92, 81, 60
Tucker, 68, 79, 88, 92
Tucker, 94, 92, 91, 91
Zabokritski, 96, 85, 91, 60
12 total names in list
*/