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How to: Populate Object Collections from Multiple Sources (LINQ)

This example shows how to merge data from different sources into a sequence of new types.

Note Note

Do not try to join in-memory data or data in the file system with data that is still in a database. Such cross-domain joins can yield undefined results because of different ways in which join operations might be defined for database queries and other types of sources. Additionally, there is a risk that such an operation could cause an out-of-memory exception if the amount of data in the database is large enough. To join data from a database to in-memory data, first call ToList or ToArray on the database query, and then perform the join on the returned collection.

To create the data file

The following example shows how to use a named type Student to store merged data from two in-memory collections of strings that simulate spreadsheet data in .csv format. The first collection of strings represents the student names and IDs, and the second collection represents the student ID (in the first column) and four exam scores. The ID is used as the foreign key.

class Student
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int ID { get; set; }
    public List<int> ExamScores { get; set; }
}

class PopulateCollection
{
    static void Main()
    {
        // These data files are defined in How to: Join Content from  
        // Dissimilar Files (LINQ). 

        // Each line of names.csv consists of a last name, a first name, and an 
        // ID number, separated by commas. For example, Omelchenko,Svetlana,111 
        string[] names = System.IO.File.ReadAllLines(@"../../../names.csv");

        // Each line of scores.csv consists of an ID number and four test  
        // scores, separated by commas. For example, 111, 97, 92, 81, 60 
        string[] scores = System.IO.File.ReadAllLines(@"../../../scores.csv");

        // Merge the data sources using a named type. 
        // var could be used instead of an explicit type. Note the dynamic 
        // creation of a list of ints for the ExamScores member. We skip  
        // the first item in the split string because it is the student ID,  
        // not an exam score.
        IEnumerable<Student> queryNamesScores =
            from nameLine in names
            let splitName = nameLine.Split(',')
            from scoreLine in scores
            let splitScoreLine = scoreLine.Split(',')
            where splitName[2] == splitScoreLine[0]
            select new Student()
            {
                FirstName = splitName[0],
                LastName = splitName[1],
                ID = Convert.ToInt32(splitName[2]),
                ExamScores = (from scoreAsText in splitScoreLine.Skip(1)
                              select Convert.ToInt32(scoreAsText)).
                              ToList()
            };

        // Optional. Store the newly created student objects in memory 
        // for faster access in future queries. This could be useful with 
        // very large data files.
        List<Student> students = queryNamesScores.ToList();

        // Display each student's name and exam score average. 
        foreach (var student in students)
        {
            Console.WriteLine("The average score of {0} {1} is {2}.",
                student.FirstName, student.LastName,
                student.ExamScores.Average());
        }

        //Keep console window open in debug mode
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output: 
    The average score of Omelchenko Svetlana is 82.5.
    The average score of O'Donnell Claire is 72.25.
    The average score of Mortensen Sven is 84.5.
    The average score of Garcia Cesar is 88.25.
    The average score of Garcia Debra is 67.
    The average score of Fakhouri Fadi is 92.25.
    The average score of Feng Hanying is 88.
    The average score of Garcia Hugo is 85.75.
    The average score of Tucker Lance is 81.75.
    The average score of Adams Terry is 85.25.
    The average score of Zabokritski Eugene is 83.
    The average score of Tucker Michael is 92.
 */

In the select clause, an object initializer is used to instantiate each new Student object by using the data from the two sources.

If you do not have to store the results of a query, anonymous types can be more convenient than named types. Named types are required if you pass the query results outside the method in which the query is executed. The following example performs the same task as the previous example, but uses anonymous types instead of named types:

// Merge the data sources by using an anonymous type. 
// Note the dynamic creation of a list of ints for the 
// ExamScores member. We skip 1 because the first string 
// in the array is the student ID, not an exam score. 
var queryNamesScores2 =
    from nameLine in names
    let splitName = nameLine.Split(',')
    from scoreLine in scores
    let splitScoreLine = scoreLine.Split(',')
    where splitName[2] == splitScoreLine[0]
    select new
    {
        First = splitName[0],
        Last = splitName[1],
        ExamScores = (from scoreAsText in splitScoreLine.Skip(1)
                      select Convert.ToInt32(scoreAsText))
                      .ToList()
    };

// Display each student's name and exam score average. 
foreach (var student in queryNamesScores2)
{
    Console.WriteLine("The average score of {0} {1} is {2}.",
        student.First, student.Last, student.ExamScores.Average());
}

  • Follow the instructions in How to: Join Content from Dissimilar Files (LINQ) to set up the source files.

  • Create a Visual Studio project that targets .NET Framework 3.5 or later. 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.

  • Copy this code into your project.

  • Press F5 to compile and run the program.

  • Press any key to exit the console window.

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