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LINQ: Grouping Data in LINQ using Group By

In this post I’ll explain how one can group data using LINQ GroupBy. GroupBy can be applied on different data types like Generic List, XML, DataTable etc.

Syntax of Group By:

var result= from c in <collection>
    group c by c.<property> into g
    select new
    {
        Key=g.Key,
        Value=g
    };

LINQ Grouping return partitioned sets of data from a collection based on a given key value, i.e. group employees by designation as:

var query = from c in employees
        group c by c.Designation;

//or same statement can be written in Lambda expression as:
var query = employees.GroupBy(c => c.Designation);

GroupBy Result: Let’s understand result returned by LINQ grouping operation.

The return collection from a GroupBy is:

IEnumerable<IGrouping<TKey, TElement>>;

The IGrouping interface inherits from IEnumerable<TElement> and has a read only key property of type TKey.

public interface IGrouping<TKey, TElmt> : IEnumerable<TElmt>
{
    TKey Key { get; }
}

Few points to consider:
·         LINQ grouping operation result is an enumerable sequence of groups with each group having a distinct key value
·         The key value is determined by the key selection expression that equates the key value for each element

Example of GroupBy:

//empmployee designations.
var empDesignations = new string[]
    {
        "Manager",
        "Senior Associate",
        "Associate",
        "Senior Associate",
        "Jr. Associate",
        "Associate",
        "Senior Associate",
        "Associate"
    };

//Group By using LINQ
Console.WriteLine("--------------- Using Plain LINQ ---------------");

(from ed in empDesignations
    group ed by ed
        into g
        select new
        {
            Key = g.Key,
            Count = g.Count()
        })
    .ToList()
    .ForEach(c => Console.WriteLine("Number of {0} is {1}", c.Key, c.Count));

//Group By using LINQ Lambda
Console.WriteLine("--------------- Using Lambda LINQ --------------");

empDesignations
    .GroupBy(g => g)
    .Select(c => new { Key = c.Key, Count = c.Count() })
    .ToList()
    .ForEach(c => Console.WriteLine("Number of {0} is {1}", c.Key, c.Count));

Output: Above LINQ queries output in command line is shown as:

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