LINQ Syntax

Let’s discuss some of the more common operations we might want to perform with LINQ. Please note that this is not an exhaustive list - just some of the most common operations you will be encountering at this time.

Data Source

For these examples, we’ll be using a data source consisting of Student objects. These are defined by the class:

public class Student {
    public string EID;
    public string First;
    public string Last;
    public double GPA;
    public int Age;    
    public string Major;
}

And the variable students is a List<Student> that we can assume is populated with initialized student objects.

We select this as our data source with the LINQ from operator. In query syntax, this would be:

var query = from student in students ...

And with method syntax, we would simply use the students list:

var query = students.SomeQueryMethod(student => ...)

To create a query from a data source using method syntax without applying any query methods (useful for chaining optional queries), we can invoke All() on the collection:

var query = students.All();

To use a different data source, we would just swap students for that source, an object that supports either the IEnumerable (usually data structures) or IQueryable (typically SQL or XML data sources) interface.

Projecting

Projecting refers to selecting specific data from a data source. For example, if we wanted to select the full name of every Student, we could do so with this query syntax:

var studentNames = from student in students select $"{student.First} {student.Last}";

Or with method syntax:

var studentNames = students.Select(student => $"{student.First} {student.Last}");

As the name is simply a string, the select above simply constructs the string, and the type of studentNames is inferred to be IEnumerable<string>.

We can also project an anonymous type. This is a special kind of object whose type is created at runtime. Anonymous types are basically collections of properties and nothing more (they cannot have methods). For example, if we wanted just the student’s full name and age, we would use this query syntax:

var studentInfo = from student in students select new {FullName = $"{student.First} {student.Last}", Age = student.Age};

or this method syntax:

var studentInfo = students.Select(student => new {FullName = $"{student.First} {student.Last}", Age = student.Age});

Finally, we could also define a new data type (i.e. class) and create an instance of it as our projection:

class StudentInfo {
    public string FullName {get; set;}
    public int Age {get; set;}
    public StudentInfo(string fullName, int age)
    {
        FullName = fullName;
        Age = age;
    }
}

Using query syntax:

var studentInfo = from student in students select new StudentInfo($"{student.First} {student.Last}", student.Age);

or this method syntax:

var studentInfo = students.Select(student => new StudentInfo($"{student.First} {student.Last}", student.Age));

Filtering

One of the most common operations you will do with a query is filter the data, so the results contain only part of the original data. This is done with the where operator takes a statement that resolves to a boolean. If this boolean expression resolves to true, then the data is included in the results; if it is false, it is excluded. For example, to find all students older than 25, we would use this query syntax:

var olderStudents = from student in students where student.Age > 25 select student;

or this method syntax:

var olderStudents = students.Where(student => student.Age > 25);

Filtering By Type

If we have a list that contains multiple types, we can filter for specific types with the where operator or the OfType operator (this is an instance where query and operator syntax vary more greatly). Consider the case where our Student class is a base class to GraduateStudent and UndergraduateStudent classes. If we wanted to get a list of only the undergraduates, we could use a where query syntax combined with an is casting test:

var undergraduates = from student in students where student is UndergraduateStudent select student;

In this case, the result would be an IEnumerable<UndergraduateStudent>. But the corresponding where in operator syntax would result in an IEnumerable<Student> that contained only UndergraduateStudent objects. To perform a cast as part of the filtering, we would instead use the OfType<T>() method:

var undergraduates = students.OfType<UndergraduateStudent>();

Ordering

Often we want to apply some form of sorting to our results, i.e. we might want to sort students by GPA. This can be done with an orderby operator. In query syntax it would be:

var studentsByGPA = from student in students orderby student.GPA select student;

And in method syntax:

var studentsByGPA = students.OrderBy(student => student.GPA);

The orderby operator sorts in ascending order (so students with the lowest grades would come first in the list). If we wanted to sort in descending order, we would need to specify descending order in our query syntax:

var studentsByGPA = from student in students orderby student.GPA descending select student;

var studentsByGPA = from student in students orderby student.GPA ascending select student;

However, in method syntax this is accomplished by a separate operator, the OrderByDescending() method:

var studentsByGPA = students.OrderByDescending(student => student.GPA);

If we need to order by multiple properties, i.e. first and last names, this is accomplished by a comma-separated list in query syntax:

var studentsByName = from student in students orderby student.Last, student.First select student;

But in method syntax, we need to use a ThenBy() operator for subsequent sorting options:

var studentsByName = students.OrderBy(student => student.Last).ThenBy(student => student.First);

We can mix and match ascending and descending sorting as well - for example, to sort students by descending GPA, then by names in alphabetical order we would use the query syntax:

var studentsByGPAAndName = from student in students orderby student.GPA descending, student.Last, student.First select student;

The corresponding method syntax would need separate operators for each sorting:

var studentsByGPAAndName = students.OrderByDescending(student => student.GPA).ThenBy(student => student.Last).ThenBy(student => student.First);

There is also a ThenByDescending() operator for chaining descending sorts.

Finally, there is also a Reverse() operator which simply reverses the order of items in the collection without sorting.

Grouping

We often want to split our results into groups, which can be accomplished with the group by operator. Consider the case where we want to split our students by the value of their Major field. We can accomplish this with query syntax:

var studentsByMajor = from student in students group student by student.Major select student;

or using method syntax:

var studentsByMajor = students.GroupBy(student => student.Major);

The result type of a grouping operation is an IEnumerable<IGrouping<TKey, TSource>>; the IGrouping is essentially a key/value pair with the key being the type we were grouping by. In the example it would be IEnumerable<IGrouping<string, Student>> (Seeing this, you can probably begin to appreciate why we normally use var for query variables).

To print out each student in each category, we’d need to iterate over this collection, and then over the groupings:

foreach(var group in studentsByMajor) {
    Console.WriteLine($"{group.Key} Students");
    foreach(var student in group) {
        Console.WriteLine($"{student.First} {student.Last}");
    }
}

Paging

A common strategy with large data sets is to separate them into pages, i.e. the first 20 items might appear on page 1, and by clicking the page 2 link, the user could view the next twenty items, and so on. This paging functionality is implemented in LINQ using the Skip() and Take() operators. The Skip() operator specifies how many records to skip over, while the Take() operator indicates how many records to include. Thus, to take the second page of students when each page displays twenty students, we would use:

var pagedStudents = students.Skip(20).Take(20);

Note that there is no query syntax corresponding to the Skip() and Take() operations, so to use them with query syntax, we wrap the query in parenthesis and invoke the methods on the result. I.e. sorting students alphabetically and then taking the third page of twenty would be:

var pagedSortedStudents = (from student in students orderby last, first select student).Skip(40).Take(20);

Existence Checks

Sometimes we want to know if a particular record exists in our data source. The Any() operator can be used to perform such a check. It evaluates to true if the query has any results, or false if it does not. Like the Skip() and Take(), it does not have a query syntax form, so it must be invoked using the method syntax. For example, to determine if we have at least one student named Bob Smith, we could use:

var bobSmithExists = (from student in students where student.First == "Bob" && student.Last == "Smith" select student).Any();

Or, in method syntax:

var bobSmithExists = students.Any(student => student.First == "Bob" && student.Last == "Smith");

Alternatively, if we wanted to retrieve Bob Smith’s record instead of simply determining if we had one, we could use First():

var bobSmith = (from student in students where student.First == "Bob" && student.Last == "Smith" select student).First();

or in method syntax:

var bobSmith = students.First(student => student.First == "Bob" && student.Last == "Smith");

This evaluates to the first matching result of the query (if we have multiple Bob Smiths, we’ll only get the first one). If there is no matching record, an InvalidOperationException is thrown. Alternatively, we can use FirstOrDefault() which returns a default value corresponding to the query data type. For classes, this would be null, so given this query:

var bobSmith = students.FirstOrDefault(student => student.First == "Bob" && student.Last == "Smith");

The value of bobSmith would be his record (if he is in the collection) or null (if he was not).

Aggregating

Sometimes we want to perform aggregate operations upon a data source, i.e. counting, summing, or averaging. As with paging, these are accomplished via method-only operators. For example, to count all students in our data source, we could use:

var studentCount = students.Count();

This can be combined with any LINQ query, i.e. the count of students with a GPA above 3.0 in query syntax would be:

var studentsAbove3GPA = (from student in students where student.GPA > 3.0 select student).Count();

or in method syntax:

var studentsAbove3GPA = students.Where(student => student.GPA > 3.0).Count();

Similarly, to compute the average GPA we would use the Average() method in conjunction with a projection. In query syntax:

var averageGPA = (from student in students select student.GPA).Average();

or in method syntax:

var averageGPA = students.Select(student => student.GPA).Average();

or we can move the selector Predicate into the Average() directly:

var averageGPA = students.Average(student => student.GPA);

We can create more complex queries to address specific questions. For example, with a group by we could compute the average GPA by major:

var gpaByMajor = from student in students group student by student.Major into majorGroup select new 
    { 
        Major = majorGroup.Key,
        AverageGPA = majorGroup.Average(student => student.GPA);
    }

The Sum() operator works similarly, summing a value. To sum the ages of all students, we could use:

var sumOfAges = (from student in students select student.Age).Sum();

or

var sumOfAges = students.Select(student => student.Age).Sum();

or

var sumOfAges = students.Sum(student => student.Age);

There are also Min() and Max() aggregate operators which select the minimum and maximum values, respectively. For example, we could find the maximum and minimum earned GPAs with:

var minGPA = students.Min(student => student.GPA);
var maxGPA = students.Max(student => student.GPA);

Finally, there is a generic Aggregate() method which provides an aggregator variable that we can use to build any kind of aggregate function. Let’s first see how it can be used to duplicate the functionality of the Sum() method:

var sumOfAges = students.Aggregate((sum, student) => sum + student.Age);

Here, sum is inferred to be an int, as student.Age is an int. So it starts at 0, and each time the Aggregate method processes a student, it adds that student’s Age into the sum.

Now let’s use this method for something new - generating a string of email addresses for a bulk mailing. Assume our email application needs a list of semicolon-separated email addresses. In that case, we could generate the emails for all students from:

var emails = students.Aggregate((emails, student) => emails + $"; {student.EID}@k-state.edu");

If we had students with EIDs “mary”, “brb30”, and “stan”, the resulting string would be:

mary@k-state.edu; brb30@k-state.edu; stan@ksu.edu