Introduction
LINQ has its own set of
Query operators which make it powerful. These exist in the System.LinqSystem.Core assembly. These are extension methods on the Enumerable and Queryable objects. Namespace within the
The standard LINQ query
operators can be classified into the following types:
Qualifiers Operators
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Any, All,
Contains
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Set Operators
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Distict,
Union, Intersect, Except
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Ordering Operators
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OrderBy, OrderByDescending , ThenBy , ThenByDescending , Reverse
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Equality Operators
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SequenceEqual
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Conversion Operators
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Cast, ToArray, ToList, ToDictionary, OfType,
SingeOrDefault
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Grouping
Operators
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GroupBy
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Joining operators
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Join, GroupJoin
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Element Operators
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First,
FirstOrDefault,ElementAt
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Partitioning Operators
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Take,
Skip, TakeWhile, SkipWhile
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Projection Operators
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Select ,
SelectMany
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Aggregate Operators
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Count, Sum
,Min, Max, Average, Aggregate
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Restriction Operators
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Where
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Concatenation
operators
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Concat
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Generation Operators
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Range, Repeat
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This article will focus
on a handful of LINQ queries that can improve our C# programming.
1. Avoiding FOREACH on Collections to Filter for Data
You can avoid the FOREACH and the IF clauses inside your FOREACH to
filter for data. Check the below sample that eliminates the confusing
conditional statements with a simpler LINQ query.
LINQ CODE
|
var filteredUsers = from
currentUser in Users
where currentUser.Active
&& currentUser.AllowLogin&& ! urrentUser.LoggedIn
select user;
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2. Select X Rows
When we have to select a
few rows from a collection, we check the loop counter with a fixed constant and
break the for loop or control the loop counter.
LINQ CODE
|
var filteredUsers =
Users.Where(u => u.Active && u.AllowLogin && !u.LoggedIn)
.Take(10)
|
3. Take the First Element from a Collection
We face lot of situations
where we have to read the first element in a collection. We would start off by
evaluating the collection with null and then its elements count and so on. Now,
there is a possibility that this collection might be empty.
LINQ CODE
|
List<string> adminUsers =
RetrieveUsersForAdminRole(RoleType.Admin);
User firstUser = adminUsers.FirstOrDefault();
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If
there are no users found, it returns the default value of the underlying type.
4. Let Go the IComparer<T> for Sorting
We no more need to write
the tough IComparer classes to sort our custom data. We can now use the OrderBy
method to order the data. Look at the sample query below for more brevity.
LINQ CODE
|
var filteredUsers = Users.Where(u
=> u.Active && u.AllowLogin && !u.LoggedIn).OrderBy( u
=> u.Name);
|
The ORDERBY function
takes in a parameter that is used to sort. If you want do a multi-sort , use
the THENBY operator.
LINQ CODE
|
var filteredUsers = Users.Where(u
=> u.Active && u.AllowLogin && !u.LoggedIn).OrderBy( u
=> u.Name).ThenBy(u => u.Location);
|
Yes, you can sort in a
Descending fashion too. LINQ provides the ORDERBYDESCENDING operator for this
very purpose.
LINQ CODE
|
var filteredUsers = Users.Where(u
=> u.Active && u.AllowLogin && !u.LoggedIn).OrderByDescending(
u => u.Name).ThenBy(u => u.Location);
|
Note: There is a ThenByDescending
operator too.
5. Do Not Use for Loops to Initialize Arrays
Do not use for loops to
initialize arrays. We do this a lot of times to write quick test code. This can
be replaced using the System.Linq.Enuemerable.Range method.
Using LinQ it is just
one line of code.
LINQ CODE
|
int[] monthsInAYearByLINQ =
System.Linq.Enumerable.Range(1, 12).ToArray();
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6. Replace Two Similar Loops with Concat
If you have to loop
through two arrays which are similar in nature, you can use the Concat
extension and write concise code. Now, consider this piece of code which uses
the Concat operator.
LINQ CODE
|
int[] firstArray
= System.Linq.Enumerable.Range(1,
12).ToArray();
int[] secondArray = System.Linq.Enumerable.Range(13,
12).ToArray();
foreach (var a in
firstArray.Concat(secondArray))
{
allElements.AppendLine(string.Format("{0}",
a));
}
|
7. Avoid Transformations
A transformation is a
technique that you follow to return a new collection that is obtained by
looping through another collection and applying filters on them.
Instead of creating a
temporary list and then filling it up with users, we can use the LINQ SELECT
and the ToArray Method to return the results for us.
LINQ CODE
|
public Users[] FindUsers(RoleType r)
{
return FindUsers()
.Select(user => new User
{
FirstName
= u.FirstName,
…
});
.Where(user => user.Role == r)
.ToArray();
}
|
8. The Let Keyword
The LET keyword lets you
build temporary variables in your LINQ query, so that your SELECT query becomes
easier to read, and maintain. It lets you predefine variables at a certain
stage of the query that can be used in the rest of the query.
For example, the
following query calculates the average twice. This can be replaced by using the
LET operator. The query will perform faster as the Average is not calculated
twice.
LINQ CODE(Simple Query)
|
var results= from store in Stores where
store.Sales.Average(s => s.Price) > 500
select new
{
Name = store.Name
StoreAveragePrice = store.
Sales.Average(s => s.Price)
};
|
See the following
snippet that uses the LET keyword.
LINQ CODE(Query using Let)
|
var results= from store in Stores
let AveragePrice =
store.Sales.Average(s => s.Price)
where AveragePrice > 500
select new
{
Name = store.Name
StoreAveragePrice = AveragePrice
};
|
9. RegEx + LINQ?
LINQ queries can help
you work with Regular Expression results and make your henceforth queries
simpler. Look at the following sample that iterates over a collection iterating
them over the regex expression.
Note that you can do
other LINQ queries on the RegEx Matched results.
LINQ CODE
|
List<string> examinerStatements
= new List<string>();
examinerStatements.Add("Mark was
present.");
examinerStatements.Add("Julie was
present.");
examinerStatements.Add("John was
absent");
System.Text.RegularExpressions.Regex
myRegEx = new System.Text.RegularExpressions.Regex("present");
var presentStudents = examinerStatements
.Where<string>(statement =>
myRegEx.IsMatch(statement))
.ToList<string>();
foreach (var examinerStatement in
presentStudents)
{
//
}
|
10. Querying ArrayList
You would have found
that you cannot run LINQ queries on an ArrayList. That is because it does not
implement the System.Collections.Generic.IEnumerable<T> interface.
LINQ CODE
|
List<string ArrayList myList = new
ArrayList();
myList.Add("One");
myList.Add("Two");
myList.Add("Three");
var results = from value in myList select
new { value };
|
You can use the Cast
Operator in such cases. The modified code snippet below shows you how to use
that.
LINQ CODE
|
ArrayList myList = new ArrayList();
myList.Add("One");
myList.Add("Two");
myList.Add("Three");
var results = from value in
myList.Cast<string>() select new { value };
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I hope these ten samples helped you learn
something new and enjoy reading.
