LINQ (Language Integrated Query) is a uniform query syntax in C# and VB.NET to retrieve data from different sources and formats. It is integrated with C# or VB, thereby eliminating the mismatch between programming languages and databases, as well as providing a single querying interface for different types of data sources.

For example, SQL is a Structured Query Language used to save and retrieve data from a database. In the same way, LINQ is a structured query syntax built-in C# and VB.NET to retrieve data from different types of data sources such as collections, ADO.Net DataSet, XML Docs, web service, and MS SQL Server, and other databases.

      string[] words = {"hello", "wonderful", "LINQ", "beautiful", "world"};

      var shortWords = from word in words where word.Length <= 5 select word;

      Dim words As String() = {"hello", "wonderful", "LINQ", "beautiful", "world"}

      Dim shortWords = From word In words _ Where word.Length <= 5 _ Select word

When the above code of C# or VB is compiled and executed, it produces the following result −

There are two syntaxes of LINQ. These are the following ones.

var longWords = words.Where( w ⇒ w.length > 10);

Dim longWords = words.Where(Function(w) w.length > 10)

var longwords = from w in words where w.length > 10;

Dim longwords = from w in words where w.length > 10

The types of LINQ are mentioned below in brief.

Apart from the above, there is also a LINQ type named PLINQ which is Microsoft’s parallel LINQ.

LINQ has a 3-layered architecture in which the uppermost layer consists of the language extensions and the bottom layer consists of data sources that are typical objects implementing IEnumerable <T> or IQueryable <T> generic interfaces. The architecture is shown below.

A query expression is nothing but a LINQ query, expressed in a form similar to that of SQL with query operators like Select, Where and OrderBy. Query expressions usually start with the keyword "From".

To access standard LINQ query operators, the namespace System. The query should be imported by default. These expressions are written within a declarative query syntax which was C# 3.0.

Below is an example to show a complete query operation that consists of data source creation, query expression definition, and query execution.

         int[] scores = new int[] { 97, 92, 81, 60 };

         IEnumerable<int> scoreQuery = from score in scores where score > 80 select score;

Overview of LINQ

LINQ is known as Language Integrated Query and it is introduced in .NET 3.5 and Visual Studio 2008. The beauty of LINQ is it provides the ability to .NET languages(like C#, VB.NET, etc.) to generate queries to retrieve data from the data source. For example, a program may get information from the student records or access employee records, etc. In, past years, such type of data is stored in a separate database from the application, and you need to learn different types of query language to access such types of data like SQL, XML, etc. And also you cannot create a query using C# language or any other .NET language.

To overcome such types of problems Microsoft developed LINQ. It attaches one, more power to the C# or .NET languages to generate a query for any LINQ compatible data source. And the best part is the syntax used to create a query is the same no matter which type of data source is used means the syntax of creating query data in a relational database is the same as that used to create query data stored in an array there is no need to use SQL or any other non-.NET language mechanism. You can also use LINQ with SQL, with XML files, with ADO.NET, with web services, and with any other database.

In C#, LINQ is present in System. Linq namespace. It provides a different type of classes and methods which supports LINQ queries. In this namespace:

The enumerable class holds standard query operators that operate on an object which executes IEnumerable<T>.

The queryable class holds standard query operators that operate on an object which executes IQueryable<T>.

The following diagram will represent the complete architectural view of LINQ.

The LINQ to Objects provider allows us to query an in-memory object such as an array, collection, and generics types. It provides many built-in functions that we can use to perform many useful operations such as filtering, ordering, and grouping with minimum code.

The LINQ to Datasets provider provides us the flexibility to query data cached in a Dataset in an easy and faster way. It also allows us to do further data manipulation operations such as searching, filtering, sorting, etc. on the Dataset using the LINQ Syntax.

The LINQ to SQL Provider is designed to work with only the SQL Server database. You can consider this as an object-relational mapping (ORM) framework which allows one-to-one mapping between the SQL Server database and related .NET Classes. These .NET classes are going to be created automatically by the wizard based on the database table.

The LINQ to XML provider is designed to work with an XML document. So, it allows us to perform different operations on XML data sources such as querying or reading, manipulating, modifying, and saving the changes to XML documents. The System.Xml.Linq namespace contains the required classes for LINQ to XML.

The LINQ to Entities provider looks like LINQ to SQL. It means it is also an object-relational mapping (ORM) framework that allows one to one, one to many, and many to many mapping between the database tables and .NET Classes. The point that you need to remember is, it is used to query any database such as SQL Server, Oracle, MySQL, DB2, etc. Now, it is called ADO.NET Entity Framework.

3. Tables are automatically created into class.

4. Columns are automatically created into properties.

5. Relationships are automatically appended to classes.

9. LINQ is part of .NET, we can use the visual studio's debugger to debug the queries.

10. It is more concise and readable, especially when filtering multiple conditions.

11. They provide powerful filtering, ordering, and grouping capabilities with a minimum of application code.

2. Lambdas and extension methods are my hammers and all problems are nails.

4. LINQ is not good to write complex queries like SQL Server.

5. There is no good way of viewing permissions.

6. Deferred execution and streaming are poorly understood.

7. A LINQ advance query is hard to understand but too many people don't understand it but 
   still use it.

8. Small data sets will take longer to build the query than execute.

9. There is an overhead for creating queries.

10. Debugging can be very tricky due to deferred execution and streaming.

In earlier to LINQ, it's necessary to learn SQL, C#, and several APIs that bind together to develop an entire application. So that those programming languages and data sources face some issues and coding also makes it difficult. Let’s see an example of several techniques was used by programmers when querying data before the arrival of Language Integrated Query.

You have already known how LINQ varies from and gets better upon ADO.NET. In addition, It helps in time-saving. Ultimately, you have known about how language integrated queries can be used for several types of data sources.

In this article, we'll shortly describe LINQ.

Overview of LINQ(Language Integrated Query)

In VB, an LINQ query is executed in the For Each Statement, and in the foreach statement for C#.

The GroupBy clause groups the elements that share a common attribute.

The standard query operators implement the IEnumerable<T> or the IQueryable<T> interface in C# and the IEnumerable(Of T) or the IQueryable(Of T) interface in Visual Basic.

The Take clause returns a specified number of elements. For example, you can use the Take clause to return two values from an array of numbers. The Skip clause skips the specified number of elements in the query and returns the rest. For example, you can use the Skip clause to skip the first four strings in an array of strings and return the remaining array of string.

The DISTINCT clause returns the result set without the duplicate values.

In compiled LINQ queries, the plan is cached in a static class and static class is a global cache. Rather than preparing the query plan from scratch, LINQ prepares plan using stating class object.

DataContext class acts as a bridge between SQL Server database and the LINQ to SQL. For accessing the database and also for changing the data in the database, it contains connections string and the functions.

Skip(): It will take an integer argument and from the given IEnumerable it skips the top n numbers

SkipWhile(): It will continue to skip the elements as far as the input condition is true. It will return all remaining elements if the condition is false

LINQ supports XML, SQL, Dataset and Objects. Through LINQ to objects or LINQ to Datasets one can use LINQ with other databases. The objects and datasets take care of database particular operations, and LINQ only needs to deal with those objects and not the database operations directly.

Lambda expression is referred as a unique function use to form delegates or expression tree types, where right side is the output and left side is the input to the method. For writing LINQ queries particularly, Lambda expression is used.

PLINQ is Parallel Language Integrated Query. In PLINQ, a query can be executed using many processors. With PLINQ, Software can be made scalable across environments during execution. Apart from supporting all operators in LINQ, PLINQ also runs many LINQ queries simultaneously as well.

The Predicate delegate is provided by the base class of the .NET library. In Predicate, we are allowed to store only methods with one input parameter and bool return type.

Action refers to the general delegates belonging to the base class library of .NET We can store only methods with input parameters and void return types in Action. Up to 16 parameters can be specified.

An Anonymous function is a function without a name. In an Anonymous function, we only define parameters and write the code in curly braces.

LINQ queries can be written for the classes that implement IEnumerable or IQueryable interfaces. The System Linq namespace provides many classes for interfaces required for LINQ queries.

If you want to use LINQ in an ASP.NET webpage, then LinqDataSource is an important part of the dataset. It is used to set the properties in the markup text, control, retrieve, and modify data. It can also be used to declaratively bind other ASP.NET controls on a page to a data source. In this manner, it is similar to SQL Datasource and ObjectDataSource controls.

The differences between Skip() and Skipwhile() extension methods are:

Lambda expression is referred to as a unique function used to form delegates or expression tree types, where the right side is the output and the left side is the input to the method. For writing, LINQ queries particularly, Lambda expression is used.

LINQ to XML provides the in-built document reform capabilities of the DOM (Document Object Model) and supports LINQ Queries. Using it, we can modify query, navigate, and save the changes of an XML document. It allows us to write queries to retrieve and navigate a collection of elements and attributes. It is quite similar to XPath and XQuery.

LINQ to SQL is a part of ADO.NET technologies. It manages the relational data as an object. LINQ to SQL converts the language integrated query in the object to SQL and forward them to the database for execution. When the database responses, the result LINQ to SQL translate them back to objects.

LINQ to SQL supports user-defined functions and stored procedure in the database.

With other programming language and C#, LINQ is used, it requires all the variables to be declared first. “FROM” clause of LINQ query defines the range or conditions to select records. So, FROM clause must appear before SELECT in LINQ.

Where clause: It allows adding some conditional filters to the query.

Let clause: It allows defining a variable and assigning it a value calculated from the data values.

Visual Basic includes the following features that support LINQ:

Anonymous types- Enables you to create a new type based on a query result

Implicitly typed variables – Enables the compiler to infer and assign a type when you declare and initialize a variable

Extension method – Enables you to extend an existing type with your own methods without modifying the type itself

Debugging: It is difficult to debug a stored procedure but as LINQ is part of.NET, visual studios debugger can be used to debug the queries

Deployment: For stored procedure, additional script should be provided but with LINQ everything gets compiled into single DLL hence deployment becomes easy

Type Safety: LINQ is type safe, so queries errors are type checked at compile time

The primary purpose of using LINQ is to retrieve complex query in a dataset.

LINQ is used to combine the values from two different data set.

It is also used to fetch a unique value from the data set.

LINQ gives a more precise way than SQL query of querying the dataset.

LINQ also provides more functionality as compared with ADO.NET.

LINQ is a programming model that is the composition of trendy-cause general query operators that assist you to paintings with records, irrespective of the records source in any .NET based programming language. It is the name given to a fixed of technology based on the combination of query abilties into any .NET language.

Language Integrated Query is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages, originally released as a major part of .NET Framework 3.5 in 2007.

LINQ

My goal is to query a DB data context using LINQ with dynamic query criteria. I am aware that one way to access an object using its field name is by converting it to an IDictionary, using C# reflection API.

Is using reflection a good idea, or should expression trees be preferred? How fast would my queries be using them both?

Is querying objects (LINQ) using expression trees faster than using reflection? How much?

Anyone know how I can do a very simple/basic subquery in LINQ like so:

SELECT a.VendorID,a.Name,
  (SELECT count(Agency_Code) from Contracts b where b.Vendor_Code = a.VendorID)


just trying to get a count of the number of contracts each vendor has

how to do a simple subquery in LINQ?

I have a table in SQL Server that have a NOT NULL Column that uses a SEQUENCE as default (NEXT VALUE FOR SeqDefa), that column is not the PK from the table.

I building a web form in ASP.NET that does a CRUD in that table, when i want to send a null value in that column to use the default in that table i get an error. that's because LINQ says that you can't send a null value to a not null column

can i get the sequence´s next value from LINQ in my form? there's another option than changing table column from not null to allowing nulls?

How to send a null value to a not null column in ASP.NET with LINQ

I have a table with a string field with such data: '1,2,4,5,8,10,15,20,35' And would like to select entries where this field contains value 5. But I can not use Contains, because I will get entries consists not only 5, but 15, 35, for example. I tried to do something like:

result = uow.Query<MyObject>().Where(x => x.MyProperty.Split(',').Contains("5")).Any();


But got the exception about expression is not supported. How can I do this?

Expression is not supported. LinQ

So basically I have created a wpf form as required, but I'm not sure on how to do the class library and what to exactly do in the library and insert it into to the wpf and use it. If anyone could help I'd really appreciate it. 

How would i go about doing the class library and inserting it into my main form, preferably with an example?

Hi lets say I have an property path looking like this

We can say that I have an article that has an Vat and Vat has an Value. Now I want to use Linq to sort a list using that property path How can I build that lambda expression when I have "Vat.Value" in a string and I want the following result

I will not always know the types of Vat and Value, sometimes its only x.Name I'm out for.

Build an Lambda expression from an propertypath

To make an array of doubles starting from 1 to 10 I currently am doing:

Dim N() As Double = Enumerable.Range(1, 10).Select(Function(x) CDbl(x)).ToArray


My technique seems inefficient and indirect. I see that ToArray has the space for arguments but whenever I try ToArray(Double) or ToArray(Of Double) it gives me an error.

What is the correct way to cast the elements to type Double when creating an array through Enumerable.Range?

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openIs querying objects (LINQ) using expression trees faster than using reflection? How much?

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openhow to do a simple subquery in LINQ?

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openHow to send a null value to a not null column in ASP.NET with LINQ

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openExpression is not supported. LinQ

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openHow would i go about doing the class library and inserting it into my main form, preferably with an example?

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openBuild an Lambda expression from an propertypath

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openIs there a nicer way to Cast the result of Enumerable.Range to type Double?

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