MSDN Documentation

LINQ (Language Integrated Query) in VB.NET

LINQ is a powerful feature in the .NET Framework that adds native data querying capabilities to the VB.NET language. It allows you to write queries against various data sources in a uniform way, regardless of whether the data is in memory, a database, or XML.

What is LINQ?

LINQ provides a consistent model for querying data. It integrates query capabilities directly into the VB.NET language, allowing you to write code that is more readable, maintainable, and less prone to errors. LINQ queries are compiled into standard .NET method calls at compile time.

Key Components of LINQ

• Query Syntax: A declarative syntax that resembles SQL.
• Method Syntax: Uses extension methods provided by LINQ to Objects.
• Query Operators: Methods that perform operations like filtering, sorting, grouping, and projection.
• Data Sources: Can include collections, databases (LINQ to SQL, LINQ to Entities), XML (LINQ to XML), ADO.NET datasets, and custom data sources.

LINQ Query Syntax Example

Here's a basic example of filtering and projecting a list of integers using LINQ query syntax:

Module LinqExample

    Sub Main()
        ' Sample data source
        Dim numbers As List(Of Integer) = New List(Of Integer) From {5, 4, 1, 3, 9, 8, 6, 7, 2, 0}

        ' LINQ query to select numbers greater than 3 and order them
        Dim query = From num In numbers
                    Where num > 3
                    Order By num
                    Select num

        ' Execute the query and display results
        Console.WriteLine("Numbers greater than 3, in ascending order:")
        For Each number In query
            Console.WriteLine(number)
        Next

        ' Example of projecting into an anonymous type
        Dim names As List(Of String) = New List(Of String) From {"Alice", "Bob", "Charlie", "David"}
        Dim projectedQuery = From name In names
                             Where name.Length > 4
                             Select New With {
                                 .FullName = name,
                                 .Initial = name.Substring(0, 1)
                             }

        Console.WriteLine(vbCrLf & "Names longer than 4 characters with their initials:")
        For Each item In projectedQuery
            Console.WriteLine($"Name: {item.FullName}, Initial: {item.Initial}")
        Next

    End Sub

End Module
                        

LINQ Method Syntax Example

The same query can be written using method syntax:

Module LinqMethodExample

    Sub Main()
        Dim numbers As List(Of Integer) = New List(Of Integer) From {5, 4, 1, 3, 9, 8, 6, 7, 2, 0}

        ' LINQ query using method syntax
        Dim query = numbers.Where(Function(num) num > 3).OrderBy(Function(num) num)

        Console.WriteLine("Numbers greater than 3, in ascending order (Method Syntax):")
        For Each number In query
            Console.WriteLine(number)
        Next
    End Sub

End Module
                        

Common LINQ Operators

LINQ provides a rich set of operators:

• Where: Filters elements based on a condition.
• Select: Projects elements into a new form.
• Orderby: Sorts elements.
• GroupBy: Groups elements based on a key.
• Join: Combines elements from two sequences.
• Count: Returns the number of elements.
• Sum, Average, Min, Max: Perform aggregate calculations.
• First, Single: Returns the first or a single element matching a condition.

LINQ Providers

LINQ is not just for in-memory collections. Different LINQ providers enable querying of other data sources:

• LINQ to Objects: For querying collections (arrays, lists, etc.).
• LINQ to SQL: For querying SQL Server databases.
• LINQ to Entities: For querying data through the Entity Framework.
• LINQ to XML: For querying XML documents.

Understanding and utilizing LINQ can significantly enhance your productivity and the quality of your VB.NET applications.