LINQ in C#

What is LINQ in C#?

LINQ (Language Integrated Query) is a C# feature that enables writing queries directly within the code to retrieve and manipulate data from various sources such as collections, databases, XML, and datasets. It offers a unified, readable, and type-safe syntax. Introduced in C# 3.0, LINQ supports in-memory objects (System.Linq), databases (Entity Framework, LINQ to SQL), XML (System.Xml.Linq), and more, with compile-time checking and IntelliSense support.

Advantages:

·       Clean and readable code

·       Unified querying syntax

·       Compile-time checking

·       IntelliSense support

·       Strong typing

Limitations:

·       May not perform well for complex queries

·       Difficult to debug long chained queries

·       SQL translation might not be optimal

 

What is Expression in C#?

Expression is a built-in class in the System.Linq.Expressions namespace.
It represents code in a tree-like structure—not as actual executable code, but as data that can be analyzed, modified, or translated (e.g., into SQL by LINQ providers).

It can be used as a return type when returning an expression tree, especially in scenarios like dynamic filtering or query generation in LINQ providers such as Entity Framework.
It is the base class from which all expression tree nodes inherit.

Example:

Expression<Func<int, bool>> expr = x => x > 10;

This does not immediately execute. Instead, it builds a tree structure like:

Lambda

 └── Body: GreaterThan

       ├── Left: x

       └── Right: 10

 

Types of Expressions (Common Subclasses)

Expression Type	Description	Example Syntax
LambdaExpression	Represents a lambda expression	x => x.Age > 18
BinaryExpression	Binary operations	x > 10, x == y
MethodCallExpression	Method calls	x.Contains("abc")
MemberExpression	Accessing a property or field	x.Age
ConstantExpression	A constant value	18, "hello"
ParameterExpression	A parameter placeholder	x => ...
UnaryExpression	Unary operations (!, -, etc.)	!x.IsActive

All of these are subclasses of the base Expression class.

 

Usage Scenarios

Expressions are used when:

• Building dynamic LINQ filters
• Creating queries at runtime
• Working with tools like Entity Framework, OData, LINQKit, and rule engines
• Writing testing frameworks and mocking tools

 

Summary

Feature	Expression (System.Linq.Expressions)
Type	Built-in abstract class
Purpose	Represents code as data (expression trees)
Usage	Build queries dynamically (LINQ, EF, filters)
Subtypes	Lambda, Binary, Member, MethodCall, etc.
Return Type	Yes, commonly used for filter expressions
Execution	Requires compilation or usage by a LINQ provider