C sharp - Expression Trees in C#

Expression Trees allow you to represent code as data.
Instead of executing a method immediately, you can store its structure in a tree format and inspect, modify, or execute it later.

They are mainly used in:

• LINQ providers (like Entity Framework)

• Dynamic query generation

• Rule engines

• Compilers and code analyzers

1. Normal Code vs Expression Tree

Normal Lambda (Compiled and Executed)

Func<int, int> square = x => x * x;
Console.WriteLine(square(5));  // 25

Here:

• Code is compiled into IL

• It executes directly

Expression Tree Version

using System.Linq.Expressions;

Expression<Func<int, int>> squareExpr = x => x * x;

Here:

• The lambda is not executed

• It is converted into a tree structure

2. What Does the Tree Look Like?

For:

x => x * x

The structure is:

Lambda
 └── Multiply
     ├── Parameter (x)
     └── Parameter (x)

Each part is represented by an object:

• ParameterExpression

• BinaryExpression

• LambdaExpression

3. Compiling an Expression Tree

To execute it:

var compiled = squareExpr.Compile();
Console.WriteLine(compiled(5));  // 25

Important:

• Expression Tree → needs .Compile() to execute

• Normal Func → executes directly

4. Why Expression Trees Are Important

They allow inspection of code logic.

Example:
Entity Framework uses them to translate C# code into SQL.

var result = db.Users.Where(u => u.Age > 18);

The lambda u => u.Age > 18 becomes an Expression Tree.

EF reads it and converts it into:

SELECT * FROM Users WHERE Age > 18

Without expression trees, this translation is impossible.

5. Building Expression Trees Manually

You can construct them programmatically.

ParameterExpression param = Expression.Parameter(typeof(int), "x");

BinaryExpression body = Expression.Multiply(param, param);

Expression<Func<int, int>> lambda =
    Expression.Lambda<Func<int, int>>(body, param);

var compiled = lambda.Compile();
Console.WriteLine(compiled(4));  // 16

This is how dynamic query engines work internally.

6. Common Expression Types

7. Expression Trees vs Delegates

8. Limitations

Expression Trees:

• Cannot represent all C# syntax

• No loops like for, while

• Limited statement support

• Designed mainly for expressions, not full methods

9. Real-World Uses

1. Entity Framework

2. LINQ Providers

3. Dynamic filtering systems

4. Code generation tools

5. Rule engines

6. ORMs

10. Advanced Concept: Expression Visitors

C# provides ExpressionVisitor class.

It allows:

• Traversing

• Modifying

• Rewriting expression trees

Example use case:
Automatically modify all x > 18 to x > 21.

This is powerful for:

• Security filters

• Multi-tenant applications

• Query rewriting