VB.NET Lambdas and Closures Cheat Sheet

# VB.NET Lambdas and Closures Cheat Sheet

1️⃣ What is a Lambda in VB.NET?

A lambda expression is an anonymous function that can be assigned to a variable or passed as an argument.

Basic Lambda Syntax

' Single-line function
Dim add As Func(Of Integer, Integer, Integer) = Function(x, y) x + y
Console.WriteLine(add(3, 5)) ' Output: 8

Lambda with a Block Body (Multi-line Lambda)

Dim sayHello As Action(Of String) = Sub(name)
    Console.WriteLine("Hello, " & name & "!")
End Sub

sayHello("Alice") ' Output: Hello, Alice!

✅ Key Notes:

Use Function when returning a value.
Use Sub when performing an action (void).
Lambdas in VB.NET are more verbose than in C# or JavaScript.

2️⃣ What is a Closure in VB.NET?

A closure is a function that remembers variables from its outer scope even after the outer function has finished execution.

Basic Closure Example: Counter

Function MakeCounter() As Func(Of Integer)
    Dim count As Integer = 0
    Return Function()
               count += 1
               Return count
           End Function
End Function

Dim counter = MakeCounter()
Console.WriteLine(counter()) ' Output: 1
Console.WriteLine(counter()) ' Output: 2

✅ Key Notes:

The inner function remembers count even after MakeCounter() has exited.
Every time counter() is called, it increments count persistently.

3️⃣ Closures with Parameters

Closures can take parameters to make them more dynamic.

Example: Counter with Step Value

Function MakeCounter(start As Integer) As Func(Of Integer, Integer)
    Dim count As Integer = start
    Return Function(step As Integer)
               count += step
               Return count
           End Function
End Function

Dim counter = MakeCounter(10)
Console.WriteLine(counter(2)) ' Output: 12
Console.WriteLine(counter(5)) ' Output: 17

✅ Key Notes:

The closure remembers count and updates it every time it’s called.
Each instance has its own state, preventing unintended side effects.

4️⃣ Closures with Objects

You can return objects with multiple functions.

Example: Counter with Reset Function

Function MakeCounter(start As Integer) As Object
    Dim count As Integer = start
    Return New With {
        .NextValue = Function()
                        count += 1
                        Return count
                    End Function,
        .Reset = Sub()
                    count = start
                 End Sub
    }
End Function

Dim counter = MakeCounter(10)
Console.WriteLine(counter.NextValue()) ' Output: 11
Console.WriteLine(counter.NextValue()) ' Output: 12
counter.Reset()
Console.WriteLine(counter.NextValue()) ' Output: 11

✅ Key Notes:

The anonymous object (New With {}) allows returning multiple related functions.
Reset() sets count back to the original start value.

5️⃣ When to Use Lambdas vs. Closures?

Feature	Use Lambda	Use Closure
Simple inline function	✅ Yes	❌ No
Captures state between calls	❌ No	✅ Yes
Used in `Where()`, `Select()`, etc.	✅ Yes	❌ No
Event handlers	✅ Yes	✅ Yes
Multiple independent instances	❌ No	✅ Yes

🚀 Final Takeaways

✅ Lambdas are useful for short, inline functions but are not as elegant as in C# or JavaScript. ✅ Closures shine when you need stateful functions that remember values between calls. ✅ Use closures to create independent, self-contained function instances.

Would you like more examples or deeper explanations? 🚀