VB.NET — Visual Basic .NET from Basics to Advanced (with C# Comparison)

In short: Both compile to the same IL and run on the same CLR with the same libraries, so performance is the same.

• Differences are syntax and features: VB.NET uses English-like keywords, is case-insensitive, line-based with End blocks, has XML literals, My namespace, WithEvents/Handles and optional late binding;
• C# uses C-style syntax, is case-sensitive and gets new language features every year while VB.NET has been feature-stable since 2020.
• Most new projects use C#; VB.NET is mainly maintained in legacy systems.

In short: Option Explicit On requires variables to be declared.

• Option Strict On disallows implicit narrowing conversions and late binding — catching type errors at compile time (strongly recommended).
• Option Infer On lets Dim x = 5 infer the type from the initializer.
• Set them per project (Compile settings) or per file at the top.

In short: A Sub performs an action and returns no value (C# void method).

• A Function returns a value with Return (or by assigning to the function name in old code).
• Both can take ByVal/ByRef, Optional and ParamArray parameters.

In short: ByVal (the default) passes a copy of the value — for reference types, a copy of the reference, so the method can modify the object but not replace the caller's variable.

• ByRef passes the variable itself, so assigning a new value inside the method changes the caller's variable (C# ref).

In short: And/Or always evaluate both operands (and also act as bitwise operators on integers).

• AndAlso/OrElse short-circuit: the second operand is evaluated only if needed.
• Use AndAlso for conditions like If obj IsNot Nothing AndAlso obj.IsActive Then to avoid NullReferenceException.

In short: Declare with Event, raise with RaiseEvent.

• Subscribe statically by declaring the field WithEvents and adding a Handles obj.EventName clause to the handler (common in WinForms/Web Forms code-behind), or dynamically with AddHandler obj.Event, AddressOf Handler and RemoveHandler (like C# +=/-=).

In short: DirectCast performs a type cast requiring an inheritance/interface relationship and throws InvalidCastException on failure (fast, like C# (T)x).

• CType performs any defined conversion including VB conversions (string to integer) and user-defined conversions.
• TryCast returns Nothing instead of throwing (reference types only, like C# as).

In short: Stabilise first: source control, Option Strict On, logging, tests on critical flows.

• Extract business logic from code-behind into class libraries (VB or new C#), target netstandard2.0 where possible so both old and new apps can use them.
• Put a reverse proxy (YARP/IIS ARR) in front and rebuild pages/features in ASP.NET Core (C#) one area at a time, sharing authentication and the database (strangler fig).
• Retire Web Forms pages as their replacements go live.