Shallow copies versus deep copies

This lesson covers the difference between shallow and deep copies in VB.NET, with the aim of avoiding bugs caused by unintentionally shared data.

Assigning a class instance to another variable usually copies the reference (the address) rather than the thing itself. That is a shallow copy, and changing one affects the other.

In the sample code, Dim shallow As Address = original makes shallow point at the same instance. Changing it with shallow.City = "Osaka" therefore changes what original.City displays too.

A common early stumble is thinking you have copied something when in fact both names share one instance. When you want genuinely separate data you have to make a deep copy — a new instance with the values copied across.

In professional work, understanding the difference matters for avoiding bugs where data is shared unintentionally — copying a template of settings and then editing it individually, for instance.

📖 Reference code

Class Address
    Public City As String
End Class

Dim original As New Address()
original.City = "Tokyo"

Dim shallow As Address = original
shallow.City = "Osaka"
Console.WriteLine(original.City)

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run VB.NET directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).