with expressions ^{C# 9.0} allowed you to create a copy of a record with some properties changed. However, this was limited to record types.

C# 10.0 extends with expressions to work with all structs and anonymous types, making it easy to create modified copies without mutating the original.

Code

Now

var point = new Point { X = 1, Y = 2 };
var moved = point with { X = 5 };

var config = new { Host = "localhost", Port = 8080 };
var prodConfig = config with { Host = "prod.example.com" };

Before

var point = new Point { X = 1, Y = 2 };
var moved = new Point { X = 5, Y = point.Y };

// No way to copy-and-modify anonymous types
var config = new { Host = "localhost", Port = 8080 };
var prodConfig = new { Host = "prod.example.com", Port = config.Port };

Notes

• The with expression performs a shallow copy then applies the specified property changes
• Especially useful with readonly structs where direct mutation is not possible
• Anonymous types are always immutable so with is the only way to derive a modified copy

More information

• with expression
• What's new in C# 10.0