Optional arguments ^{C# 4.0} allow parameters to a method to assume a default in the absence of an explicit passed value.

C# 12.0 extends this to lambda expressions ^{C# 3.0}.

Code

Now

var increment = (int x, int y = 1) => x + y;

Before

var increment = (int x, int y) => x + y;
var incrementBy1 = (int x) => increment(x, 1);

Notes

• The default value is part of the lambda's inferred delegate type, so it only applies when called through var (or a delegate type declaring the same default); assigning the lambda to Func<int, int, int> drops it and produces warning CS9099
• C# 12 also allows params arrays on lambda parameters, such as (params int[] values) => values.Sum()

More information

• What's new in C# 12.0
• Language reference