From 0f793f2b5d9a37bc560b49c9dbef72569ea4cb4a Mon Sep 17 00:00:00 2001 From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com> Date: Fri, 1 Aug 2025 19:35:06 +0000 Subject: [PATCH 1/5] Initial plan From 0b4ebe69627f2a0d37058d12d7969876ad20db1c Mon Sep 17 00:00:00 2001 From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com> Date: Fri, 1 Aug 2025 19:42:35 +0000 Subject: [PATCH 2/5] Add explanation for contravariance behavior with anonymous functions Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com> --- ...e-for-func-and-action-generic-delegates.md | 52 ++++++++++++++++++- 1 file changed, 51 insertions(+), 1 deletion(-) diff --git a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md index 0e638aced2008..2257ac8128f9c 100644 --- a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md +++ b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md @@ -78,7 +78,57 @@ class Program } } ``` - + +## Contravariance and Anonymous Functions + +When working with anonymous functions (lambda expressions), you might encounter counterintuitive behavior related to contravariance. Consider the following example: + +```csharp +public class Person +{ + public virtual void ReadContact() { /*...*/ } +} + +public class Employee : Person +{ + public override void ReadContact() { /*...*/ } +} + +class Program +{ + private static void Main() + { + var personReadContact = (Person p) => p.ReadContact(); + + // This works - contravariance allows assignment + Action employeeReadContact = personReadContact; + + // This causes a compile error: CS1661 + // Action employeeReadContact2 = (Person p) => p.ReadContact(); + } +} +``` + +This behavior seems contradictory: if contravariance allows assigning a delegate that accepts a base type (`Person`) to a delegate variable expecting a derived type (`Employee`), why does direct assignment of the lambda expression fail? + +The key difference is **type inference**. In the first case, the lambda expression is first assigned to a variable with explicit type `var`, which causes the compiler to infer the lambda's type as `Action`. The subsequent assignment to `Action` succeeds because of contravariance rules for delegates. + +In the second case, the compiler cannot directly infer that the lambda expression `(Person p) => p.ReadContact()` should have type `Action` when it's being assigned to `Action`. The type inference rules for anonymous functions don't automatically apply contravariance during the initial type determination. + +### Workarounds + +To make direct assignment work, you can use explicit casting: + +```csharp +// Explicit cast to the desired delegate type +Action employeeReadContact = (Action)((Person p) => p.ReadContact()); + +// Or specify the lambda parameter type that matches the target delegate +Action employeeReadContact2 = (Employee e) => e.ReadContact(); +``` + +This behavior illustrates the difference between delegate contravariance (which works after types are established) and lambda expression type inference (which occurs during compilation). + ## See also - [Covariance and Contravariance (C#)](./index.md) From 0dcf2e18ed5978bb72fb17406e06ae09f0d74368 Mon Sep 17 00:00:00 2001 From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com> Date: Mon, 4 Aug 2025 18:00:06 +0000 Subject: [PATCH 3/5] Remove "explicit" when describing var keyword usage The var keyword represents implicit typing, not explicit typing. This correction addresses the terminology issue pointed out in the code review. Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com> --- .../using-variance-for-func-and-action-generic-delegates.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md index 2257ac8128f9c..2f74c553466da 100644 --- a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md +++ b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md @@ -111,7 +111,7 @@ class Program This behavior seems contradictory: if contravariance allows assigning a delegate that accepts a base type (`Person`) to a delegate variable expecting a derived type (`Employee`), why does direct assignment of the lambda expression fail? -The key difference is **type inference**. In the first case, the lambda expression is first assigned to a variable with explicit type `var`, which causes the compiler to infer the lambda's type as `Action`. The subsequent assignment to `Action` succeeds because of contravariance rules for delegates. +The key difference is **type inference**. In the first case, the lambda expression is first assigned to a variable with type `var`, which causes the compiler to infer the lambda's type as `Action`. The subsequent assignment to `Action` succeeds because of contravariance rules for delegates. In the second case, the compiler cannot directly infer that the lambda expression `(Person p) => p.ReadContact()` should have type `Action` when it's being assigned to `Action`. The type inference rules for anonymous functions don't automatically apply contravariance during the initial type determination. From 6a161464c9c1ac60e273558c45595c7d26b0d140 Mon Sep 17 00:00:00 2001 From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com> Date: Mon, 4 Aug 2025 18:16:55 +0000 Subject: [PATCH 4/5] Add periods to code comments for proper punctuation Co-authored-by: BillWagner <493969+BillWagner@users.noreply.github.com> --- .../using-variance-for-func-and-action-generic-delegates.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md index 2f74c553466da..027a33a997f85 100644 --- a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md +++ b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md @@ -120,10 +120,10 @@ In the second case, the compiler cannot directly infer that the lambda expressio To make direct assignment work, you can use explicit casting: ```csharp -// Explicit cast to the desired delegate type +// Explicit cast to the desired delegate type. Action employeeReadContact = (Action)((Person p) => p.ReadContact()); -// Or specify the lambda parameter type that matches the target delegate +// Or specify the lambda parameter type that matches the target delegate. Action employeeReadContact2 = (Employee e) => e.ReadContact(); ``` From caa926d8db9cf905adee27136294a89c14c2ed44 Mon Sep 17 00:00:00 2001 From: Bill Wagner Date: Tue, 5 Aug 2025 10:06:12 -0400 Subject: [PATCH 5/5] Apply suggestions from code review Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com> --- .../using-variance-for-func-and-action-generic-delegates.md | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md index 027a33a997f85..daa28e8719f1a 100644 --- a/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md +++ b/docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-for-func-and-action-generic-delegates.md @@ -79,7 +79,7 @@ class Program } ``` -## Contravariance and Anonymous Functions +## Contravariance and anonymous functions When working with anonymous functions (lambda expressions), you might encounter counterintuitive behavior related to contravariance. Consider the following example: @@ -100,10 +100,10 @@ class Program { var personReadContact = (Person p) => p.ReadContact(); - // This works - contravariance allows assignment + // This works - contravariance allows assignment. Action employeeReadContact = personReadContact; - // This causes a compile error: CS1661 + // This causes a compile error: CS1661. // Action employeeReadContact2 = (Person p) => p.ReadContact(); } }