Record types can seal ToString in C# 10. (#24957)

* Add information on sealed ToString

Fixes #24077

Beginning in C# 10.0, records may declare a `ToString` method as `sealed`. This prevents derived records from creating an override. Effectively, that means the `ToString` output will not include the runtime type information. (All members and values are displayed, because derived records will still have a PrintMembers method generated.)

* add note on ToString.

* fix a build warning introduced

This was introduced in an earlier PR

* Update docs/csharp/language-reference/builtin-types/record.md

Co-authored-by: Youssef Victor <[email protected]>

* Apply suggestions from code review

Co-authored-by: Tom Dykstra <[email protected]>

Co-authored-by: Youssef Victor <[email protected]>
Co-authored-by: Tom Dykstra <[email protected]>

Author: 3 people

docs/csharp/language-reference/attributes/general.md

@@ -45,7 +45,7 @@ The string provided as the first argument to the attribute constructor will be d
 
 In C# 10, you can use constant string interpolation and the `nameof` operator to ensure the names match:
 
-:::code language="csharp" source="snippets/ObsoleteExample.cs" ID="Snippet2" :::
+:::code language="csharp" source="snippets/ObsoleteExample.cs" id="Snippet2" :::
 
 ## `AttributeUsage` attribute
 

docs/csharp/language-reference/builtin-types/record.md

@@ -1,7 +1,7 @@
 ---
 title: "Records - C# reference"
 description: Learn about the record type in C#
-ms.date: 02/25/2021
+ms.date: 07/01/2021
 f1_keywords: 
   - "record_CSharpKeyword"
 helpviewer_keywords: 
@@ -136,7 +136,7 @@ The `ToString` override creates a <xref:System.Text.StringBuilder> object with t
 
 :::code language="csharp" source="snippets/shared/RecordType.cs" id="ToStringOverrideDefault":::
 
-You can provide your own implementation of `PrintMembers` or the `ToString` override. Examples are provided in the [`PrintMembers` formatting in derived records](#printmembers-formatting-in-derived-records) section later in this article.
+You can provide your own implementation of `PrintMembers` or the `ToString` override. Examples are provided in the [`PrintMembers` formatting in derived records](#printmembers-formatting-in-derived-records) section later in this article. In C# 10 and later, your implementation of `ToString` may include the `sealed` modifier, which prevents the compiler from synthesizing a `ToString` implementation for any derived records. Effectively, that means the `ToString` output will not include the runtime type information. (All members and values are displayed, because derived records will still have a PrintMembers method generated.)
 
 ## Inheritance
 
@@ -185,6 +185,9 @@ Here is an example of code that replaces the synthesized `PrintMembers` methods,
 
 :::code language="csharp" source="snippets/shared/RecordType.cs" id="PrintMembersImplementation":::
 
+> [!NOTE]
+> In C# 10.0 and later, the compiler will synthesize `PrintMembers` when a base record has sealed the `ToString` method. You can also create your own implementation of `PrintMembers`.
+
 ### Deconstructor behavior in derived records
 
 The `Deconstruct` method of a derived record returns the values of all positional properties of the compile-time type. If the variable type is a base record, only the base record properties are deconstructed unless the object is cast to the derived type. The following example demonstrates calling a deconstructor on a derived record.

docs/csharp/whats-new/tutorials/records.md

@@ -1,7 +1,7 @@
 ---
 title: Use record types - C# tutorial
 description: Learn about how to use record types, build hierarchies of records, and when to choose records over classes.
-ms.date: 11/12/2020
+ms.date: 07/01/2021
 ---
 # Create record types
 
@@ -125,6 +125,8 @@ You declare a `PrintMembers` method in the `DegreeDays` record that doesn't prin
 
 The signature declares a `virtual protected` method to match the compiler's version. Don't worry if you get the accessors wrong; the language enforces the correct signature. If you forget the correct modifiers for any synthesized method, the compiler issues warnings or errors that help you get the right signature.
 
+In C# 10.0 and later, you can declare the `ToString` method as `sealed` in a record type. That prevents derived records from providing a new implementation. Derived records will still contain the `PrintMembers` override. You would do this if you didn't want the `ToString` method to display the runtime type of the record. In the preceding example, you'd lose the information on where the record was measuring heating or cooling degree days.
+
 ## Non-destructive mutation
 
 The synthesized members in a positional record don't modify the state of the record. The goal is that you can more easily create immutable records. Look again at the preceding declarations for `HeatingDegreeDays` and `CoolingDegreeDays`. The members added perform computations on the values for the record, but don't mutate state. Positional records make it easier for you to create immutable reference types.

docs/csharp/whats-new/tutorials/snippets/record-types/record-types.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net5.0</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <RootNamespace>record_types</RootNamespace>
   </PropertyGroup>