Update code snippets to use .NET 6 features where appropriate

[tdykstra]

.NET 6 features to implement in code snippets where appropriate:

• Implicit usings
• Nullable reference types (NRT)
• File-scoped namespace declarations
• Top-level statements

Not all snippets will be upgraded. For some, the benefit may not be enough to justify the time that would have to be spent.

The following project files target .NET 6 but omit implicit usings and/or NRT:

• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/program-structure/snippets/top-level-statements-1/top-level-statements.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/program-structure/snippets/top-level-statements-2/top-level-statements.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/program-structure/snippets/top-level-statements-3/top-level-statements.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/program-structure/snippets/top-level-statements-4/top-level-statements.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/program-structure/snippets/top-level-statements-5/top-level-statements.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/types/snippets/anonymous-types/anonymous-types.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/types/snippets/namespaces/namespaces.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/keywords/snippets/keywords.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/operators/snippets/patterns/patterns.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/snippets/iterators/iterators.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/whats-new/tutorials/snippets/record-types/record-types.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/serialization/snippets/system-text-json-supported-collection-types/csharp/SystemTextJsonSamples.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/serialization/snippets/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter/csharp/SystemTextJsonSamples.csproj

The following project files target .NET Core 3.1:

• https://github.com/dotnet/docs/blob/main/docs/core/testing/snippets/order-unit-tests/csharp/MSTest.Project/MSTest.Project.csproj
• https://github.com/dotnet/docs/blob/main/docs/core/testing/snippets/order-unit-tests/csharp/NUnit.TestProject/NUnit.Project.csproj
• https://github.com/dotnet/docs/blob/main/docs/core/testing/snippets/order-unit-tests/csharp/XUnit.TestProject/XUnit.Project.csproj
• https://github.com/dotnet/docs/blob/main/docs/core/tutorials/snippets/library-with-visual-studio/csharp/ShowCase/ShowCase.csproj
• https://github.com/dotnet/docs/blob/main/docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTest/StringLibraryTest.csproj
• https://github.com/dotnet/docs/blob/main/docs/core/tutorials/snippets/with-visual-studio/csharp/HelloWorld.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/tutorials/snippets/object-oriented-programming/object-oriented-programming.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/tutorials/snippets/safelycast/asandis/asandis.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/tutorials/snippets/safelycast/nullablepatternmatching/nullablepatternmatching.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/tutorials/snippets/safelycast/patternmatching/patternmatching.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/arrays/snippets/arrays.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/concepts/async/snippets/index/AsyncBreakfast-final/AsyncBreakfast.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/whats-new/tutorials/snippets/generate-consume-asynchronous-streams/finished/IssuePRreport.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/roslyn-sdk/tutorials/snippets/how-to-write-csharp-analyzer-code-fix/MakeConst/MakeConst.Test/MakeConst.Test.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/generics/snippets/generics.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/concepts/async/snippets/index/AsyncBreakfast-V3/AsyncBreakfast.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/concepts/async/snippets/index/AsyncBreakfast-V2/AsyncBreakfast.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/concepts/async/snippets/index/AsyncBreakfast-starter/AsyncBreakfast.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/concepts/async/snippets/index/AsyncBreakfast-final/AsyncBreakfast.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/whats-new/tutorials/snippets/generate-consume-asynchronous-streams/start/IssuePRreport.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/whats-new/tutorials/snippets/generate-consume-asynchronous-streams/finished/IssuePRreport.csproj
• https://github.com/dotnet/docs/blob/main/docs/csharp/roslyn-sdk/tutorials/snippets/how-to-write-csharp-analyzer-code-fix/MakeConstTestProject/MakeConstTestProject/MakeConstTestProject.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/base-types/snippets/character-encoding-introduction/csharp/RuneSamples.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/base-types/snippets/best-practices-strings/csharp/tostring/tostring.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/base-types/snippets/best-practices-strings/csharp/formattable/formattable.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/assembly/snippets/inspect-contents-using-metadataloadcontext/AssemblySnippets.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/assembly/snippets/inspect-contents-using-metadataloadcontext/AssemblySnippets.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/custom-writing-with-utf8jsonwriter/custom-writing-with-utf8jsonwriter.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/custom-reading-with-utf8jsonreader/custom-reading-with-utf8jsonreader.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/computing-with-jsondocument-valid/computing-with-jsondocument-valid.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/computing-with-jsondocument-error/computing-with-jsondocument-error.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/deserializing-with-jsonserializer-valid/deserializing-with-jsonserializer-valid.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/deserializing-with-jsonserializer-error/deserializing-with-jsonserializer-error.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/datetime-converter-examples/example1/example1.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/datetime-converter-examples/example2/example2.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/datetime-converter-examples/example3/example3.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/writing-with-utf8jsonwriter/writing-with-utf8jsonwriter.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/serializing-with-jsonserializer/serializing-with-jsonserializer.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/reading-with-utf8jsonreader-valid/reading-with-utf8jsonreader-valid.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/reading-with-utf8jsonreader-error/reading-with-utf8jsonreader-error.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/deserializing-with-jsonserializer-valid/deserializing-with-jsonserializer-valid.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/datetime/snippets/system-text-json-support/csharp/deserializing-with-jsonserializer-error/deserializing-with-jsonserializer-error.csproj
• https://github.com/dotnet/docs/blob/main/docs/standard/io/snippets/pipelines_1/Pipes.csproj

The following folders directly or indirectly contain the indicated number of project files that target .NET 5:

• https://github.com/dotnet/docs/blob/main/docs/core/ 3
• https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/ 17
• https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/ 4
• https://github.com/dotnet/docs/blob/main/docs/csharp/programming-guide/ 32
• https://github.com/dotnet/docs/blob/main/docs/csharp/whats-new/ 1
• https://github.com/dotnet/docs/blob/main/docs/devops/ 1
• https://github.com/dotnet/docs/blob/main/docs/framework/ 1
• https://github.com/dotnet/docs/blob/main/docs/standard/asynchronous-programming-patterns/ 1
• https://github.com/dotnet/docs/blob/main/docs/standard/base-types/ 3
• https://github.com/dotnet/docs/blob/main/docs/standard/exceptions/ 2
• https://github.com/dotnet/docs/blob/main/docs/standard/garbage-collection/ 1
• https://github.com/dotnet/docs/blob/main/docs/standard/io/ 5
• https://github.com/dotnet/docs/blob/main/docs/standard/parallel-programming/ 1
• https://github.com/dotnet/docs/blob/main/docs/standard/security/ 3
• https://github.com/dotnet/docs/blob/main/docs/standard/serialization/ 4