A .NET guideline specifies that an application should never throw a NullReferenceException. However, many applications and libraries do. The NullReferenceException is the most common exception happening. That’s why C# 8 tries to get rid of it. With C# 8, reference types are not null be default. This is a big change, and a great feature. However, what about all the legacy code? Can old libraries be used with C# 8 applications, and can C# 7 applications make use of C# 8 libraries?

This article demonstrates how C# 8 allows mixing old and new assemblies. 

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C# 11 is the next version of C# coming in .NET 7, and it is introducing a warning wave that issues a warning when a type is declared with all lower-case letters. This is being done so that in the future the language can begin moving away from conditional keywords and instead use full on keywords instead. The warning is alerting customers to types that may become keywords in future versions. 

No. We don't have any plans to bundle .NET into the OS and we're trying really hard to avoid this. 

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  <PropertyGroup>
    <AnalysisLevel>latest</AnalysisLevel>
    <AnalysisMode>AllEnabledByDefault</AnalysisMode>
    <CodeAnalysisTreatWarningsAsErrors>true</CodeAnalysisTreatWarningsAsErrors>
    <EnableNETAnalyzers>true</EnableNETAnalyzers>
    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
    <Nullable>enable</Nullable>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <RunAnalyzersDuringBuild>true</RunAnalyzersDuringBuild>
    <RunAnalyzersDuringLiveAnalysis>true</RunAnalyzersDuringLiveAnalysis>
    <!--
      CA2007: Consider calling ConfigureAwait on the awaited task
      MA0004: Use Task.ConfigureAwait(false) as the current SynchronizationContext is not needed
      CA1056: Change the type of property 'Url' from 'string' to 'System.Uri'
      CA1054: Change the type of parameter of the method to allow a Uri to be passed as a 'System.Uri' object
      CA1055: Change the return type of method from 'string' to 'System.Uri'
    -->
    <NoWarn>$(NoWarn);CA2007;CA1056;CA1054;CA1055;MA0004</NoWarn>
    <NoError>$(NoError);CA2007;CA1056;CA1054;CA1055;MA0004</NoError>
    <Deterministic>true</Deterministic>
    <Features>strict</Features>
    <ReportAnalyzer>true</ReportAnalyzer>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="5.0.1">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>

    <PackageReference Include="Meziantou.Analyzer" Version="1.0.639">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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    <PackageReference Include="Microsoft.VisualStudio.Threading.Analyzers" Version="16.8.55">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>

    <PackageReference Include="AsyncFixer" Version="1.3.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>

    <PackageReference Include="Asyncify" Version="0.9.7">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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    <PackageReference Include="ClrHeapAllocationAnalyzer" Version="3.0.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
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    <PackageReference Include="SonarAnalyzer.CSharp" Version="8.16.0.25740">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
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  <ItemGroup>
    <AdditionalFiles Include="$(MSBuildThisFileDirectory)BannedSymbols.txt" Link="Properties/BannedSymbols.txt" />
  </ItemGroup>
</Project>

# https://github.com/dotnet/roslyn-analyzers/blob/master/src/Microsoft.CodeAnalysis.BannedApiAnalyzers/BannedApiAnalyzers.Help.md
P:System.DateTime.Now;Use System.DateTime.UtcNow instead
P:System.DateTimeOffset.Now;Use System.DateTimeOffset.UtcNow instead
P:System.DateTimeOffset.DateTime;Use System.DateTimeOffset.UtcDateTime instead

[*.cs]

# MA0026 : Complete the task
dotnet_diagnostic.MA0026.severity = suggestion

# CA1308: In method 'urlToLower', replace the call to 'ToLowerInvariant' with 'ToUpperInvariant' (CA1308)
dotnet_diagnostic.CA1308.severity = suggestion

# CA1040: Avoid empty interfaces
dotnet_diagnostic.CA1040.severity = suggestion

# CA1829 Use the "Count" property instead of Enumerable.Count()
dotnet_diagnostic.CA1829.severity = suggestion

# Use 'Count' property here instead.
dotnet_diagnostic.S2971.severity = suggestion

# S1135 : Complete the task
dotnet_diagnostic.S1135.severity = suggestion

# S2479: Replace the control character at position 7 by its escape sequence
dotnet_diagnostic.S2479.severity = suggestion

# CA2007: Consider calling ConfigureAwait on the awaited task
dotnet_diagnostic.CA2007.severity = none

# MA0004: Use Task.ConfigureAwait(false) as the current SynchronizationContext is not needed
dotnet_diagnostic.MA0004.severity = none

# CA1056: Change the type of property 'Url' from 'string' to 'System.Uri'
dotnet_diagnostic.CA1056.severity = suggestion

# CA1054: Change the type of parameter of the method to allow a Uri to be passed as a 'System.Uri' object
dotnet_diagnostic.CA1054.severity = suggestion

# CA1055: Change the return type of method from 'string' to 'System.Uri'
dotnet_diagnostic.CA1055.severity = suggestion

# S4457: Split this method into two, one handling parameters check and the other handling the asynchronous code.
dotnet_diagnostic.S4457.severity = none

# AsyncFixer01: Unnecessary async/await usage
dotnet_diagnostic.AsyncFixer01.severity = suggestion

# AsyncFixer02: Long-running or blocking operations inside an async method
dotnet_diagnostic.AsyncFixer02.severity = error

# VSTHRD103: Call async methods when in an async method
dotnet_diagnostic.VSTHRD103.severity = error

# AsyncFixer03: Fire & forget async void methods
dotnet_diagnostic.AsyncFixer03.severity = error

# VSTHRD100: Avoid async void methods
dotnet_diagnostic.VSTHRD100.severity = error

# VSTHRD101: Avoid unsupported async delegates
dotnet_diagnostic.VSTHRD101.severity = error

# VSTHRD107: Await Task within using expression
dotnet_diagnostic.VSTHRD107.severity = error

# AsyncFixer04: Fire & forget async call inside a using block
dotnet_diagnostic.AsyncFixer04.severity = error

# VSTHRD110: Observe result of async calls
dotnet_diagnostic.VSTHRD110.severity = error

# VSTHRD002: Avoid problematic synchronous waits
dotnet_diagnostic.VSTHRD002.severity = suggestion

# MA0045: Do not use blocking call (make method async)
dotnet_diagnostic.MA0045.severity = suggestion

# AsyncifyInvocation: Use Task Async
dotnet_diagnostic.AsyncifyInvocation.severity = error

# AsyncifyVariable: Use Task Async
dotnet_diagnostic.AsyncifyVariable.severity = error

# VSTHRD111: Use ConfigureAwait(bool)
dotnet_diagnostic.VSTHRD111.severity = none

# MA0022: Return Task.FromResult instead of returning null
dotnet_diagnostic.MA0022.severity = error

# VSTHRD114: Avoid returning a null Task
dotnet_diagnostic.VSTHRD114.severity = error

# VSTHRD200: Use "Async" suffix for async methods
dotnet_diagnostic.VSTHRD200.severity = suggestion

# MA0040: Specify a cancellation token
dotnet_diagnostic.MA0032.severity = suggestion

# MA0040: Flow the cancellation token when available
dotnet_diagnostic.MA0040.severity = suggestion

# MA0079: Use a cancellation token using .WithCancellation()
dotnet_diagnostic.MA0079.severity = suggestion

# MA0080: Use a cancellation token using .WithCancellation()
dotnet_diagnostic.MA0080.severity = error

#AsyncFixer05: Downcasting from a nested task to an outer task.
dotnet_diagnostic.AsyncFixer05.severity = error

# ClrHeapAllocationAnalyzer ----------------------------------------------------
# HAA0301: Closure Allocation Source
dotnet_diagnostic.HAA0301.severity = suggestion

# HAA0601: Value type to reference type conversion causing boxing allocation
dotnet_diagnostic.HAA0601.severity = suggestion

# HAA0302: Display class allocation to capture closure
dotnet_diagnostic.HAA0302.severity = suggestion

# HAA0101: Array allocation for params parameter
dotnet_diagnostic.HAA0101.severity = suggestion

# HAA0603: Delegate allocation from a method group
dotnet_diagnostic.HAA0603.severity = suggestion

# HAA0602: Delegate on struct instance caused a boxing allocation
dotnet_diagnostic.HAA0602.severity = suggestion

# HAA0401: Possible allocation of reference type enumerator
dotnet_diagnostic.HAA0401.severity = silent

# HAA0303: Lambda or anonymous method in a generic method allocates a delegate instance
dotnet_diagnostic.HAA0303.severity = silent

# HAA0102: Non-overridden virtual method call on value type
dotnet_diagnostic.HAA0102.severity = silent

# HAA0502: Explicit new reference type allocation
dotnet_diagnostic.HAA0502.severity = none

# HAA0505: Initializer reference type allocation
dotnet_diagnostic.HAA0505.severity = silent

dotnet_diagnostic.CA1308.severity = suggestion

[SuppressMessage("Microsoft.Usage", "CA1052:Type 'Program' is a static holder type but is neither static nor NotInheritable",
  Justification = "We need it for our integration tests this way.")]
[SuppressMessage("Microsoft.Usage", "RCS1102:Type 'Program' is a static holder type but is neither static nor NotInheritable",
  Justification = "We need it for our integration tests this way.")]
[SuppressMessage("Microsoft.Usage", "S1118:Type 'Program' is a static holder type but is neither static nor NotInheritable",
  Justification = "We need it for our integration tests this way.")]
public class Program { }

• We have two new refactorings for global usings, Extract Global Using and Inline Global Using, with Find Usages support for this feature.
• For nullable reference types, we’ve split the “should never be null” warning into two categories: one for those who look at NRT annotation and one for those who have a runtime check for null. We’ve also added a setting to enable runtime enforced not null warnings only, and supported [MemberNotNull] and [MemberNotNullWhen] annotations.
• We’ve implemented generic attributes support for C#11. 

It provides all the features available in the GitHub plain-text editor, including Markdown input. For features like tables, it offers a much easier experience in comparison to plain-text Markdown and allows users to be more productive.  

I have always wanted to learn how to make an OS from scratch. In college I was taught how to implement advanced features (pagination, semaphores, memory management, etc)