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Field use cases

Initial community discussion thread: dotnet/csharplang#140

First-access ("lazy") initialization

public List<int> Prop => field ??= new();

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Nullability analysis with the field keyword

Properties that use the field keyword will not have warnings such as

For properties that have a get accessor with a body and which use field, analyze using this pseudocode, providing the same warnings as we would with accessors as local functions.

void Prop()
{
   T? field = default;

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declare @schemaName sysname = 'dbo'
declare @tableName sysname = 'ItemPrices'
declare @rowCondition nvarchar(max) = 'PriceCode in (''SEEDS'', ''YEAREND'')'


declare @builder nvarchar(max);
select @builder = coalesce(@builder + '

', '') + 'if exists(
    select *' + QueryWithoutSelect + ')

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flowchart
    ICollection --- IEnumerable
    IList --- ICollection
    IEnumerableT[IEnumerable<T>] --- IEnumerable
    ICollectionT[ICollection<T>] --- IEnumerableT
    IListT[IList<T>] --- ICollectionT
    IReadOnlyCollectionT[IReadOnlyCollection<T>] --- IEnumerableT
    IReadOnlyListT[IReadOnlyList<T>] --- IReadOnlyCollectionT

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/// <summary>
/// Tracks how many references there are to a disposable object, and disposes it when there are none remaining.
/// </summary>
public sealed class RefCountingDisposer
{
    private readonly IDisposable disposable;
    private uint refCount = 1;
    private readonly object lockObject = new();

    /// <summary>

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using System;
using System.Threading;

internal sealed class DeferredAction
{
    private static readonly Action SentinelNextSetShouldPost = () => { };

    private readonly SynchronizationContext synchronizationContext;
    private Action scheduledAction = SentinelNextSetShouldPost;

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#if !NET7_0_OR_GREATER
namespace System.Runtime.CompilerServices;
#endif

#if !NET5_0_OR_GREATER
internal static class IsExternalInit { }
#endif

#if !NET7_0_OR_GREATER
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Field | AttributeTargets.Property | AttributeTargets.Struct, Inherited = false)]

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Open types in nameof

[TODO: follow standard sections, add examples, flesh everything out]

Motivation

This is a small feature that removes a common frustration: why do I have to pick a generic type argument when the choice has no effect on the evaluation of the expression? It's very odd to require something to be specified within an operand when it has no impact on the result. Notably, typeof does not suffer from this limitation.

It's not just about code that better expresses itself. Once some arbitrary type argument has been chosen in a nameof expression, such as object?, changing a constraint on a type parameter can break uses of nameof unnecessarily. Insult becomes added to injury in this scenario. Satisfying the type parameter requires declaring a dummy class to implement an interface which is constraining the type parameter. Now there's unused metadata and a strange name invented, all for the purpose of adding a type argument to the nameof expression, a type argument which `na

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Get-NetNeighbor -AddressFamily IPv4 |
Select-Object -ExpandProperty IPAddress -Unique |
ForEach-Object -ThrottleLimit 20 -Parallel {
    try {
        (Resolve-DnsName $_ -LlmnrNetbiosOnly -ErrorAction Stop).NameHost
    } catch { }
}

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using System.Reflection;
using System.Reflection.Emit;

public static class CommandVerifier
{
    private static readonly AsyncLocal<Action<object?[]>?> ActiveInterceptor = new();
    private static readonly FieldInfo ActiveInterceptorField = typeof(CommandVerifier).GetField(nameof(ActiveInterceptor), BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.DeclaredOnly)!;

    public static async Task<object?[]> VerifyArgumentsAsync(Func<Task> invokeInterceptedDelegateAsync)
    {