Use [MemberNotNull] to reduce use of = null!;

src/libraries/Common/src/System/Net/DebugCriticalHandleMinusOneIsInvalid.cs

@@ -14,20 +14,14 @@ namespace System.Net
     //
     internal abstract class DebugCriticalHandleMinusOneIsInvalid : CriticalHandleMinusOneIsInvalid
     {
-        private string _trace = null!;
+        private string _trace;
 
         protected DebugCriticalHandleMinusOneIsInvalid() : base()
-        {
-            Trace();
-        }
-
-        private void Trace()
         {
             _trace = "WARNING! GC-ed  >>" + this.GetType().FullName + "<< (should be explicitly closed) \r\n";
             if (NetEventSource.IsEnabled) NetEventSource.Info(this, "Creating SafeHandle");
 #if TRACE_VERBOSE
-            string stacktrace = Environment.StackTrace;
-            _trace += stacktrace;
+            _trace += Environment.StackTrace;
 #endif //TRACE_VERBOSE
         }
 

src/libraries/Common/src/System/Net/DebugCriticalHandleZeroOrMinusOneIsInvalid.cs

@@ -14,20 +14,14 @@ namespace System.Net
     //
     internal abstract class DebugCriticalHandleZeroOrMinusOneIsInvalid : CriticalHandleZeroOrMinusOneIsInvalid
     {
-        private string _trace = null!;
+        private string _trace;
 
         protected DebugCriticalHandleZeroOrMinusOneIsInvalid() : base()
-        {
-            Trace();
-        }
-
-        private void Trace()
         {
             _trace = "WARNING! GC-ed  >>" + this.GetType().FullName + "<< (should be explicitly closed) \r\n";
             if (NetEventSource.IsEnabled) NetEventSource.Info(this, "Creating SafeHandle");
 #if TRACE_VERBOSE
-            string stacktrace = Environment.StackTrace;
-            _trace += stacktrace;
+            _trace += Environment.StackTrace;
 #endif //TRACE_VERBOSE
         }
 

src/libraries/Common/src/System/Net/DebugSafeHandleMinusOneIsInvalid.cs

@@ -14,20 +14,14 @@ namespace System.Net
     //
     internal abstract class DebugSafeHandleMinusOneIsInvalid : SafeHandleMinusOneIsInvalid
     {
-        private string _trace = null!; // initialized by helper called from ctor
+        private string _trace;
 
         protected DebugSafeHandleMinusOneIsInvalid(bool ownsHandle) : base(ownsHandle)
-        {
-            Trace();
-        }
-
-        private void Trace()
         {
             _trace = "WARNING! GC-ed  >>" + this.GetType().FullName + "<< (should be explicitly closed) \r\n";
             if (NetEventSource.IsEnabled) NetEventSource.Info(this, "Creating SafeHandle");
 #if TRACE_VERBOSE
-            string stacktrace = Environment.StackTrace;
-            _trace += stacktrace;
+            _trace += Environment.StackTrace;
 #endif //TRACE_VERBOSE
         }
 

src/libraries/Common/src/System/Net/NTAuthentication.Common.cs

@@ -24,7 +24,7 @@ internal partial class NTAuthentication
         private ContextFlagsPal _contextFlags;
 
         private bool _isCompleted;
-        private string _package = null!;
+        private string _package;
         private string? _lastProtocolName;
         private string? _protocolName;
         private string? _clientSpecifiedSpn;
@@ -98,6 +98,7 @@ internal NTAuthentication(bool isServer, string package, NetworkCredential crede
             Initialize(isServer, package, credential, spn, requestedContextFlags, channelBinding);
         }
 
+        [MemberNotNull(nameof(_package))]
         private void Initialize(bool isServer, string package, NetworkCredential credential, string? spn, ContextFlagsPal requestedContextFlags, ChannelBinding? channelBinding)
         {
             if (NetEventSource.IsEnabled) NetEventSource.Enter(this, package, spn, requestedContextFlags);

src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/BlockingCollection.cs

@@ -43,14 +43,14 @@ namespace System.Collections.Concurrent
     [DebuggerDisplay("Count = {Count}, Type = {_collection}")]
     public class BlockingCollection<T> : IEnumerable<T>, ICollection, IDisposable, IReadOnlyCollection<T>
     {
-        private IProducerConsumerCollection<T> _collection = null!;
+        private IProducerConsumerCollection<T> _collection;
         private int _boundedCapacity;
         private const int NON_BOUNDED = -1;
         private SemaphoreSlim? _freeNodes;
-        private SemaphoreSlim _occupiedNodes = null!;
+        private SemaphoreSlim _occupiedNodes;
         private bool _isDisposed;
-        private CancellationTokenSource _consumersCancellationTokenSource = null!;
-        private CancellationTokenSource _producersCancellationTokenSource = null!;
+        private CancellationTokenSource _consumersCancellationTokenSource;
+        private CancellationTokenSource _producersCancellationTokenSource;
 
         private volatile int _currentAdders;
         private const int COMPLETE_ADDING_ON_MASK = unchecked((int)0x80000000);
@@ -211,6 +211,10 @@ public BlockingCollection(IProducerConsumerCollection<T> collection)
         /// <param name="collection">The collection to use as the underlying data store.</param>
         /// <param name="boundedCapacity">The bounded size of the collection.</param>
         /// <param name="collectionCount">The number of items currently in the underlying collection.</param>
+        [MemberNotNull(nameof(_collection))]
+        [MemberNotNull(nameof(_consumersCancellationTokenSource))]
+        [MemberNotNull(nameof(_producersCancellationTokenSource))]
+        [MemberNotNull(nameof(_occupiedNodes))]
         private void Initialize(IProducerConsumerCollection<T> collection, int boundedCapacity, int collectionCount)
         {
             Debug.Assert(boundedCapacity > 0 || boundedCapacity == NON_BOUNDED);

src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameObjectCollectionBase.cs

@@ -11,6 +11,7 @@
 
 #pragma warning disable 618 // obsolete types, namely IHashCodeProvider
 
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Runtime.Serialization;
 
@@ -24,9 +25,9 @@ namespace System.Collections.Specialized
     public abstract class NameObjectCollectionBase : ICollection, ISerializable, IDeserializationCallback
     {
         private bool _readOnly = false;
-        private ArrayList _entriesArray = null!; // initialized in Reset method, called from constructor
+        private ArrayList _entriesArray;
         private IEqualityComparer _keyComparer;
-        private volatile Hashtable _entriesTable = null!; // initialized in Reset method, called from constructor
+        private volatile Hashtable _entriesTable;
         private volatile NameObjectEntry? _nullKeyEntry;
         private KeysCollection? _keys;
         private int _version;
@@ -96,6 +97,8 @@ public virtual void OnDeserialization(object? sender)
         // Private helpers
         //
 
+        [MemberNotNull(nameof(_entriesArray))]
+        [MemberNotNull(nameof(_entriesTable))]
         private void Reset()
         {
             _entriesArray = new ArrayList();
@@ -104,6 +107,8 @@ private void Reset()
             _version++;
         }
 
+        [MemberNotNull(nameof(_entriesArray))]
+        [MemberNotNull(nameof(_entriesTable))]
         private void Reset(int capacity)
         {
             _entriesArray = new ArrayList(capacity);

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Hosting/AssemblyCatalog.cs

@@ -25,7 +25,7 @@ public class AssemblyCatalog : ComposablePartCatalog, ICompositionElement
     {
         private readonly object _thisLock = new object();
         private readonly ICompositionElement _definitionOrigin;
-        private volatile Assembly _assembly = null!; // Always initiialized with helper
+        private volatile Assembly _assembly;
         private volatile ComposablePartCatalog? _innerCatalog = null;
         private int _isDisposed = 0;
 
@@ -387,6 +387,7 @@ public AssemblyCatalog(Assembly assembly, ICompositionElement definitionOrigin)
             _definitionOrigin = definitionOrigin;
         }
 
+        [MemberNotNull(nameof(_assembly))]
         private void InitializeAssemblyCatalog(Assembly assembly)
         {
             if (assembly.ReflectionOnly)

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Hosting/DirectoryCatalog.cs

@@ -23,16 +23,16 @@ namespace System.ComponentModel.Composition.Hosting
     public partial class DirectoryCatalog : ComposablePartCatalog, INotifyComposablePartCatalogChanged, ICompositionElement
     {
         private readonly Lock _thisLock = new Lock();
-        private readonly ICompositionElement? _definitionOrigin = null;
-        private ComposablePartCatalogCollection _catalogCollection = null!; // Always initialized with Initialize()
-        private Dictionary<string, AssemblyCatalog> _assemblyCatalogs = null!;
-        private volatile bool _isDisposed = false;
-        private string _path = null!;
-        private string _fullPath = null!;
-        private string _searchPattern = null!;
-        private ReadOnlyCollection<string> _loadedFiles = null!;
+        private readonly ICompositionElement? _definitionOrigin;
+        private ComposablePartCatalogCollection _catalogCollection;
+        private Dictionary<string, AssemblyCatalog> _assemblyCatalogs;
+        private volatile bool _isDisposed;
+        private string _path;
+        private string _fullPath;
+        private string _searchPattern;
+        private ReadOnlyCollection<string> _loadedFiles;
 
-        private readonly ReflectionContext? _reflectionContext = null;
+        private readonly ReflectionContext? _reflectionContext;
 
         /// <summary>
         ///     Creates a catalog of <see cref="ComposablePartDefinition"/>s based on all the *.dll files
@@ -745,6 +745,12 @@ private static string GetFullPath(string path)
             return RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? fullPath.ToUpperInvariant() : fullPath;
         }
 
+        [MemberNotNull(nameof(_path))]
+        [MemberNotNull(nameof(_fullPath))]
+        [MemberNotNull(nameof(_searchPattern))]
+        [MemberNotNull(nameof(_assemblyCatalogs))]
+        [MemberNotNull(nameof(_catalogCollection))]
+        [MemberNotNull(nameof(_loadedFiles))]
         private void Initialize(string path, string searchPattern)
         {
             _path = path;

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs

@@ -18,7 +18,7 @@ internal class ImportType
 
         private readonly Type _type;
         private readonly bool _isAssignableCollectionType;
-        private Type _contractType = null!; // Initialized in Initialize()
+        private Type _contractType;
         private Func<Export, object>? _castSingleValue;
         private readonly bool _isOpenGeneric = false;
 
@@ -129,6 +129,7 @@ public static bool IsDescendentOf(Type type, Type baseType)
             return IsGenericDescendentOf(type, baseType.GetGenericTypeDefinition());
         }
 
+        [MemberNotNull(nameof(_contractType))]
         private void Initialize(Type type)
         {
             if (!type.IsGenericType)

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

@@ -92,7 +92,7 @@ public partial class Activity : IDisposable
         /// reasonable, but arguments (e.g. specific accounts etc), should not be in
         /// the name but rather in the tags.
         /// </summary>
-        public string OperationName { get; } = null!;
+        public string OperationName { get; }
 
         /// <summary>Gets or sets the display name of the Activity</summary>
         /// <remarks>
@@ -334,7 +334,6 @@ public Activity(string operationName)
             if (string.IsNullOrEmpty(operationName))
             {
                 NotifyError(new ArgumentException(SR.OperationNameInvalid));
-                return;
             }
 
             OperationName = operationName;

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Unix.cs

@@ -12,6 +12,7 @@
 ===========================================================*/
 
 using System;
+using System.Diagnostics;
 
 namespace Microsoft.Win32.SafeHandles
 {
@@ -24,7 +25,7 @@ public sealed partial class SafeProcessHandle : SafeHandleZeroOrMinusOneIsInvali
         // Process.{Safe}Handle to initalize and use a WaitHandle to successfully use it on
         // Unix as well to wait for the process to complete.
 
-        private readonly SafeWaitHandle _handle = null!;
+        private readonly SafeWaitHandle? _handle;
         private readonly bool _releaseRef;
 
         internal SafeProcessHandle(int processId, SafeWaitHandle handle) :
@@ -41,6 +42,7 @@ protected override bool ReleaseHandle()
         {
             if (_releaseRef)
             {
+                Debug.Assert(_handle != null);
                 _handle.DangerousRelease();
             }
             return true;

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/PerformanceCounterLib.cs

@@ -9,6 +9,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
 using System.Threading;
@@ -192,19 +193,22 @@ private Dictionary<int, string> GetStringTable(bool isHelp)
         internal class PerformanceMonitor
         {
 #if FEATURE_REGISTRY
-            private RegistryKey _perfDataKey = null!; // will be initialized by Init() method
+            private RegistryKey _perfDataKey;
 #endif
             private readonly string _machineName;
 
             internal PerformanceMonitor(string machineName)
             {
                 _machineName = machineName;
+#if FEATURE_REGISTRY
                 Init();
+#endif
             }
 
+#if FEATURE_REGISTRY
+            [MemberNotNull(nameof(_perfDataKey))]
             private void Init()
             {
-#if FEATURE_REGISTRY
                 if (ProcessManager.IsRemoteMachine(_machineName))
                 {
                     _perfDataKey = RegistryKey.OpenRemoteBaseKey(RegistryHive.PerformanceData, _machineName);
@@ -213,8 +217,8 @@ private void Init()
                 {
                     _perfDataKey = Registry.PerformanceData;
                 }
-#endif
             }
+#endif
 
             // Win32 RegQueryValueEx for perf data could deadlock (for a Mutex) up to 2mins in some
             // scenarios before they detect it and exit gracefully. In the mean time, ERROR_BUSY,

src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/SourceFilter.cs

@@ -4,12 +4,13 @@
 
 using System;
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Diagnostics
 {
     public class SourceFilter : TraceFilter
     {
-        private string _src = null!;
+        private string _src;
 
         public SourceFilter(string source)
         {
@@ -31,6 +32,7 @@ public string Source
             {
                 return _src;
             }
+            [MemberNotNull(nameof(_src))]
             set
             {
                 if (value == null)

src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/SwitchAttribute.cs

@@ -2,18 +2,18 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
-using System.Reflection;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 
 namespace System.Diagnostics
 {
     [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Constructor |
                     AttributeTargets.Event | AttributeTargets.Method | AttributeTargets.Property)]
     public sealed class SwitchAttribute : Attribute
     {
-        private Type _type = null!; // Initialized using property
-        private string _name = null!;
+        private Type _type;
+        private string _name;
 
         public SwitchAttribute(string switchName, Type switchType)
         {
@@ -24,6 +24,7 @@ public SwitchAttribute(string switchName, Type switchType)
         public string SwitchName
         {
             get { return _name; }
+            [MemberNotNull(nameof(_name))]
             set
             {
                 if (value == null)
@@ -38,6 +39,7 @@ public string SwitchName
         public Type SwitchType
         {
             get { return _type; }
+            [MemberNotNull(nameof(_type))]
             set
             {
                 if (value == null)

src/libraries/System.Diagnostics.TraceSource/src/System/Diagnostics/SwitchLevelAttribute.cs

@@ -2,12 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace System.Diagnostics
 {
     [AttributeUsage(AttributeTargets.Class)]
     public sealed class SwitchLevelAttribute : Attribute
     {
-        private Type _type = null!;
+        private Type _type;
 
         public SwitchLevelAttribute(Type switchLevelType)
         {
@@ -17,6 +19,7 @@ public SwitchLevelAttribute(Type switchLevelType)
         public Type SwitchLevelType
         {
             get { return _type; }
+            [MemberNotNull(nameof(_type))]
             set
             {
                 if (value == null)