More CoreLib code cleanup

src/System.Private.CoreLib/shared/Internal/Win32/RegistryKey.cs

@@ -29,7 +29,7 @@ internal sealed class RegistryKey : IDisposable
         private const int MaxKeyLength = 255;
         private const int MaxValueLength = 16383;
 
-        private SafeRegistryHandle _hkey;
+        private readonly SafeRegistryHandle _hkey;
 
         private RegistryKey(SafeRegistryHandle hkey)
         {

src/System.Private.CoreLib/shared/System/AggregateException.cs

@@ -25,7 +25,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class AggregateException : Exception
     {
-        private ReadOnlyCollection<Exception> m_innerExceptions; // Complete set of exceptions. Do not rename (binary serialization)
+        private readonly ReadOnlyCollection<Exception> m_innerExceptions; // Complete set of exceptions. Do not rename (binary serialization)
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AggregateException"/> class.
@@ -258,7 +258,7 @@ protected AggregateException(SerializationInfo info, StreamingContext context) :
             }
 
             Exception[]? innerExceptions = info.GetValue("InnerExceptions", typeof(Exception[])) as Exception[];
-            if (innerExceptions == null)
+            if (innerExceptions is null)
             {
                 throw new SerializationException(SR.AggregateException_DeserializationFailure);
             }

src/System.Private.CoreLib/shared/System/ArgumentException.cs

@@ -23,7 +23,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentException : SystemException
     {
-        private string? _paramName;
+        private readonly string? _paramName;
 
         // Creates a new ArgumentException with its message
         // string set to the empty string.

src/System.Private.CoreLib/shared/System/ArgumentOutOfRangeException.cs

@@ -22,7 +22,7 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentOutOfRangeException : ArgumentException
     {
-        private object? _actualValue;
+        private readonly object? _actualValue;
 
         // Creates a new ArgumentOutOfRangeException with its message
         // string set to a default message explaining an argument was out of range.

src/System.Private.CoreLib/shared/System/Array.Enumerators.cs

@@ -10,11 +10,11 @@ namespace System
 {
     internal sealed class ArrayEnumerator : IEnumerator, ICloneable
     {
-        private Array array;
+        private readonly Array array;
         private int index;
-        private int endIndex;
-        private int startIndex;    // Save for Reset.
-        private int[] _indices;    // The current position in a multidim array
+        private readonly int endIndex;
+        private readonly int startIndex;    // Save for Reset.
+        private readonly int[] _indices;    // The current position in a multidim array
         private bool _complete;
 
         internal ArrayEnumerator(Array array, int index, int count)
@@ -110,7 +110,7 @@ internal sealed class SZArrayEnumerator : IEnumerator, ICloneable
     {
         private readonly Array _array;
         private int _index;
-        private int _endIndex; // Cache Array.Length, since it's a little slow.
+        private readonly int _endIndex; // Cache Array.Length, since it's a little slow.
 
         internal SZArrayEnumerator(Array array)
         {

src/System.Private.CoreLib/shared/System/Array.cs

@@ -936,9 +936,8 @@ public static int IndexOf(Array array, object? value, int startIndex, int count)
                 return retVal;
 #endif
 
-            object[]? objArray = array as object[];
             int endIndex = startIndex + count;
-            if (objArray != null)
+            if (array is object[] objArray)
             {
                 if (value == null)
                 {
@@ -1120,9 +1119,8 @@ public static int LastIndexOf(Array array, object? value, int startIndex, int co
                 return retVal;
 #endif
 
-            object[]? objArray = array as object[];
             int endIndex = startIndex - count + 1;
-            if (objArray != null)
+            if (array is object[] objArray)
             {
                 if (value == null)
                 {

src/System.Private.CoreLib/shared/System/AttributeUsageAttribute.cs

@@ -19,7 +19,7 @@ namespace System
     [AttributeUsage(AttributeTargets.Class, Inherited = true)]
     public sealed class AttributeUsageAttribute : Attribute
     {
-        private AttributeTargets _attributeTarget = AttributeTargets.All; // Defaults to all
+        private readonly AttributeTargets _attributeTarget = AttributeTargets.All; // Defaults to all
         private bool _allowMultiple = false; // Defaults to false
         private bool _inherited = true; // Defaults to true
 

src/System.Private.CoreLib/shared/System/BadImageFormatException.cs

@@ -21,8 +21,8 @@ namespace System
     [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public partial class BadImageFormatException : SystemException
     {
-        private string? _fileName;  // The name of the corrupt PE file.
-        private string? _fusionLog;  // fusion log (when applicable)
+        private readonly string? _fileName;  // The name of the corrupt PE file.
+        private readonly string? _fusionLog;  // fusion log (when applicable)
 
         public BadImageFormatException()
             : base(SR.Arg_BadImageFormatException)

src/System.Private.CoreLib/shared/System/Buffers/ConfigurableArrayPool.cs

@@ -74,7 +74,7 @@ public override T[] Rent(int minimumLength)
             }
 
             var log = ArrayPoolEventSource.Log;
-            T[]? buffer = null;
+            T[]? buffer;
 
             int index = Utilities.SelectBucketIndex(minimumLength);
             if (index < _buckets.Length)

src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.O.cs

@@ -49,7 +49,7 @@ private static bool TryFormatDateTimeO(DateTime value, TimeSpan offset, Span<byt
             bytesWritten = bytesRequired;
 
             // Hoist most of the bounds checks on buffer.
-            { var unused = destination[MinimumBytesNeeded - 1]; }
+            { _ = destination[MinimumBytesNeeded - 1]; }
 
             FormattingHelpers.WriteFourDecimalDigits((uint)value.Year, destination, 0);
             destination[4] = Utf8Constants.Minus;

src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Date.cs

@@ -105,23 +105,14 @@ public static bool TryFormat(DateTimeOffset value, Span<byte> destination, out i
                 offset = value.Offset;
             }
 
-            switch (symbol)
+            return symbol switch
             {
-                case 'R':
-                    return TryFormatDateTimeR(value.UtcDateTime, destination, out bytesWritten);
-
-                case 'l':
-                    return TryFormatDateTimeL(value.UtcDateTime, destination, out bytesWritten);
-
-                case 'O':
-                    return TryFormatDateTimeO(value.DateTime, value.Offset, destination, out bytesWritten);
-
-                case 'G':
-                    return TryFormatDateTimeG(value.DateTime, offset, destination, out bytesWritten);
-
-                default:
-                    return FormattingHelpers.TryFormatThrowFormatException(out bytesWritten);
-            }
+                'R' => TryFormatDateTimeR(value.UtcDateTime, destination, out bytesWritten),
+                'l' => TryFormatDateTimeL(value.UtcDateTime, destination, out bytesWritten),
+                'O' => TryFormatDateTimeO(value.DateTime, value.Offset, destination, out bytesWritten),
+                'G' => TryFormatDateTimeG(value.DateTime, offset, destination, out bytesWritten),
+                _ => FormattingHelpers.TryFormatThrowFormatException(out bytesWritten),
+            };
         }
 
         /// <summary>
@@ -149,23 +140,14 @@ public static bool TryFormat(DateTime value, Span<byte> destination, out int byt
         {
             char symbol = FormattingHelpers.GetSymbolOrDefault(format, 'G');
 
-            switch (symbol)
+            return symbol switch
             {
-                case 'R':
-                    return TryFormatDateTimeR(value, destination, out bytesWritten);
-
-                case 'l':
-                    return TryFormatDateTimeL(value, destination, out bytesWritten);
-
-                case 'O':
-                    return TryFormatDateTimeO(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
-
-                case 'G':
-                    return TryFormatDateTimeG(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten);
-
-                default:
-                    return FormattingHelpers.TryFormatThrowFormatException(out bytesWritten);
-            }
+                'R' => TryFormatDateTimeR(value, destination, out bytesWritten),
+                'l' => TryFormatDateTimeL(value, destination, out bytesWritten),
+                'O' => TryFormatDateTimeO(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten),
+                'G' => TryFormatDateTimeG(value, Utf8Constants.NullUtcOffset, destination, out bytesWritten),
+                _ => FormattingHelpers.TryFormatThrowFormatException(out bytesWritten),
+            };
         }
     }
 }

src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs

@@ -117,7 +117,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
             // because it may have an observable side effect (throwing).
             // We use 8 instead of 7 so that we also capture the dash if we're asked to insert one.
 
-            { var unused = destination[8]; }
+            { _ = destination[8]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte03, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte02, destination, 2, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte01, destination, 4, FormattingHelpers.HexCasing.Lowercase);
@@ -133,7 +133,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(8);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte05, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte04, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -147,7 +147,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte07, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte06, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -161,7 +161,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[4]; }
+            { _ = destination[4]; }
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte08, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte09, destination, 2, FormattingHelpers.HexCasing.Lowercase);
 
@@ -175,7 +175,7 @@ public static bool TryFormat(Guid value, Span<byte> destination, out int bytesWr
                 destination = destination.Slice(4);
             }
 
-            { var unused = destination[11]; } // can't hoist bounds check on the final brace (if exists)
+            { _ = destination[11]; } // can't hoist bounds check on the final brace (if exists)
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte10, destination, 0, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte11, destination, 2, FormattingHelpers.HexCasing.Lowercase);
             FormattingHelpers.WriteHexByte(guidAsBytes.Byte12, destination, 4, FormattingHelpers.HexCasing.Lowercase);

src/System.Private.CoreLib/shared/System/Buffers/Text/Utf8Parser/Utf8Parser.Date.cs

@@ -121,26 +121,15 @@ public static bool TryParse(ReadOnlySpan<byte> source, out DateTime value, out i
         /// </exceptions>
         public static bool TryParse(ReadOnlySpan<byte> source, out DateTimeOffset value, out int bytesConsumed, char standardFormat = default)
         {
-            switch (standardFormat)
+            return standardFormat switch
             {
-                case 'R':
-                    return TryParseDateTimeOffsetR(source, NoFlipCase, out value, out bytesConsumed);
-
-                case 'l':
-                    return TryParseDateTimeOffsetR(source, FlipCase, out value, out bytesConsumed);
-
-                case 'O':
-                    return TryParseDateTimeOffsetO(source, out value, out bytesConsumed, out _);
-
-                case default(char):
-                    return TryParseDateTimeOffsetDefault(source, out value, out bytesConsumed);
-
-                case 'G':
-                    return TryParseDateTimeG(source, out DateTime _, out value, out bytesConsumed);
-
-                default:
-                    return ParserHelpers.TryParseThrowFormatException(out value, out bytesConsumed);
-            }
+                'R' => TryParseDateTimeOffsetR(source, NoFlipCase, out value, out bytesConsumed),
+                'l' => TryParseDateTimeOffsetR(source, FlipCase, out value, out bytesConsumed),
+                'O' => TryParseDateTimeOffsetO(source, out value, out bytesConsumed, out _),
+                default(char) => TryParseDateTimeOffsetDefault(source, out value, out bytesConsumed),
+                'G' => TryParseDateTimeG(source, out DateTime _, out value, out bytesConsumed),
+                _ => ParserHelpers.TryParseThrowFormatException(out value, out bytesConsumed),
+            };
         }
 
         private const uint FlipCase = 0x00000020u;  // XOR mask to flip the case of a letter.

src/System.Private.CoreLib/shared/System/CLSCompliantAttribute.cs

@@ -16,7 +16,7 @@ namespace System
     [AttributeUsage(AttributeTargets.All, Inherited = true, AllowMultiple = false)]
     public sealed class CLSCompliantAttribute : Attribute
     {
-        private bool _compliant;
+        private readonly bool _compliant;
 
         public CLSCompliantAttribute(bool isCompliant)
         {

src/System.Private.CoreLib/shared/System/Collections/ArrayList.cs

@@ -772,7 +772,7 @@ public virtual void TrimToSize()
         // Note this requires reimplementing half of ArrayList...
         private class IListWrapper : ArrayList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal IListWrapper(IList list)
             {
@@ -983,8 +983,7 @@ public override void InsertRange(int index, ICollection c)
 
                 if (c.Count > 0)
                 {
-                    ArrayList? al = _list as ArrayList;
-                    if (al != null)
+                    if (_list is ArrayList al)
                     {
                         // We need to special case ArrayList.
                         // When c is a range of _list, we need to handle this in a special way.
@@ -1235,8 +1234,8 @@ public void Reset()
 
         private class SyncArrayList : ArrayList
         {
-            private ArrayList _list;
-            private object _root;
+            private readonly ArrayList _list;
+            private readonly object _root;
 
             internal SyncArrayList(ArrayList list)
                 : base(false)
@@ -1590,8 +1589,8 @@ public override void TrimToSize()
 
         private class SyncIList : IList
         {
-            private IList _list;
-            private object _root;
+            private readonly IList _list;
+            private readonly object _root;
 
             internal SyncIList(IList list)
             {
@@ -1719,7 +1718,7 @@ public virtual void RemoveAt(int index)
 
         private class FixedSizeList : IList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal FixedSizeList(IList l)
             {
@@ -2031,7 +2030,7 @@ public override void TrimToSize()
 
         private class ReadOnlyList : IList
         {
-            private IList _list;
+            private readonly IList _list;
 
             internal ReadOnlyList(IList l)
             {
@@ -2343,12 +2342,12 @@ public override void TrimToSize()
         // made to the list while an enumeration is in progress.
         private sealed class ArrayListEnumerator : IEnumerator, ICloneable
         {
-            private ArrayList _list;
+            private readonly ArrayList _list;
             private int _index;
-            private int _endIndex;       // Where to stop.
-            private int _version;
+            private readonly int _endIndex;       // Where to stop.
+            private readonly int _version;
             private object? _currentElement;
-            private int _startIndex;     // Save this for Reset.
+            private readonly int _startIndex;     // Save this for Reset.
 
             internal ArrayListEnumerator(ArrayList list, int index, int count)
             {
@@ -2404,7 +2403,7 @@ public void Reset()
         private class Range : ArrayList
         {
             private ArrayList _baseList;
-            private int _baseIndex;
+            private readonly int _baseIndex;
             private int _baseSize;
             private int _baseVersion;
 
@@ -2819,13 +2818,13 @@ public override void TrimToSize()
 
         private sealed class ArrayListEnumeratorSimple : IEnumerator, ICloneable
         {
-            private ArrayList _list;
+            private readonly ArrayList _list;
             private int _index;
-            private int _version;
+            private readonly int _version;
             private object? _currentElement;
-            private bool _isArrayList;
+            private readonly bool _isArrayList;
             // this object is used to indicate enumeration has not started or has terminated
-            private static object s_dummyObject = new object();
+            private static readonly object s_dummyObject = new object();
 
             internal ArrayListEnumeratorSimple(ArrayList list)
             {
@@ -2910,7 +2909,7 @@ public void Reset()
 
         internal class ArrayListDebugView
         {
-            private ArrayList _arrayList;
+            private readonly ArrayList _arrayList;
 
             public ArrayListDebugView(ArrayList arrayList)
             {