Add some more missing 'readonly' modifiers

src/WinRT.Runtime/ComWrappersSupport.net5.cs

@@ -41,7 +41,7 @@ private static DefaultComWrappers DefaultComWrappersInstance
  internal static Type CreateRCWType;
 
  private static ComWrappers _comWrappers;
- private static object _comWrappersLock = new object();
+ private static readonly object _comWrappersLock = new object();
  private static ComWrappers ComWrappers
  {
  get

src/WinRT.Runtime/Marshalers.cs

@@ -79,8 +79,8 @@ internal struct HSTRING_HEADER
 
  public ref struct Pinnable
  {
- private HSTRING_HEADER _header;
- private string _value;
+ private readonly HSTRING_HEADER _header;
+ private readonly string _value;
 #if DEBUG
  private bool _pinned;
 #endif
@@ -116,7 +116,7 @@ public unsafe IntPtr GetAbi()
  Marshal.ThrowExceptionForHR(Platform.WindowsCreateStringReference(
  (ushort*)Unsafe.AsPointer(ref Unsafe.AsRef(in GetPinnableReference())),
  _value.Length,
- (IntPtr*)Unsafe.AsPointer(ref _header),
+ (IntPtr*)Unsafe.AsPointer(ref Unsafe.AsRef(in _header)),
  &hstring));
  return hstring;
  }

src/WinRT.Runtime/MonoSupport.cs

@@ -13,7 +13,7 @@ namespace WinRT
 {
  internal static class Mono
  {
- static unsafe Lazy<bool> _usingMono = new Lazy<bool>(() =>
+ static readonly unsafe Lazy<bool> _usingMono = new Lazy<bool>(() =>
  {
  IntPtr modulePtr = Platform.LoadLibraryExW("mono-2.0-bdwgc.dll", IntPtr.Zero, 0);
  if (modulePtr == IntPtr.Zero) return false;
@@ -75,7 +75,7 @@ struct MonoInternalThread_x64
 
  public sealed class ThreadContext : IDisposable
  {
- static Lazy<HashSet<IntPtr>> _foreignThreads = new Lazy<HashSet<IntPtr>>();
+ static readonly Lazy<HashSet<IntPtr>> _foreignThreads = new Lazy<HashSet<IntPtr>>();
 
  readonly IntPtr _threadPtr = IntPtr.Zero;
 

src/WinRT.Runtime/Projections/Bindable.net5.cs

@@ -873,7 +873,7 @@ public IEnumerator GetEnumerator()
  public sealed class ToAbiHelper : IBindableVector
 #pragma warning restore CA2257
  {
- private global::System.Collections.IList _list;
+ private readonly global::System.Collections.IList _list;
 
  public ToAbiHelper(global::System.Collections.IList list) => _list = list;