Replace default(T) != null with typeof(T).IsValueType

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

@@ -1671,10 +1671,7 @@ private static int GetDefaultChunkSize<TSource>()
  {
  int chunkSize;
 
- // Because of the lack of typeof(T).IsValueType we need two pieces of information
- // to determine this. default(T) will return a non null for Value Types, except those
- // using Nullable<>, that is why we need a second condition.
- if (default(TSource) != null || Nullable.GetUnderlyingType(typeof(TSource)) != null)
+ if (typeof(TSource).IsValueType)
  {
  // Marshal.SizeOf fails for value types that don't have explicit layouts. We
  // just fall back to some arbitrary constant in that case. Is there a better way?

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableExtensions.cs

@@ -17,6 +17,9 @@ internal static partial class ImmutableExtensions
  {
  internal static bool IsValueType<T>()
  {
+#if NETCOREAPP
+ return typeof(T).IsValueType;
+#else
  if (default(T) != null)
  {
  return true;
@@ -29,6 +32,7 @@ internal static bool IsValueType<T>()
  }
 
  return false;
+#endif
  }
 
 #if EqualsStructurally

src/libraries/System.Collections/src/System/Collections/Generic/HashSet.cs

@@ -266,7 +266,7 @@ public bool Contains(T item)
  {
  int hashCode = item == null ? 0 : InternalGetHashCode(item.GetHashCode());
 
- if (default(T) != null)
+ if (typeof(T).IsValueType)
  {
  // see note at "HashSet" level describing why "- 1" appears in for loop
  for (int i = buckets[hashCode % buckets.Length] - 1; i >= 0; i = slots[i].next)
@@ -368,7 +368,7 @@ public bool Remove(T item)
  hashCode = item == null ? 0 : InternalGetHashCode(item.GetHashCode());
  bucket = hashCode % _buckets!.Length;
 
- if (default(T) != null)
+ if (typeof(T).IsValueType)
  {
  for (i = _buckets[bucket] - 1; i >= 0; last = i, i = slots[i].next)
  {
@@ -1341,7 +1341,7 @@ private bool AddIfNotPresent(T value)
  hashCode = value == null ? 0 : InternalGetHashCode(value.GetHashCode());
  bucket = hashCode % _buckets!.Length;
 
- if (default(T) != null)
+ if (typeof(T).IsValueType)
  {
  for (int i = _buckets[bucket] - 1; i >= 0; i = slots[i].next)
  {
@@ -1577,7 +1577,7 @@ private int InternalIndexOf(T item)
  {
  int hashCode = item == null ? 0 : InternalGetHashCode(item.GetHashCode());
 
- if (default(T) != null)
+ if (typeof(T).IsValueType)
  {
  // see note at "HashSet" level describing why "- 1" appears in for loop
  for (int i = buckets[hashCode % buckets.Length] - 1; i >= 0; i = slots[i].next)

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Scheduling/Scheduling.cs

@@ -75,10 +75,7 @@ internal static int GetDefaultChunkSize<T>()
  {
  int chunkSize;
 
- // Because of the lack of typeof(T).IsValueType we need two pieces of information
- // to determine this. default(T) will return a non null for Value Types, except those
- // using Nullable<>, that is why we need a second condition.
- if (default(T) != null || Nullable.GetUnderlyingType(typeof(T)) != null)
+ if (typeof(T).IsValueType)
  {
  // Marshal.SizeOf fails for value types that don't have explicit layouts. We
  // just fall back to some arbitrary constant in that case. Is there a better way?

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs

@@ -250,7 +250,7 @@ public bool ContainsValue(TValue value)
  }
  else
  {
- if (default(TValue) != null)
+ if (typeof(TValue).IsValueType)
  {
  // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
  for (int i = 0; i < _count; i++)
@@ -344,7 +344,7 @@ private ref TValue FindValue(TKey key)
  int i = GetBucket(hashCode);
  Entry[]? entries = _entries;
  uint collisionCount = 0;
- if (default(TKey) != null)
+ if (typeof(TKey).IsValueType)
  {
  // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
 
@@ -495,7 +495,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
 
  if (comparer == null)
  {
- if (default(TKey) != null)
+ if (typeof(TKey).IsValueType)
  {
  // ValueType: Devirtualize with EqualityComparer<TValue>.Default intrinsic
  while (true)
@@ -658,7 +658,7 @@ private bool TryInsert(TKey key, TValue value, InsertionBehavior behavior)
  _version++;
 
  // Value types never rehash
- if (default(TKey) == null && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
+ if (!typeof(TKey).IsValueType && collisionCount > HashHelpers.HashCollisionThreshold && comparer is NonRandomizedStringEqualityComparer)
  {
  // If we hit the collision threshold we'll need to switch to the comparer which is using randomized string hashing
  // i.e. EqualityComparer<string>.Default.
@@ -720,7 +720,7 @@ private void Resize()
  private void Resize(int newSize, bool forceNewHashCodes)
  {
  // Value types never rehash
- Debug.Assert(!forceNewHashCodes || default(TKey) == null);
+ Debug.Assert(!forceNewHashCodes || !typeof(TKey).IsValueType);
  Debug.Assert(_entries != null, "_entries should be non-null");
  Debug.Assert(newSize >= _entries.Length);
 
@@ -729,7 +729,7 @@ private void Resize(int newSize, bool forceNewHashCodes)
  int count = _count;
  Array.Copy(_entries, entries, count);
 
- if (default(TKey) == null && forceNewHashCodes)
+ if (!typeof(TKey).IsValueType && forceNewHashCodes)
  {
  for (int i = 0; i < count; i++)
  {

src/libraries/System.Private.CoreLib/src/System/Memory.cs

@@ -53,7 +53,7 @@ public Memory(T[]? array)
  this = default;
  return; // returns default
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 
  _object = array;
@@ -71,7 +71,7 @@ internal Memory(T[]? array, int start)
  this = default;
  return; // returns default
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
  if ((uint)start > (uint)array.Length)
  ThrowHelper.ThrowArgumentOutOfRangeException();
@@ -103,7 +103,7 @@ public Memory(T[]? array, int start, int length)
  this = default;
  return; // returns default
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 #if TARGET_64BIT
  // See comment in Span<T>.Slice for how this works.

src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

@@ -34,7 +34,7 @@ public static Span<T> AsSpan<T>(this T[]? array, int start)
  ThrowHelper.ThrowArgumentOutOfRangeException();
  return default;
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
  if ((uint)start > (uint)array.Length)
  ThrowHelper.ThrowArgumentOutOfRangeException();
@@ -56,7 +56,7 @@ public static Span<T> AsSpan<T>(this T[]? array, Index startIndex)
  return default;
  }
 
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 
  int actualIndex = startIndex.GetOffset(array.Length);
@@ -83,7 +83,7 @@ public static Span<T> AsSpan<T>(this T[]? array, Range range)
  return default;
  }
 
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 
  (int start, int length) = range.GetOffsetAndLength(array.Length);

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.cs

@@ -27,7 +27,7 @@ public static T[] GetSubArray<T>(T[] array, Range range)
 
  (int offset, int length) = range.GetOffsetAndLength(array.Length);
 
- if (default(T) != null || typeof(T[]) == array.GetType())
+ if (typeof(T).IsValueType || typeof(T[]) == array.GetType())
  {
  // We know the type of the array to be exactly T[].
 

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs

@@ -522,7 +522,7 @@ public static Memory<T> CreateFromPinnedArray<T>(T[]? array, int start, int leng
  ThrowHelper.ThrowArgumentOutOfRangeException();
  return default;
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
  if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
  ThrowHelper.ThrowArgumentOutOfRangeException();

src/libraries/System.Private.CoreLib/src/System/Span.cs

@@ -50,7 +50,7 @@ public Span(T[]? array)
  this = default;
  return; // returns default
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 
  _pointer = new ByReference<T>(ref MemoryMarshal.GetArrayDataReference(array));
@@ -79,7 +79,7 @@ public Span(T[]? array, int start, int length)
  this = default;
  return; // returns default
  }
- if (default(T) == null && array.GetType() != typeof(T[]))
+ if (!typeof(T).IsValueType && array.GetType() != typeof(T[]))
  ThrowHelper.ThrowArrayTypeMismatchException();
 #if TARGET_64BIT
  // See comment in Span<T>.Slice for how this works.