Update ICollection<T> usage to IReadOnlyCollection<T> where applicable (

dotnet#101469)

Anywhere we're casting to `ICollection<T>` just to use its `Count`, we can instead now cast to `IReadOnlyCollection<T>`, as the former inherits the latter as of .NET 9. This expands the set of types that can light-up with the check; in a couple of places it also lets us remove what would then be a duplicative check. We can do the same for `IList<T>` and `IReadOnlyList<T>`.

While dispatch via the DIM could result in an extra interface dispatch for types that weren't previously implementing the read-only interface, a) our collection types already implement both, and b) these cases all represent fast paths where the extra savings from the faster path should more than make up for additional call overheads. I audited for anywhere we were missing explicit implementations and added a few corner-cases in.

src/libraries/Common/src/System/Diagnostics/DiagnosticsHelper.cs

@@ -35,14 +35,22 @@ internal static bool CompareTags(List<KeyValuePair<string, object?>>? sortedTags
             int size = count / (sizeof(ulong) * 8) + 1;
             BitMapper bitMapper = new BitMapper(size <= 100 ? stackalloc ulong[size] : new ulong[size]);
 
+#if NET9_0_OR_GREATER // ICollection<T> : IReadOnlyCollection<T> on .NET 9+
+            if (tags2 is IReadOnlyCollection<KeyValuePair<string, object?>> tagsCol)
+#else
             if (tags2 is ICollection<KeyValuePair<string, object?>> tagsCol)
+#endif
             {
                 if (tagsCol.Count != count)
                 {
                     return false;
                 }
 
+#if NET9_0_OR_GREATER // IList<T> : IReadOnlyList<T> on .NET 9+
+                if (tagsCol is IReadOnlyList<KeyValuePair<string, object?>> secondList)
+#else
                 if (tagsCol is IList<KeyValuePair<string, object?>> secondList)
+#endif
                 {
                     for (int i = 0; i < count; i++)
                     {

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenSetInternalBase.cs

@@ -32,7 +32,11 @@ private protected override bool IsProperSubsetOfCore(IEnumerable<T> other)
         {
             Debug.Assert(_thisSet.Count != 0, "EmptyFrozenSet should have been used.");
 
+#if NET9_0_OR_GREATER // ICollection<T> : IReadOnlyCollection<T> on .NET 9+
+            if (other is IReadOnlyCollection<T> otherAsCollection)
+#else
             if (other is ICollection<T> otherAsCollection)
+#endif
             {
                 int otherCount = otherAsCollection.Count;
 
@@ -59,7 +63,11 @@ private protected override bool IsProperSupersetOfCore(IEnumerable<T> other)
         {
             Debug.Assert(_thisSet.Count != 0, "EmptyFrozenSet should have been used.");
 
+#if NET9_0_OR_GREATER // ICollection<T> : IReadOnlyCollection<T> on .NET 9+
+            if (other is IReadOnlyCollection<T> otherAsCollection)
+#else
             if (other is ICollection<T> otherAsCollection)
+#endif
             {
                 int otherCount = otherAsCollection.Count;
 
@@ -103,7 +111,11 @@ private protected override bool IsSupersetOfCore(IEnumerable<T> other)
             Debug.Assert(_thisSet.Count != 0, "EmptyFrozenSet should have been used.");
 
             // Try to compute the answer based purely on counts.
+#if NET9_0_OR_GREATER // ICollection<T> : IReadOnlyCollection<T> on .NET 9+
+            if (other is IReadOnlyCollection<T> otherAsCollection)
+#else
             if (other is ICollection<T> otherAsCollection)
+#endif
             {
                 int otherCount = otherAsCollection.Count;
 

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

@@ -38,11 +38,13 @@ internal static bool TryGetCount<T>(this IEnumerable sequence, out int count)
                 return true;
             }
 
+#if !NET9_0_OR_GREATER // ICollection<T> : IReadOnlyCollection<T> on .NET 9+
             if (sequence is ICollection<T> collectionOfT)
             {
                 count = collectionOfT.Count;
                 return true;
             }
+#endif
 
             if (sequence is IReadOnlyCollection<T> readOnlyCollection)
             {

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

@@ -462,7 +462,7 @@ public void EnqueueRange(IEnumerable<TElement> elements, TPriority priority)
             ArgumentNullException.ThrowIfNull(elements);
 
             int count;
-            if (elements is ICollection<TElement> collection &&
+            if (elements is IReadOnlyCollection<TElement> collection &&
                 (count = collection.Count) > _nodes.Length - _size)
             {
                 Grow(checked(_size + count));

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

@@ -1323,7 +1323,7 @@ public bool Overlaps(IEnumerable<T> other)
             if (Count == 0)
                 return false;
 
-            if (other is ICollection<T> c && c.Count == 0)
+            if (other is IReadOnlyCollection<T> c && c.Count == 0)
                 return false;
 
             SortedSet<T>? asSorted = other as SortedSet<T>;

src/libraries/System.Linq.Expressions/src/System/Dynamic/ExpandoObject.cs

@@ -17,7 +17,7 @@ namespace System.Dynamic
     /// <summary>
     /// Represents an object with members that can be dynamically added and removed at runtime.
     /// </summary>
-    public sealed class ExpandoObject : IDynamicMetaObjectProvider, IDictionary<string, object?>, INotifyPropertyChanged
+    public sealed class ExpandoObject : IDynamicMetaObjectProvider, IDictionary<string, object?>, IReadOnlyDictionary<string, object?>, INotifyPropertyChanged
     {
         private static readonly MethodInfo s_expandoTryGetValue =
             typeof(RuntimeOps).GetMethod(nameof(RuntimeOps.ExpandoTryGetValue))!;
@@ -618,6 +618,10 @@ System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
 
         ICollection<object?> IDictionary<string, object?>.Values => new ValueCollection(this);
 
+        IEnumerable<string> IReadOnlyDictionary<string, object?>.Keys => new KeyCollection(this);
+
+        IEnumerable<object?> IReadOnlyDictionary<string, object?>.Values => new ValueCollection(this);
+
         object? IDictionary<string, object?>.this[string key]
         {
             get
@@ -636,6 +640,18 @@ System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
             }
         }
 
+        object? IReadOnlyDictionary<string, object?>.this[string key]
+        {
+            get
+            {
+                if (!TryGetValueForKey(key, out object? value))
+                {
+                    throw System.Linq.Expressions.Error.KeyDoesNotExistInExpando(key);
+                }
+                return value;
+            }
+        }
+
         void IDictionary<string, object?>.Add(string key, object? value)
         {
             this.TryAddMember(key, value);
@@ -650,6 +666,15 @@ System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
             return index >= 0 && data[index] != Uninitialized;
         }
 
+        bool IReadOnlyDictionary<string, object?>.ContainsKey(string key)
+        {
+            ArgumentNullException.ThrowIfNull(key);
+
+            ExpandoData data = _data;
+            int index = data.Class.GetValueIndexCaseSensitive(key);
+            return index >= 0 && data[index] != Uninitialized;
+        }
+
         bool IDictionary<string, object?>.Remove(string key)
         {
             ArgumentNullException.ThrowIfNull(key);
@@ -662,6 +687,11 @@ System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
             return TryGetValueForKey(key, out value);
         }
 
+        bool IReadOnlyDictionary<string, object?>.TryGetValue(string key, out object? value)
+        {
+            return TryGetValueForKey(key, out value);
+        }
+
         #endregion
 
         #region ICollection<KeyValuePair<string, object>> Members

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

@@ -1852,7 +1852,7 @@ public static int Count<TSource>(this ParallelQuery<TSource> source)
             // If the data source is a collection, we can just return the count right away.
             if (source is ParallelEnumerableWrapper<TSource> sourceAsWrapper)
             {
-                if (sourceAsWrapper.WrappedEnumerable is ICollection<TSource> sourceAsCollection)
+                if (sourceAsWrapper.WrappedEnumerable is IReadOnlyCollection<TSource> sourceAsCollection)
                 {
                     return sourceAsCollection.Count;
                 }
@@ -1923,7 +1923,7 @@ public static long LongCount<TSource>(this ParallelQuery<TSource> source)
             // If the data source is a collection, we can just return the count right away.
             if (source is ParallelEnumerableWrapper<TSource> sourceAsWrapper)
             {
-                if (sourceAsWrapper.WrappedEnumerable is ICollection<TSource> sourceAsCollection)
+                if (sourceAsWrapper.WrappedEnumerable is IReadOnlyCollection<TSource> sourceAsCollection)
                 {
                     return sourceAsCollection.Count;
                 }

src/libraries/System.Linq/src/System/Linq/AnyAll.cs

@@ -15,7 +15,7 @@ public static bool Any<TSource>(this IEnumerable<TSource> source)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
-            if (source is ICollection<TSource> gc)
+            if (source is IReadOnlyCollection<TSource> gc)
             {
                 return gc.Count != 0;
             }

src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs

@@ -127,7 +127,7 @@ public override int GetCount(bool onlyIfCheap)
                     return count == -1 ? -1 : count + 1;
                 }
 
-                return !onlyIfCheap || _source is ICollection<TSource> ? _source.Count() + 1 : -1;
+                return !onlyIfCheap || _source is IReadOnlyCollection<TSource> ? _source.Count() + 1 : -1;
             }
 
             public override TSource? TryGetFirst(out bool found)
@@ -276,7 +276,7 @@ public override int GetCount(bool onlyIfCheap)
                     return count == -1 ? -1 : count + _appendCount + _prependCount;
                 }
 
-                return !onlyIfCheap || _source is ICollection<TSource> ? _source.Count() + _appendCount + _prependCount : -1;
+                return !onlyIfCheap || _source is IReadOnlyCollection<TSource> ? _source.Count() + _appendCount + _prependCount : -1;
             }
         }
     }

src/libraries/System.Linq/src/System/Linq/Count.cs

@@ -15,7 +15,7 @@ public static int Count<TSource>(this IEnumerable<TSource> source)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
-            if (source is ICollection<TSource> collectionoft)
+            if (source is IReadOnlyCollection<TSource> collectionoft)
             {
                 return collectionoft.Count;
             }
@@ -101,7 +101,7 @@ public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> s
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
-            if (source is ICollection<TSource> collectionoft)
+            if (source is IReadOnlyCollection<TSource> collectionoft)
             {
                 count = collectionoft.Count;
                 return true;

src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.SpeedOpt.cs

@@ -30,7 +30,7 @@ public override List<TSource> ToList()
             public override int GetCount(bool onlyIfCheap)
             {
                 int count;
-                if (!onlyIfCheap || _source is ICollection<TSource> || _source is ICollection)
+                if (!onlyIfCheap || _source is IReadOnlyCollection<TSource> || _source is ICollection)
                 {
                     count = _source.Count();
                 }

src/libraries/System.Linq/src/System/Linq/ElementAt.cs

@@ -16,7 +16,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int i
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 return list[index];
             }
@@ -115,7 +115,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, Index
 
         private static TSource? TryGetElementAt<TSource>(this IEnumerable<TSource> source, int index, out bool found)
         {
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 return (found = (uint)index < (uint)list.Count) ?
                     list[index] :

src/libraries/System.Linq/src/System/Linq/First.cs

@@ -78,7 +78,7 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
 
         private static TSource? TryGetFirstNonIterator<TSource>(IEnumerable<TSource> source, out bool found)
         {
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 if (list.Count > 0)
                 {

src/libraries/System.Linq/src/System/Linq/Grouping.cs

@@ -351,7 +351,7 @@ public interface IGrouping<out TKey, out TElement> : IEnumerable<TElement>
 
     [DebuggerDisplay("Key = {Key}")]
     [DebuggerTypeProxy(typeof(SystemLinq_GroupingDebugView<,>))]
-    internal sealed class Grouping<TKey, TElement> : IGrouping<TKey, TElement>, IList<TElement>
+    internal sealed class Grouping<TKey, TElement> : IGrouping<TKey, TElement>, IList<TElement>, IReadOnlyList<TElement>
     {
         internal readonly TKey _key;
         internal readonly int _hashCode;
@@ -398,6 +398,8 @@ public IEnumerator<TElement> GetEnumerator()
 
         int ICollection<TElement>.Count => _count;
 
+        int IReadOnlyCollection<TElement>.Count => _count;
+
         bool ICollection<TElement>.IsReadOnly => true;
 
         void ICollection<TElement>.Add(TElement item) => ThrowHelper.ThrowNotSupportedException();
@@ -431,5 +433,18 @@ TElement IList<TElement>.this[int index]
 
             set => ThrowHelper.ThrowNotSupportedException();
         }
+
+        TElement IReadOnlyList<TElement>.this[int index]
+        {
+            get
+            {
+                if ((uint)index >= (uint)_count)
+                {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
+                }
+
+                return _elements[index];
+            }
+        }
     }
 }

src/libraries/System.Linq/src/System/Linq/Last.cs

@@ -77,7 +77,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
 
         private static TSource? TryGetLastNonIterator<TSource>(IEnumerable<TSource> source, out bool found)
         {
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 int count = list.Count;
                 if (count > 0)
@@ -126,7 +126,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
                 return ordered.TryGetLast(predicate, out found);
             }
 
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 for (int i = list.Count - 1; i >= 0; --i)
                 {

src/libraries/System.Linq/src/System/Linq/OrderedEnumerable.SpeedOpt.cs

@@ -58,7 +58,7 @@ public override int GetCount(bool onlyIfCheap)
                     return iterator.GetCount(onlyIfCheap);
                 }
 
-                return !onlyIfCheap || _source is ICollection<TElement> || _source is ICollection ? _source.Count() : -1;
+                return !onlyIfCheap || _source is IReadOnlyCollection<TElement> || _source is ICollection ? _source.Count() : -1;
             }
 
             internal TElement[] ToArray(int minIdx, int maxIdx)

src/libraries/System.Linq/src/System/Linq/Reverse.SpeedOpt.cs

@@ -30,7 +30,7 @@ public override int GetCount(bool onlyIfCheap) =>
 
             public override TSource? TryGetElementAt(int index, out bool found)
             {
-                if (_source is IList<TSource> list)
+                if (_source is IReadOnlyList<TSource> list)
                 {
                     int count = list.Count;
                     if ((uint)index < (uint)count)
@@ -59,7 +59,7 @@ public override int GetCount(bool onlyIfCheap) =>
                 {
                     return iterator.TryGetLast(out found);
                 }
-                else if (_source is IList<TSource> list)
+                else if (_source is IReadOnlyList<TSource> list)
                 {
                     int count = list.Count;
                     if (count > 0)
@@ -95,7 +95,7 @@ public override int GetCount(bool onlyIfCheap) =>
                 {
                     return iterator.TryGetFirst(out found);
                 }
-                else if (_source is IList<TSource> list)
+                else if (_source is IReadOnlyList<TSource> list)
                 {
                     if (list.Count > 0)
                     {

src/libraries/System.Linq/src/System/Linq/SequenceEqual.cs

@@ -22,7 +22,7 @@ public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnum
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.second);
             }
 
-            if (first is ICollection<TSource> firstCol && second is ICollection<TSource> secondCol)
+            if (first is IReadOnlyCollection<TSource> firstCol && second is IReadOnlyCollection<TSource> secondCol)
             {
                 if (first.TryGetSpan(out ReadOnlySpan<TSource> firstSpan) && second.TryGetSpan(out ReadOnlySpan<TSource> secondSpan))
                 {
@@ -34,7 +34,7 @@ public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnum
                     return false;
                 }
 
-                if (firstCol is IList<TSource> firstList && secondCol is IList<TSource> secondList)
+                if (firstCol is IReadOnlyList<TSource> firstList && secondCol is IReadOnlyList<TSource> secondList)
                 {
                     comparer ??= EqualityComparer<TSource>.Default;
 

src/libraries/System.Linq/src/System/Linq/Single.cs

@@ -69,7 +69,7 @@ public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
-            if (source is IList<TSource> list)
+            if (source is IReadOnlyList<TSource> list)
             {
                 switch (list.Count)
                 {

src/libraries/System.Net.Security/src/System/Security/Authentication/ExtendedProtection/ServiceNameCollection.cs

@@ -147,7 +147,7 @@ private void AddIfNew(string serviceName)
         /// </summary>
         private static int GetCountOrOne(IEnumerable collection)
         {
-            ICollection<string>? c = collection as ICollection<string>;
+            IReadOnlyCollection<string>? c = collection as IReadOnlyCollection<string>;
             return c != null ? c.Count : 1;
         }
 

src/libraries/System.Private.CoreLib/src/System/Collections/Concurrent/ConcurrentQueue.cs

@@ -89,7 +89,7 @@ public ConcurrentQueue(IEnumerable<T> collection)
             // case we round its length up to a power of 2, as all segments must
             // be a power of 2 in length.
             int length = InitialSegmentLength;
-            if (collection is ICollection<T> c)
+            if (collection is IReadOnlyCollection<T> c)
             {
                 int count = c.Count;
                 if (count > length)