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

Author: stephentoub

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;