Fix IDE0251: Make member readonly

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

@@ -308,7 +308,7 @@ public bool MoveNext()
                 return false;
             }
 
-            public T Current
+            public readonly T Current
             {
                 get
                 {
@@ -327,7 +327,7 @@ void IEnumerator.Reset()
                 _current = _start - 1;
             }
 
-            public void Dispose()
+            public readonly void Dispose()
             {
             }
         }

src/libraries/System.Private.CoreLib/src/System/Buffers/MemoryHandle.cs

@@ -32,7 +32,7 @@ public MemoryHandle(void* pointer, GCHandle handle = default, IPinnable? pinnabl
         /// Returns the pointer to memory, where the memory is assumed to be pinned and hence the address won't change.
         /// </summary>
         [CLSCompliant(false)]
-        public void* Pointer => _pointer;
+        public readonly void* Pointer => _pointer;
 
         /// <summary>
         /// Frees the pinned handle and releases IPinnable.

src/libraries/System.Private.CoreLib/src/System/Collections/DictionaryEntry.cs

@@ -26,13 +26,13 @@ public DictionaryEntry(object key, object? value)
 
         public object Key
         {
-            get => _key;
+            readonly get => _key;
             set => _key = value;
         }
 
         public object? Value
         {
-            get => _value;
+            readonly get => _value;
             set => _value = value;
         }
 
@@ -43,7 +43,7 @@ public void Deconstruct(out object key, out object? value)
             value = Value;
         }
 
-        public override string ToString() =>
+        public override readonly string ToString() =>
             KeyValuePair.PairToString(_key, _value);
     }
 }

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

@@ -1807,11 +1807,11 @@ public bool MoveNext()
                 return false;
             }
 
-            public KeyValuePair<TKey, TValue> Current => _current;
+            public readonly KeyValuePair<TKey, TValue> Current => _current;
 
-            public void Dispose() { }
+            public readonly void Dispose() { }
 
-            object? IEnumerator.Current
+            readonly object? IEnumerator.Current
             {
                 get
                 {
@@ -1840,7 +1840,7 @@ void IEnumerator.Reset()
                 _current = default;
             }
 
-            DictionaryEntry IDictionaryEnumerator.Entry
+            readonly DictionaryEntry IDictionaryEnumerator.Entry
             {
                 get
                 {
@@ -1853,7 +1853,7 @@ DictionaryEntry IDictionaryEnumerator.Entry
                 }
             }
 
-            object IDictionaryEnumerator.Key
+            readonly object IDictionaryEnumerator.Key
             {
                 get
                 {
@@ -1866,7 +1866,7 @@ object IDictionaryEnumerator.Key
                 }
             }
 
-            object? IDictionaryEnumerator.Value
+            readonly object? IDictionaryEnumerator.Value
             {
                 get
                 {
@@ -2022,7 +2022,7 @@ internal Enumerator(Dictionary<TKey, TValue> dictionary)
                     _currentKey = default;
                 }
 
-                public void Dispose() { }
+                public readonly void Dispose() { }
 
                 public bool MoveNext()
                 {
@@ -2047,9 +2047,9 @@ public bool MoveNext()
                     return false;
                 }
 
-                public TKey Current => _currentKey!;
+                public readonly TKey Current => _currentKey!;
 
-                object? IEnumerator.Current
+                readonly object? IEnumerator.Current
                 {
                     get
                     {
@@ -2216,7 +2216,7 @@ internal Enumerator(Dictionary<TKey, TValue> dictionary)
                     _currentValue = default;
                 }
 
-                public void Dispose() { }
+                public readonly void Dispose() { }
 
                 public bool MoveNext()
                 {
@@ -2240,9 +2240,9 @@ public bool MoveNext()
                     return false;
                 }
 
-                public TValue Current => _currentValue!;
+                public readonly TValue Current => _currentValue!;
 
-                object? IEnumerator.Current
+                readonly object? IEnumerator.Current
                 {
                     get
                     {

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

@@ -450,7 +450,7 @@ internal static IAlternateEqualityComparer<TAlternate, T> GetAlternateComparer(H
             /// <summary>Adds the specified element to a set.</summary>
             /// <param name="item">The element to add to the set.</param>
             /// <returns>true if the element is added to the set; false if the element is already present.</returns>
-            public bool Add(TAlternate item)
+            public readonly bool Add(TAlternate item)
             {
                 HashSet<T> set = Set;
                 IAlternateEqualityComparer<TAlternate, T> comparer = GetAlternateComparer(set);
@@ -538,7 +538,7 @@ public bool Add(TAlternate item)
             /// <summary>Removes the specified element from a set.</summary>
             /// <param name="item">The element to remove.</param>
             /// <returns>true if the element is successfully found and removed; otherwise, false.</returns>
-            public bool Remove(TAlternate item)
+            public readonly bool Remove(TAlternate item)
             {
                 HashSet<T> set = Set;
                 IAlternateEqualityComparer<TAlternate, T> comparer = GetAlternateComparer(set);
@@ -623,7 +623,7 @@ public bool TryGetValue(TAlternate equalValue, [MaybeNullWhen(false)] out T actu
             }
 
             /// <summary>Finds the item in the set and returns a reference to the found item, or a null reference if not found.</summary>
-            internal ref readonly T FindValue(TAlternate item)
+            internal readonly ref readonly T FindValue(TAlternate item)
             {
                 HashSet<T> set = Set;
                 IAlternateEqualityComparer<TAlternate, T> comparer = GetAlternateComparer(set);
@@ -1834,11 +1834,11 @@ public bool MoveNext()
                 return false;
             }
 
-            public T Current => _current;
+            public readonly T Current => _current;
 
-            public void Dispose() { }
+            public readonly void Dispose() { }
 
-            object? IEnumerator.Current
+            readonly object? IEnumerator.Current
             {
                 get
                 {

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

@@ -1199,7 +1199,7 @@ internal Enumerator(List<T> list)
                 _current = default;
             }
 
-            public void Dispose()
+            public readonly void Dispose()
             {
             }
 
@@ -1228,7 +1228,7 @@ private bool MoveNextRare()
                 return false;
             }
 
-            public T Current => _current!;
+            public readonly T Current => _current!;
 
             object? IEnumerator.Current
             {

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

@@ -490,7 +490,7 @@ public T Current
                 }
             }
 
-            private void ThrowEnumerationNotStartedOrEnded()
+            private readonly void ThrowEnumerationNotStartedOrEnded()
             {
                 Debug.Assert(_index == -1 || _index == -2);
                 throw new InvalidOperationException(_index == -1 ? SR.InvalidOperation_EnumNotStarted : SR.InvalidOperation_EnumEnded);

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

@@ -22,7 +22,7 @@ public ValueListBuilder(Span<T> initialSpan)
 
         public int Length
         {
-            get => _pos;
+            readonly get => _pos;
             set
             {
                 Debug.Assert(value >= 0);
@@ -138,12 +138,12 @@ private void AddWithResize(T item)
             _pos = pos + 1;
         }
 
-        public ReadOnlySpan<T> AsSpan()
+        public readonly ReadOnlySpan<T> AsSpan()
         {
             return _span.Slice(0, _pos);
         }
 
-        public bool TryCopyTo(Span<T> destination, out int itemsWritten)
+        public readonly bool TryCopyTo(Span<T> destination, out int itemsWritten)
         {
             if (_span.Slice(0, _pos).TryCopyTo(destination))
             {

src/libraries/System.Private.CoreLib/src/System/Decimal.DecCalc.cs

@@ -61,29 +61,29 @@ private struct DecCalc
 
             private uint High
             {
-                get => uhi;
+                readonly get => uhi;
                 set => uhi = value;
             }
 
             private uint Low
             {
-                get => ulo;
+                readonly get => ulo;
                 set => ulo = value;
             }
 
             private uint Mid
             {
-                get => umid;
+                readonly get => umid;
                 set => umid = value;
             }
 
-            private bool IsNegative => (int)uflags < 0;
+            private readonly bool IsNegative => (int)uflags < 0;
 
-            private int Scale => (byte)(uflags >> ScaleShift);
+            private readonly int Scale => (byte)(uflags >> ScaleShift);
 
             private ulong Low64
             {
-                get => ulomid;
+                readonly get => ulomid;
                 set => ulomid = value;
             }
 
@@ -2520,7 +2520,7 @@ public ulong Low64
                     get => ((ulong)U1 << 32) | U0;
                     set { U1 = (uint)(value >> 32); U0 = (uint)value; }
 #else
-                    get => ulo64LE;
+                    readonly get => ulo64LE;
                     set => ulo64LE = value;
 #endif
                 }
@@ -2534,7 +2534,7 @@ public ulong High64
                     get => ((ulong)U2 << 32) | U1;
                     set { U2 = (uint)(value >> 32); U1 = (uint)value; }
 #else
-                    get => uhigh64LE;
+                    readonly get => uhigh64LE;
                     set => uhigh64LE = value;
 #endif
                 }
@@ -2563,7 +2563,7 @@ public ulong Low64
                     get => ((ulong)U1 << 32) | U0;
                     set { U1 = (uint)(value >> 32); U0 = (uint)value; }
 #else
-                    get => ulo64LE;
+                    readonly get => ulo64LE;
                     set => ulo64LE = value;
 #endif
                 }
@@ -2574,7 +2574,7 @@ public ulong High64
                     get => ((ulong)U3 << 32) | U2;
                     set { U3 = (uint)(value >> 32); U2 = (uint)value; }
 #else
-                    get => uhigh64LE;
+                    readonly get => uhigh64LE;
                     set => uhigh64LE = value;
 #endif
                 }
@@ -2609,7 +2609,7 @@ public ulong Low64
                     get => ((ulong)U1 << 32) | U0;
                     set { U1 = (uint)(value >> 32); U0 = (uint)value; }
 #else
-                    get => ulo64LE;
+                    readonly get => ulo64LE;
                     set => ulo64LE = value;
 #endif
                 }
@@ -2620,7 +2620,7 @@ public ulong Mid64
                     get => ((ulong)U3 << 32) | U2;
                     set { U3 = (uint)(value >> 32); U2 = (uint)value; }
 #else
-                    get => umid64LE;
+                    readonly get => umid64LE;
                     set => umid64LE = value;
 #endif
                 }
@@ -2631,7 +2631,7 @@ public ulong High64
                     get => ((ulong)U5 << 32) | U4;
                     set { U5 = (uint)(value >> 32); U4 = (uint)value; }
 #else
-                    get => uhigh64LE;
+                    readonly get => uhigh64LE;
                     set => uhigh64LE = value;
 #endif
                 }

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

@@ -115,7 +115,7 @@ internal InvocationListEnumerator(MulticastDelegate? d)
             /// <summary>
             /// Implements the IEnumerator pattern.
             /// </summary>
-            public TDelegate Current
+            public readonly TDelegate Current
             {
                 get => _current!;
             }
@@ -141,7 +141,7 @@ public bool MoveNext()
             /// </summary>
             /// <returns>An IEnumerator instance that can be used to iterate through the invocation targets of the delegate.</returns>
             [EditorBrowsable(EditorBrowsableState.Never)] // Only here to make foreach work
-            public System.Delegate.InvocationListEnumerator<TDelegate> GetEnumerator() => this;
+            public readonly System.Delegate.InvocationListEnumerator<TDelegate> GetEnumerator() => this;
         }
 
         public object? DynamicInvoke(params object?[]? args)

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventPipe.cs

@@ -98,7 +98,7 @@ internal static void MarshalToNative(EventPipeProviderConfiguration managed, ref
                 native.m_pFilterData = (char*)Marshal.StringToCoTaskMemUni(managed.FilterData);
             }
 
-            internal void Release()
+            internal readonly void Release()
             {
                 if (m_pProviderName != null)
                 {

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventPipeMetadataGenerator.cs

@@ -373,7 +373,7 @@ private static unsafe bool GenerateMetadataForProperty(PropertyAnalysis property
             return true;
         }
 
-        internal unsafe bool GenerateMetadataV2(byte* pMetadataBlob, ref uint offset, uint blobSize)
+        internal readonly unsafe bool GenerateMetadataV2(byte* pMetadataBlob, ref uint offset, uint blobSize)
         {
             if (TypeInfo == null)
                 return false;
@@ -564,7 +564,7 @@ internal static bool GetTypeInfoFromType(Type? type, out TraceLoggingTypeInfo? t
             }
         }
 
-        internal bool GetMetadataLength(out uint size)
+        internal readonly bool GetMetadataLength(out uint size)
         {
             size = 0;
 
@@ -667,7 +667,7 @@ private static TypeCode GetTypeCodeExtended(Type parameterType)
             return Type.GetTypeCode(parameterType);
         }
 
-        internal bool GetMetadataLengthV2(out uint size)
+        internal readonly bool GetMetadataLengthV2(out uint size)
         {
             return GetMetadataLengthForNamedTypeV2(ParameterName, TypeInfo, out size);
         }