Add some xml comments

src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/ListDictionary.cs

@@ -5,13 +5,10 @@
 
 namespace System.Collections.Specialized
 {
-    /// <devdoc>
-    ///  <para>
-    ///    This is a simple implementation of IDictionary using a singly linked list. This
-    ///    will be smaller and faster than a Hashtable if the number of elements is 10 or less.
-    ///    This should not be used if performance is important for large numbers of elements.
-    ///  </para>
-    /// </devdoc>
+    /// <summary>
+    /// Implements IDictionary using a singly linked list.
+    /// Recommended for collections that typically include fewer than 10 items.
+    /// </summary>
     [Serializable]
     [System.Runtime.CompilerServices.TypeForwardedFrom("System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ListDictionary : IDictionary

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

@@ -1,21 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class representing an AV that was deemed unsafe and may have corrupted the application.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
+    /// <summary>
+    /// The exception that is thrown when there is an attempt to read or write protected memory.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class AccessViolationException : SystemException

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

@@ -1,25 +1,16 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for invalid arguments to a method.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
-    // The ArgumentException is thrown when an argument does not meet
-    // the contract of the method.  Ideally it should give a meaningful error
-    // message describing what was wrong and which parameter is incorrect.
+    /// <summary>
+    /// The exception that is thrown when a null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentException : SystemException

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

@@ -1,24 +1,16 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for null arguments to a method.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
-    // The ArgumentException is thrown when an argument
-    // is null when it shouldn't be.
+    /// <summary>
+    /// The exception that is thrown when a null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentNullException : ArgumentException

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

@@ -1,15 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for method arguments outside of the legal range.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.Serialization;
 using System.Runtime.CompilerServices;
@@ -19,8 +10,9 @@
 
 namespace System
 {
-    // The ArgumentOutOfRangeException is thrown when an argument
-    // is outside the legal range for that argument.
+    /// <summary>
+    /// The exception that is thrown when the value of an argument is outside the allowable range of values as defined by the invoked method.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArgumentOutOfRangeException : ArgumentException

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

@@ -1,23 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for bad arithmetic conditions!
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
-    // The ArithmeticException is thrown when overflow or underflow
-    // occurs.
+    /// <summary>
+    /// The exception that is thrown for errors in an arithmetic, casting, or conversion operation.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArithmeticException : SystemException

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

@@ -1,23 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: The arrays are of different primitive types.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
-    // The ArrayMismatchException is thrown when an attempt to store
-    // an object of the wrong type within an array occurs.
+    /// <summary>
+    /// The exception that is thrown when there is an attempt to read or write protected memory.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ArrayTypeMismatchException : SystemException

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

@@ -1,22 +1,16 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*============================================================
-**
-**
-**
-** Purpose: Exception to an invalid dll or executable format.
-**
-**
-===========================================================*/
-
 using System.ComponentModel;
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
+    /// <summary>
+    /// The exception that is thrown when there is an attempt to read or write protected memory.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public partial class BadImageFormatException : SystemException

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

@@ -1,19 +1,16 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*============================================================
-**
-** Purpose: Default IComparer implementation.
-**
-===========================================================*/
-
 using System.ComponentModel;
 using System.Globalization;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System.Collections
 {
+    /// <summary>
+    /// Compares two objects for equivalence, where string comparisons are case-sensitive.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class Comparer : IComparer, ISerializable

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

@@ -1,22 +1,18 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-** Purpose: A circular-array implementation of a generic queue.
-**
-**
-=============================================================================*/
-
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
 namespace System.Collections.Generic
 {
-    // A simple Queue of generic objects.  Internally it is implemented as a
-    // circular buffer, so Enqueue can be O(n).  Dequeue is O(1).
+    /// <summary>
+    /// Represents a first-in, first-out collection of objects.
+    /// </summary>
+    /// <remarks>
+    /// Implemented as a circular buffer, so Enqueue and Dequeue are typically O(1).
+    /// </remarks>
     [DebuggerTypeProxy(typeof(QueueDebugView<>))]
     [DebuggerDisplay("Count = {Count}")]
     [Serializable]

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

@@ -1,23 +1,13 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
-/*============================================================
-**
-**
-**
-**
-**
-** Purpose: List for exceptions.
-**
-**
-===========================================================*/
-
 using System.Runtime.CompilerServices;
 
 namespace System.Collections
 {
-    ///    This is a simple implementation of IDictionary using a singly linked list. This
-    ///    will be smaller and faster than a Hashtable if the number of elements is 10 or less.
-    ///    This should not be used if performance is important for large numbers of elements.
+    /// <summary>
+    /// Implements IDictionary using a singly linked list.
+    /// Recommended for collections that typically include fewer than 10 items.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     // Needs to be public to support binary serialization compatibility

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

@@ -1,19 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-** Purpose: The exception class for a misaligned access exception
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
+    /// <summary>
+    /// The exception that is thrown when there is an attempt to read or write protected memory.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public sealed class DataMisalignedException : SystemException

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

@@ -1,21 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for bad arithmetic conditions!
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
+    /// <summary>
+    /// The exception that is thrown when there is an attempt to divide an integral or <see cref="decimal"/> value by zero.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class DivideByZeroException : ArithmeticException

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

@@ -1,16 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*============================================================
-**
-**
-**
-** Purpose: A representation of an IEEE double precision
-**          floating point number.
-**
-**
-===========================================================*/
-
 using System.Buffers.Binary;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
@@ -22,6 +12,9 @@
 
 namespace System
 {
+    /// <summary>
+    /// Represents a double-precision floating-point number.
+    /// </summary>
     [Serializable]
     [StructLayout(LayoutKind.Sequential)]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]

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

@@ -1,23 +1,15 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-/*=============================================================================
-**
-**
-**
-** Purpose: Exception class for duplicate objects in WaitAll/WaitAny.
-**
-**
-=============================================================================*/
-
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
 
 namespace System
 {
-    // The DuplicateWaitObjectException is thrown when an object
-    // appears more than once in the list of objects to WaitAll or WaitAny.
+    /// <summary>
+    /// The exception that is thrown when an object appears more than once in an array of synchronization objects.
+    /// </summary>
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class DuplicateWaitObjectException : ArgumentException