Make internal & private classes sealed when possible. Avoid double lookup in dictionary

src/ICSharpCode.SharpZipLib/Core/StringBuilderPool.cs

@@ -3,7 +3,7 @@
 
 namespace ICSharpCode.SharpZipLib.Core
 {
-	internal class StringBuilderPool
+	internal sealed class StringBuilderPool
 	{
 		public static StringBuilderPool Instance { get; } = new StringBuilderPool();
 		private readonly ConcurrentQueue<StringBuilder> pool = new ConcurrentQueue<StringBuilder>();

src/ICSharpCode.SharpZipLib/Encryption/PkzipClassic.cs

@@ -130,7 +130,7 @@ protected void Reset()
 	/// <summary>
 	/// PkzipClassic CryptoTransform for encryption.
 	/// </summary>
-	internal class PkzipClassicEncryptCryptoTransform : PkzipClassicCryptoBase, ICryptoTransform
+	internal sealed class PkzipClassicEncryptCryptoTransform : PkzipClassicCryptoBase, ICryptoTransform
 	{
 		/// <summary>
 		/// Initialise a new instance of <see cref="PkzipClassicEncryptCryptoTransform"></see>
@@ -240,7 +240,7 @@ public void Dispose()
 	/// <summary>
 	/// PkzipClassic CryptoTransform for decryption.
 	/// </summary>
-	internal class PkzipClassicDecryptCryptoTransform : PkzipClassicCryptoBase, ICryptoTransform
+	internal sealed class PkzipClassicDecryptCryptoTransform : PkzipClassicCryptoBase, ICryptoTransform
 	{
 		/// <summary>
 		/// Initialise a new instance of <see cref="PkzipClassicDecryptCryptoTransform"></see>.

src/ICSharpCode.SharpZipLib/Encryption/ZipAESStream.cs

@@ -15,7 +15,7 @@ namespace ICSharpCode.SharpZipLib.Encryption
 	/// Based on information from http://www.winzip.com/aes_info.htm
 	/// and http://www.gladman.me.uk/cryptography_technology/fileencrypt/
 	/// </remarks>
-	internal class ZipAESStream : CryptoStream
+	internal sealed class ZipAESStream : CryptoStream
 	{
 		/// <summary>
 		/// Constructor

src/ICSharpCode.SharpZipLib/Encryption/ZipAESTransform.cs

@@ -6,7 +6,7 @@ namespace ICSharpCode.SharpZipLib.Encryption
 	/// <summary>
 	/// Transforms stream using AES in CTR mode
 	/// </summary>
-	internal class ZipAESTransform : ICryptoTransform
+	internal sealed class ZipAESTransform : ICryptoTransform
 	{
 		private const int PWD_VER_LENGTH = 2;
 

src/ICSharpCode.SharpZipLib/Zip/Compression/DeflaterHuffman.cs

@@ -60,7 +60,7 @@ public class DeflaterHuffman
 		private static short[] staticDCodes;
 		private static byte[] staticDLength;
 
-		private class Tree
+		private sealed class Tree
 		{
 			#region Instance Fields
 

src/ICSharpCode.SharpZipLib/Zip/Compression/InflaterDynHeader.cs

@@ -4,7 +4,7 @@
 
 namespace ICSharpCode.SharpZipLib.Zip.Compression
 {
-	internal class InflaterDynHeader
+	internal sealed class InflaterDynHeader
 	{
 		#region Constants
 

src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs

@@ -2679,9 +2679,9 @@ private void CopyEntryDataDirect(ZipUpdate update, Stream stream, bool updateCrc
 		private int FindExistingUpdate(ZipEntry entry)
 		{
 			int result = -1;
-			if (updateIndex_.ContainsKey(entry.Name))
+			if (updateIndex_.TryGetValue(entry.Name, out int value))
 			{
-				result = (int)updateIndex_[entry.Name];
+				result = value;
 			}
 			/*
 						// This is slow like the coming of the next ice age but takes less storage and may be useful
@@ -2705,9 +2705,9 @@ private int FindExistingUpdate(string fileName, bool isEntryName = false)
 
 			string convertedName = !isEntryName ? GetTransformedFileName(fileName) : fileName;
 
-			if (updateIndex_.ContainsKey(convertedName))
+			if (updateIndex_.TryGetValue(convertedName, out int value))
 			{
-				result = (int)updateIndex_[convertedName];
+				result = value;
 			}
 
 			/*
@@ -3030,7 +3030,7 @@ private void UpdateCommentOnly()
 		/// <summary>
 		/// Class used to sort updates.
 		/// </summary>
-		private class UpdateComparer : IComparer<ZipUpdate>
+		private sealed class UpdateComparer : IComparer<ZipUpdate>
 		{
 			/// <summary>
 			/// Compares two objects and returns a value indicating whether one is
@@ -3252,7 +3252,7 @@ private void CheckUpdating()
 		/// <summary>
 		/// Represents a pending update to a Zip file.
 		/// </summary>
-		private class ZipUpdate
+		private sealed class ZipUpdate
 		{
 			#region Constructors
 
@@ -3911,7 +3911,7 @@ private static void WriteEncryptionHeader(Stream stream, long crcValue)
 		/// <summary>
 		/// Represents a string from a <see cref="ZipFile"/> which is stored as an array of bytes.
 		/// </summary>
-		private class ZipString
+		private sealed class ZipString
 		{
 			#region Constructors
 
@@ -4086,7 +4086,7 @@ public void Dispose()
 		/// An <see cref="UncompressedStream"/> is a stream that you can write uncompressed data
 		/// to and flush, but cannot read, seek or do anything else to.
 		/// </summary>
-		private class UncompressedStream : Stream
+		private sealed class UncompressedStream : Stream
 		{
 			#region Constructors
 
@@ -4241,7 +4241,7 @@ private readonly
 		/// A <see cref="PartialInputStream"/> is an <see cref="InflaterInputStream"/>
 		/// whose data is only a part or subsection of a file.
 		/// </summary>
-		private class PartialInputStream : Stream
+		private sealed class PartialInputStream : Stream
 		{
 			#region Constructors