Enable explicit null checking of reference types

ICSharpCode.SharpZipLib.sln.DotSettings

@@ -0,0 +1,5 @@
+<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=IV/@EntryIndexedValue">IV</s:String>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=decryptor/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=Deflater/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=encryptor/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

appveyor.yml

@@ -1,5 +1,5 @@
 version: '{build}'
-image: Visual Studio 2017
+image: Visual Studio 2019
 configuration:
 - Debug
 - Release

src/ICSharpCode.SharpZipLib/BZip2/BZip2InputStream.cs

@@ -1,6 +1,7 @@
 using ICSharpCode.SharpZipLib.Checksum;
 using System;
 using System.IO;
+using ICSharpCode.SharpZipLib.Core;
 
 namespace ICSharpCode.SharpZipLib.BZip2
 {
@@ -9,7 +10,7 @@ namespace ICSharpCode.SharpZipLib.BZip2
 	/// </summary>
 	public class BZip2InputStream : Stream
 	{
-		#region Constants
+#region Constants
 
 		private const int START_BLOCK_STATE = 1;
 		private const int RAND_PART_A_STATE = 2;
@@ -19,9 +20,9 @@ public class BZip2InputStream : Stream
 		private const int NO_RAND_PART_B_STATE = 6;
 		private const int NO_RAND_PART_C_STATE = 7;
 
-		#endregion Constants
+#endregion Constants
 
-		#region Instance Fields
+#region Instance Fields
 
 		/*--
 		index of the last char in the block, so
@@ -55,8 +56,8 @@ The current block size is 100000 * this number.
 		private byte[] selector = new byte[BZip2Constants.MaximumSelectors];
 		private byte[] selectorMtf = new byte[BZip2Constants.MaximumSelectors];
 
-		private int[] tt;
-		private byte[] ll8;
+		private int[] tt = Empty.Array<int>();
+		private byte[] ll8 = Empty.Array<byte>();
 
 		/*--
 		freq table collected to save a pass over the data
@@ -87,7 +88,7 @@ during decompression.
 		private int i2, j2;
 		private byte z;
 
-		#endregion Instance Fields
+#endregion Instance Fields
 
 		/// <summary>
 		/// Construct instance for reading from stream
@@ -119,7 +120,7 @@ public BZip2InputStream(Stream stream)
 		/// </summary>
 		public bool IsStreamOwner { get; set; } = true;
 
-		#region Stream Overrides
+#region Stream Overrides
 
 		/// <summary>
 		/// Gets a value indicating if the stream supports reading
@@ -222,7 +223,7 @@ public override void SetLength(long value)
 		/// <param name="offset">The offset to start obtaining data from.</param>
 		/// <param name="count">The number of bytes of data to write.</param>
 		/// <exception cref="NotSupportedException">Any access</exception>
-		public override void Write(byte[] buffer, int offset, int count)
+		public override void Write(byte[]? buffer, int offset, int count)
 		{
 			throw new NotSupportedException("BZip2InputStream Write not supported");
 		}
@@ -316,7 +317,7 @@ public override int ReadByte()
 			return retChar;
 		}
 
-		#endregion Stream Overrides
+#endregion Stream Overrides
 
 		private void MakeMaps()
 		{
@@ -746,7 +747,7 @@ cache misses.
 
 		private void SetupBlock()
 		{
-			int[] cftab = new int[257];
+			int[]? cftab = new int[257];
 
 			cftab[0] = 0;
 			Array.Copy(unzftab, 0, cftab, 1, 256);

src/ICSharpCode.SharpZipLib/BZip2/BZip2OutputStream.cs

@@ -1,6 +1,7 @@
 using ICSharpCode.SharpZipLib.Checksum;
 using System;
 using System.IO;
+using ICSharpCode.SharpZipLib.Core;
 
 namespace ICSharpCode.SharpZipLib.BZip2
 {
@@ -79,11 +80,11 @@ The current block size is 100000 * this number.
 		private char[] selector = new char[BZip2Constants.MaximumSelectors];
 		private char[] selectorMtf = new char[BZip2Constants.MaximumSelectors];
 
-		private byte[] block;
-		private int[] quadrant;
-		private int[] zptr;
-		private short[] szptr;
-		private int[] ftab;
+		private byte[] block = Empty.Array<byte>();
+		private int[] quadrant = Empty.Array<int>();
+		private int[] zptr = Empty.Array<int>();
+		private short[] szptr = Empty.Array<short>();
+		private int[] ftab = Empty.Array<int>();
 
 		private int nMTF;
 
@@ -266,7 +267,7 @@ public override int ReadByte()
 		/// <returns>The total number of bytes read. This might be less than the number of bytes
 		/// requested if that number of bytes are not currently available, or zero
 		/// if the end of the stream is reached.</returns>
-		public override int Read(byte[] buffer, int offset, int count)
+		public override int Read(byte[]? buffer, int offset, int count)
 		{
 			throw new NotSupportedException("BZip2OutputStream Read not supported");
 		}
@@ -710,14 +711,14 @@ private void SendMTFValues()
 				remF -= aFreq;
 			}
 
-			int[][] rfreq = new int[BZip2Constants.GroupCount][];
+			int[][]? rfreq = new int[BZip2Constants.GroupCount][];
 			for (int i = 0; i < BZip2Constants.GroupCount; ++i)
 			{
 				rfreq[i] = new int[BZip2Constants.MaximumAlphaSize];
 			}
 
-			int[] fave = new int[BZip2Constants.GroupCount];
-			short[] cost = new short[BZip2Constants.GroupCount];
+			int[]? fave = new int[BZip2Constants.GroupCount];
+			short[]? cost = new short[BZip2Constants.GroupCount];
 			/*---
 			Iterate up to N_ITERS times to improve the tables.
 			---*/
@@ -1691,7 +1692,7 @@ private void AllocateCompressStructures()
 			zptr = new int[n];
 			ftab = new int[65537];
 
-			if (block == null || quadrant == null || zptr == null || ftab == null)
+			if (block == Empty.Array<byte>() || quadrant == null || zptr == null || ftab == null)
 			{
 				//		int totalDraw = (n + 1 + NUM_OVERSHOOT_BYTES) + (n + NUM_OVERSHOOT_BYTES) + n + 65537;
 				//		compressOutOfMemory ( totalDraw, n );

src/ICSharpCode.SharpZipLib/Core/EmptyRefs.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using ICSharpCode.SharpZipLib.Zip;
+
+namespace ICSharpCode.SharpZipLib.Core
+{
+	internal static class Empty
+	{
+#if NET45
+		internal static class EmptyArray<T>
+		{
+			public static readonly T[] Value = new T[0];
+		}
+		public static T[] Array<T>() => EmptyArray<T>.Value;
+#else
+		public static T[] Array<T>() => System.Array.Empty<T>();
+#endif
+	}
+}

src/ICSharpCode.SharpZipLib/Core/Exceptions/StreamDecodingException.cs

@@ -5,7 +5,7 @@ namespace ICSharpCode.SharpZipLib
 {
 	/// <summary>
 	/// Indicates that an error occured during decoding of a input stream due to corrupt
-	/// data or (unintentional) library incompability.
+	/// data or (unintentional) library incompatibility.
 	/// </summary>
 	[Serializable]
 	public class StreamDecodingException : SharpZipBaseException