Make collection-typed fields readonly where possible

also I widened them all to the relevant interface

this was done with a naive regex replace, followed by reverting lines until all
the build errors were gone; the regex was something like
s@(public|internal|protected|private) ((?:List|Dictionary)<[^=\n{]+= )@$1
readonly I$2@g

src/BizHawk.Client.Common/DisplayManager/FilterManager.cs

@@ -40,8 +40,8 @@ namespace BizHawk.Client.Common.FilterManager
 	{
 		private readonly Dictionary<string, BaseFilter> _filterNameIndex = new Dictionary<string, BaseFilter>();
 
-		public List<BaseFilter> Filters = new List<BaseFilter>();
-		public List<ProgramStep> Program = new List<ProgramStep>();
+		public readonly IList<BaseFilter> Filters = new List<BaseFilter>();
+		public readonly IList<ProgramStep> Program = new List<ProgramStep>();
 
 		public BaseFilter this[string name]
 		{

src/BizHawk.Client.Common/movie/tasproj/StateDictionary.cs

@@ -9,7 +9,7 @@ namespace BizHawk.Client.Common
 {
 	internal class TempFileStateDictionary : IDictionary<int, byte[]>, IDisposable
 	{ 
-		private Dictionary<int, Stream> _streams = new Dictionary<int, Stream>();
+		private readonly IDictionary<int, Stream> _streams = new Dictionary<int, Stream>();
 
 		public byte[] this[int key]
 		{

src/BizHawk.Emulation.Cores/Computers/AmstradCPC/AmstradCPC.cs

@@ -107,8 +107,8 @@ namespace BizHawk.Emulation.Cores.Computers.AmstradCPC
 		public CPCBase _machine;
 
 		public List<GameInfo> _gameInfo;
-		public List<GameInfo> _tapeInfo = new List<GameInfo>();
-		public List<GameInfo> _diskInfo = new List<GameInfo>();
+		public readonly IList<GameInfo> _tapeInfo = new List<GameInfo>();
+		public readonly IList<GameInfo> _diskInfo = new List<GameInfo>();
 
 		private SoundProviderMixer SoundMixer;
 

src/BizHawk.Emulation.Cores/Computers/AmstradCPC/Media/Tape/CDT/CdtConverter.cs

@@ -29,7 +29,7 @@ namespace BizHawk.Emulation.Cores.Computers.AmstradCPC
 		/// <summary>
 		/// Working list of generated tape data blocks
 		/// </summary>
-		private List<TapeDataBlock> _blocks = new List<TapeDataBlock>();
+		private readonly IList<TapeDataBlock> _blocks = new List<TapeDataBlock>();
 
 		/// <summary>
 		/// Position counter

src/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Media/Tape/PZX/PzxConverter.cs

@@ -30,7 +30,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 		/// <summary>
 		/// Working list of generated tape data blocks
 		/// </summary>
-		private List<TapeDataBlock> _blocks = new List<TapeDataBlock>();
+		private readonly IList<TapeDataBlock> _blocks = new List<TapeDataBlock>();
 
 		/// <summary>
 		/// Position counter
@@ -40,7 +40,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 		/// <summary>
 		/// Object to keep track of loops - this assumes there is only one loop at a time
 		/// </summary>
-		private List<KeyValuePair<int, int>> _loopCounter = new List<KeyValuePair<int, int>>();
+		private readonly IList<KeyValuePair<int, int>> _loopCounter = new List<KeyValuePair<int, int>>();
 
 		private readonly DatacorderDevice _datacorder;
 

src/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Media/Tape/TZX/TzxConverter.cs

@@ -29,7 +29,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 		/// <summary>
 		/// Working list of generated tape data blocks
 		/// </summary>
-		private List<TapeDataBlock> _blocks = new List<TapeDataBlock>();
+		private readonly IList<TapeDataBlock> _blocks = new List<TapeDataBlock>();
 
 		/// <summary>
 		/// Position counter

src/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/ZXSpectrum.cs

@@ -159,8 +159,8 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 
 		public List<GameInfo> _gameInfo;
 
-		public List<GameInfo> _tapeInfo = new List<GameInfo>();
-		public List<GameInfo> _diskInfo = new List<GameInfo>();
+		public readonly IList<GameInfo> _tapeInfo = new List<GameInfo>();
+		public readonly IList<GameInfo> _diskInfo = new List<GameInfo>();
 
 		private SyncSoundMixer SoundMixer;
 

src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/FDS/FDSInspector.cs

@@ -16,7 +16,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 		private static readonly byte[] Check = FTXT.E.GetBytes("FDS\x1a");
 		private static readonly byte[] CheckAlt = FTXT.E.GetBytes("\x01*NIN");
 
-		public List<FDSDisk> Disks = new List<FDSDisk>();
+		public readonly IList<FDSDisk> Disks = new List<FDSDisk>();
 
 		public byte[] ExtraData;
 
@@ -63,7 +63,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 	{
 		private static readonly byte[] Check = FTXT.E.GetBytes("*NINTENDO-HVC*");
 
-		public List<FDSChunk> Chunks = new List<FDSChunk>();
+		public readonly IList<FDSChunk> Chunks = new List<FDSChunk>();
 
 		public byte[] HeaderData;
 		public byte[] ExtraData;

src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GenDbgHlp.cs

@@ -43,7 +43,7 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 		private IntPtr DllBase;
 
 		private readonly List<Symbol> SymbolsByAddr = new List<Symbol>();
-		private Dictionary<string, Symbol> SymbolsByName = new Dictionary<string, Symbol>();
+		private readonly IDictionary<string, Symbol> SymbolsByName = new Dictionary<string, Symbol>();
 
 		private readonly byte[][] data = new byte[10][];
 

src/BizHawk.Emulation.Cores/Consoles/Sony/PSX/Octoshock.cs

@@ -22,6 +22,7 @@ using Newtonsoft.Json;
 
 using BizHawk.Emulation.Common;
 using BizHawk.Common;
+using BizHawk.Common.CollectionExtensions;
 using BizHawk.Emulation.DiscSystem;
 
 #pragma warning disable 649 //adelikat: Disable dumb warnings until this file is complete
@@ -480,7 +481,7 @@ namespace BizHawk.Emulation.Cores.Sony.PSX
 		public bool CurrentTrayOpen { get; private set; }
 		public int CurrentDiscIndexMounted { get; private set; }
 
-		public List<string> HackyDiscButtons = new List<string>();
+		public readonly IList<string> HackyDiscButtons = new List<string>();
 
 		public IEmulatorServiceProvider ServiceProvider { get; private set; }
 

src/BizHawk.Emulation.Cores/Consoles/Sony/PSX/PSF.cs

@@ -8,8 +8,8 @@ namespace BizHawk.Emulation.Cores.Sony.PSX
 {
 	public class PSF
 	{
-		public Dictionary<string, string> TagsDictionary = new Dictionary<string, string>();
-		public List<string> LooseTags = new List<string>();
+		public readonly IDictionary<string, string> TagsDictionary = new Dictionary<string, string>();
+		public readonly IList<string> LooseTags = new List<string>();
 
 		public byte[] Data;
 		public byte[] LibData;

src/BizHawk.Emulation.Cores/FileID.cs

@@ -83,7 +83,7 @@ namespace BizHawk.Emulation.Cores
 		/// <summary>
 		/// extra information which could be easily gotten during the file ID (region, suspected homebrew, CRC invalid, etc.)
 		/// </summary>
-		public Dictionary<string, object> ExtraInfo = new Dictionary<string, object>();
+		public readonly IDictionary<string, object> ExtraInfo = new Dictionary<string, object>();
 	}
 
 	public class FileIDResults : List<FileIDResult>

src/BizHawk.Emulation.Cores/Libretro/LibretroCore_Description.cs

@@ -37,7 +37,7 @@ namespace BizHawk.Emulation.Cores.Libretro
 		/// <summary>
 		/// Variables defined by the core
 		/// </summary>
-		public Dictionary<string, VariableDescription> Variables = new Dictionary<string, VariableDescription>();
+		public readonly IDictionary<string, VariableDescription> Variables = new Dictionary<string, VariableDescription>();
 	}
 
 	public class VariableDescription

src/BizHawk.Emulation.DiscSystem/Disc.cs

@@ -60,7 +60,7 @@ namespace BizHawk.Emulation.DiscSystem
 		/// <summary>
 		/// Free-form optional memos about the disc
 		/// </summary>
-		public Dictionary<string, object> Memos = new Dictionary<string, object>();
+		public readonly IDictionary<string, object> Memos = new Dictionary<string, object>();
 
 		public void Dispose()
 		{
@@ -104,7 +104,7 @@ namespace BizHawk.Emulation.DiscSystem
 		/// <summary>
 		/// Disposable resources (blobs, mostly) referenced by this disc
 		/// </summary>
-		internal List<IDisposable> DisposableResources = new List<IDisposable>();
+		internal readonly IList<IDisposable> DisposableResources = new List<IDisposable>();
 
 		/// <summary>
 		/// The sectors on the disc. Don't use this directly! Use the SectorSynthProvider instead.

src/BizHawk.Emulation.DiscSystem/DiscFormats/Blobs/RiffMaster.cs

@@ -229,7 +229,7 @@ namespace BizHawk.Emulation.DiscSystem
 
 		public class RiffContainer_INFO : RiffContainer
 		{
-			public Dictionary<string, string> dictionary = new Dictionary<string, string>();
+			public readonly IDictionary<string, string> dictionary = new Dictionary<string, string>();
 			public RiffContainer_INFO() { type = "INFO"; }
 
 			/// <exception cref="FormatException"><paramref name="rc"/>.<see cref="RiffContainer.subchunks"/> contains a chunk that does not inherit <see cref="RiffSubchunk"/></exception>

src/BizHawk.Emulation.DiscSystem/DiscFormats/CCD_format.cs

@@ -39,22 +39,22 @@ namespace BizHawk.Emulation.DiscSystem
 			/// <summary>
 			/// The [Session] sections
 			/// </summary>
-			public List<CCDSession> Sessions = new List<CCDSession>();
+			public readonly IList<CCDSession> Sessions = new List<CCDSession>();
 
 			/// <summary>
 			/// The [Entry] sctions
 			/// </summary>
-			public List<CCDTocEntry> TOCEntries = new List<CCDTocEntry>();
+			public readonly IList<CCDTocEntry> TOCEntries = new List<CCDTocEntry>();
 
 			/// <summary>
 			/// The [TRACK] sections
 			/// </summary>
-			public List<CCDTrack> Tracks = new List<CCDTrack>();
+			public readonly IList<CCDTrack> Tracks = new List<CCDTrack>();
 
 			/// <summary>
 			/// The [TRACK] sections, indexed by number
 			/// </summary>
-			public Dictionary<int, CCDTrack> TracksByNumber = new Dictionary<int, CCDTrack>();
+			public readonly IDictionary<int, CCDTrack> TracksByNumber = new Dictionary<int, CCDTrack>();
 		}
 
 		/// <summary>
@@ -126,7 +126,7 @@ namespace BizHawk.Emulation.DiscSystem
 			/// <summary>
 			/// The indexes specified for the track (these are 0-indexed)
 			/// </summary>
-			public Dictionary<int, int> Indexes = new Dictionary<int, int>();
+			public readonly IDictionary<int, int> Indexes = new Dictionary<int, int>();
 		}
 
 		/// <summary>

src/BizHawk.Emulation.DiscSystem/DiscFormats/CUE/CUE_Compile.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.Collections.Generic;
+using System.Linq;
 
 using BizHawk.Common;
 
@@ -104,11 +105,11 @@ namespace BizHawk.Emulation.DiscSystem.CUE
 		public CueTrackFlags Flags = CueTrackFlags.None;
 		public CueTrackType TrackType = CueTrackType.Unknown;
 
-		public List<CompiledCueIndex> Indexes = new List<CompiledCueIndex>();
+		public readonly IList<CompiledCueIndex> Indexes = new List<CompiledCueIndex>();
 
 		public override string ToString()
 		{
-			var idx = Indexes.Find((i) => i.Number == 1);
+			var idx = Indexes.FirstOrDefault(cci => cci.Number == 1);
 			if (idx.Number != 1) return $"T#{Number:D2} NO INDEX 1";
 			var indexlist = string.Join("|", Indexes);
 			return $"T#{Number:D2} {BlobIndex}:{idx.FileMSF} ({indexlist})";

src/BizHawk.Emulation.DiscSystem/DiscFormats/M3U_file.cs

@@ -63,7 +63,7 @@ namespace BizHawk.Emulation.DiscSystem
 			return true;
 		} //Parse()
 
-		public List<Entry> Entries = new List<Entry>();
+		public readonly IList<Entry> Entries = new List<Entry>();
 
 		public void Rebase(string basepath)
 		{

src/BizHawk.Emulation.DiscSystem/DiscFormats/MDS_Format.cs

@@ -35,12 +35,12 @@ namespace BizHawk.Emulation.DiscSystem
 			/// <summary>
 			/// List of MDS session blocks
 			/// </summary>
-			public List<ASession> Sessions = new List<ASession>();
+			public readonly IList<ASession> Sessions = new List<ASession>();
 
 			/// <summary>
 			/// List of track blocks
 			/// </summary>
-			public List<ATrack> Tracks = new List<ATrack>();
+			public readonly IList<ATrack> Tracks = new List<ATrack>();
 
 			/// <summary>
 			/// Current parsed session objects
@@ -50,7 +50,7 @@ namespace BizHawk.Emulation.DiscSystem
 			/// <summary>
 			/// Calculated MDS TOC entries (still to be parsed into BizHawk)
 			/// </summary>
-			public List<ATOCEntry> TOCEntries = new List<ATOCEntry>();
+			public readonly IList<ATOCEntry> TOCEntries = new List<ATOCEntry>();
 			
 		}
 

src/BizHawk.Emulation.DiscSystem/DiscFormats/SBI_format.cs

@@ -19,7 +19,7 @@ namespace BizHawk.Emulation.DiscSystem.SBI
 		/// <summary>
 		/// a list of patched ABAs
 		/// </summary>
-		public List<int> ABAs = new List<int>();
+		public readonly IList<int> ABAs = new List<int>();
 
 		/// <summary>
 		/// 12 values (Q subchannel data) for every patched ABA; -1 means unpatched

src/BizHawk.Emulation.DiscSystem/DiscStructure.cs

@@ -13,7 +13,7 @@ namespace BizHawk.Emulation.DiscSystem
 		/// This is a 1-indexed list of sessions (session 1 is at [1])
 		/// Support for multiple sessions is thoroughly not working yet
 		/// </summary>
-		public List<Session> Sessions = new List<Session>();
+		public readonly IList<Session> Sessions = new List<Session>();
 
 		public class Session
 		{
@@ -43,7 +43,7 @@ namespace BizHawk.Emulation.DiscSystem
 			/// For a disc with "3 tracks", Tracks.Count will be 5: it includes that lead-in track as well as the leadout track.
 			/// Perhaps we should turn this into a special collection type with no Count or Length, or a method to GetTrack()
 			/// </summary>
-			public List<Track> Tracks = new List<Track>();
+			public readonly IList<Track> Tracks = new List<Track>();
 
 			/// <summary>
 			/// A reference to the first information track (Track 1)