Fixed thread safety issues as per code review comments.

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -362,6 +362,7 @@
     <Compile Include="TypeSystem\Implementation\MetadataTypeDefinition.cs" />
     <Compile Include="TypeSystem\Implementation\MetadataTypeParameter.cs" />
     <Compile Include="TypeSystem\Implementation\SpecializedParameter.cs" />
+    <Compile Include="TypeSystem\Implementation\ThreeState.cs" />
     <Compile Include="TypeSystem\MetadataModule.cs" />
     <Compile Include="TypeSystem\ModifiedType.cs" />
     <Compile Include="TypeSystem\Implementation\PinnedType.cs" />

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataField.cs

@@ -42,7 +42,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		object constantValue;
 		IType type;
 		bool isVolatile; // initialized together with this.type
-		bool? isDecimalConstant;
+		// this can't be bool? as bool? is not thread-safe from torn reads
+		byte decimalConstantState;
 
 		internal MetadataField(MetadataModule module, FieldDefinitionHandle handle)
 		{
@@ -53,9 +54,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			var def = module.metadata.GetFieldDefinition(handle);
 			this.attributes = def.Attributes;
 
-			if ((attributes & (FieldAttributes.Static | FieldAttributes.InitOnly)) != (FieldAttributes.Static | FieldAttributes.InitOnly)) {
-				isDecimalConstant = false;
-			}
+			if ((attributes & (FieldAttributes.Static | FieldAttributes.InitOnly)) != (FieldAttributes.Static | FieldAttributes.InitOnly))
+				decimalConstantState = ThreeState.False;
 		}
 
 		public EntityHandle MetadataToken => handle;
@@ -193,11 +193,11 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		bool IsDecimalConstant {
 			get {
-				if (isDecimalConstant == null) {
+				if (decimalConstantState == ThreeState.Unknown) {
 					var fieldDef = module.metadata.GetFieldDefinition(handle);
-					isDecimalConstant = DecimalConstantHelper.IsDecimalConstant(module, fieldDef.GetCustomAttributes());
+					decimalConstantState = ThreeState.From(DecimalConstantHelper.IsDecimalConstant(module, fieldDef.GetCustomAttributes()));
 				}
-				return (bool)isDecimalConstant;
+				return decimalConstantState == ThreeState.True;
 			}
 		}
 

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataParameter.cs

@@ -37,8 +37,9 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		// lazy-loaded:
 		string name;
-		bool? hasConstantValueInSignature;
-		bool? isDecimalConstant;
+		// these can't be bool? as bool? is not thread-safe from torn reads
+		byte constantValueInSignatureState;
+		byte decimalConstantState;
 
 		internal MetadataParameter(MetadataModule module, IParameterizedMember owner, IType type, ParameterHandle handle)
 		{
@@ -50,7 +51,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			var param = module.metadata.GetParameter(handle);
 			this.attributes = param.Attributes;
 			if (!IsOptional)
-				isDecimalConstant = false; // only optional parameters can be constants
+				decimalConstantState = ThreeState.False; // only optional parameters can be constants
 		}
 
 		public EntityHandle MetadataToken => handle;
@@ -125,25 +126,25 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public bool HasConstantValueInSignature {
 			get {
-				if (hasConstantValueInSignature == null) {
+				if (constantValueInSignatureState == ThreeState.Unknown) {
 					if (IsDecimalConstant) {
-						hasConstantValueInSignature = DecimalConstantHelper.AllowsDecimalConstants(module);
+						constantValueInSignatureState = ThreeState.From(DecimalConstantHelper.AllowsDecimalConstants(module));
 					}
 					else {
-						hasConstantValueInSignature = !module.metadata.GetParameter(handle).GetDefaultValue().IsNil;
+						constantValueInSignatureState = ThreeState.From(!module.metadata.GetParameter(handle).GetDefaultValue().IsNil);
 					}
 				}
-				return (bool)hasConstantValueInSignature;
+				return constantValueInSignatureState == ThreeState.True;
 			}
 		}
 
 		bool IsDecimalConstant {
 			get {
-				if (isDecimalConstant == null) {
+				if (decimalConstantState == ThreeState.Unknown) {
 					var parameterDef = module.metadata.GetParameter(handle);
-					isDecimalConstant = DecimalConstantHelper.IsDecimalConstant(module, parameterDef.GetCustomAttributes());
+					decimalConstantState = ThreeState.From(DecimalConstantHelper.IsDecimalConstant(module, parameterDef.GetCustomAttributes()));
 				}
-				return (bool)isDecimalConstant;
+				return decimalConstantState == ThreeState.True;
 			}
 		}
 

ICSharpCode.Decompiler/TypeSystem/Implementation/ThreeState.cs

@@ -0,0 +1,33 @@
+// Copyright (c) 2018 Daniel Grunwald
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+namespace ICSharpCode.Decompiler.TypeSystem.Implementation
+{
+	/// <summary>
+	/// Constants used instead of <see cref="bool?"/> 
+	/// in multithreaded code, as <c>bool?</c> might produce torn reads.
+	/// </summary>
+	static class ThreeState
+	{
+		public const byte Unknown = 0;
+		public const byte False = 1;
+		public const byte True = 2;
+
+		public static byte From(bool value) => value ? True : False;
+	}
+}