Fix NullReferenceException in TypeSystemAstBuilder when attribute argument decoding fails.

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -1288,7 +1288,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(attribute);
 			attribute.Type.AcceptVisitor(this);
-			if (attribute.Arguments.Count != 0 || !attribute.GetChildByRole(Roles.LPar).IsNull) {
+			if (attribute.Arguments.Count != 0 || attribute.HasArgumentList) {
 				Space(policy.SpaceBeforeMethodCallParentheses);
 				WriteCommaSeparatedListInParenthesis(attribute.Arguments, policy.SpaceWithinMethodCallParentheses);
 			}

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -471,9 +471,16 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				mt.MemberName = mt.MemberName.Substring(0, mt.MemberName.Length - 9);
 			}
 			var parameters = attribute.Constructor?.Parameters ?? EmptyList<IParameter>.Instance;
-			foreach (var (arg, p) in attribute.FixedArguments.ZipLongest(parameters)) {
+			for (int i = 0; i < attribute.FixedArguments.Length; i++) {
+				var arg = attribute.FixedArguments[i];
+				var p = (i < parameters.Count) ? parameters[i] : null;
 				attr.Arguments.Add(ConvertConstantValue(p?.Type ?? arg.Type, arg.Type, arg.Value));
 			}
+			if (attribute.HasDecodeErrors) {
+				attr.HasArgumentList = true;
+				attr.AddChild(new Comment("Could not decode attribute arguments.", CommentType.MultiLine), Roles.Comment);
+				attr.AddChild(new CSharpTokenNode(TextLocation.Empty, Roles.RPar), Roles.RPar);
+			}
 			if (attribute.NamedArguments.Length > 0) {
 				InitializedObjectResolveResult targetResult = new InitializedObjectResolveResult(attribute.AttributeType);
 				foreach (var namedArg in attribute.NamedArguments) {

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -274,7 +274,6 @@
     <Compile Include="Documentation\GetPotentiallyNestedClassTypeReference.cs" />
     <Compile Include="Documentation\IdStringMemberReference.cs" />
     <Compile Include="Documentation\IdStringProvider.cs" />
-    <Compile Include="Documentation\XmlDocKeyProvider.cs" />
     <Compile Include="Documentation\XmlDocLoader.cs" />
     <Compile Include="Documentation\XmlDocumentationProvider.cs" />
     <Compile Include="IL\ControlFlow\AsyncAwaitDecompiler.cs" />

ICSharpCode.Decompiler/TypeSystem/IAttribute.cs

@@ -40,6 +40,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		IMethod Constructor { get; }
 
+		/// <summary>
+		/// Gets whether there were errors decoding the attribute.
+		/// </summary>
+		bool HasDecodeErrors { get; }
+
 		/// <summary>
 		/// Gets the positional arguments.
 		/// </summary>

ICSharpCode.Decompiler/TypeSystem/Implementation/CustomAttribute.cs

@@ -38,6 +38,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		// lazy-loaded:
 		CustomAttributeValue<IType> value;
 		bool valueDecoded;
+		bool hasDecodeErrors;
 
 		internal CustomAttribute(MetadataModule module, IMethod attrCtor, CustomAttributeHandle handle)
 		{
@@ -65,6 +66,13 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 		}
 
+		public bool HasDecodeErrors {
+			get {
+				DecodeValue();
+				return hasDecodeErrors;
+			}
+		}
+
 		void DecodeValue()
 		{
 			lock (this) {
@@ -80,6 +88,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 						ImmutableArray<CustomAttributeTypedArgument<IType>>.Empty,
 						ImmutableArray<CustomAttributeNamedArgument<IType>>.Empty
 					);
+					hasDecodeErrors = true;
 					valueDecoded = true; // in case of errors, never try again.
 				}
 			}

ICSharpCode.Decompiler/TypeSystem/Implementation/DefaultAttribute.cs

@@ -67,6 +67,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			get { return attributeType; }
 		}
 
+		bool IAttribute.HasDecodeErrors => false;
+
 		public IMethod Constructor {
 			get {
 				IMethod ctor = this.constructor;