Fix #1785: Only use System.MathF, if it is available in the used framework.

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1044,9 +1044,14 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				if (isDouble)
 					mathType = compilation.FindType(typeof(Math));
 				else {
-					mathType = compilation.FindType(new TopLevelTypeName("System", "MathF")).GetDefinition();
-					if (mathType == null || !mathType.GetFields(f => f.Name == "PI" && f.IsConst).Any() || !mathType.GetFields(f => f.Name == "E" && f.IsConst).Any())
+					mathType = compilation.FindType(new TopLevelTypeName("System", "MathF"));
+					var typeDef = mathType.GetDefinition();
+					if (typeDef == null
+						|| !typeDef.IsDirectImportOf(compilation.MainModule)
+						|| !typeDef.GetFields(f => f.Name == "PI" && f.IsConst).Any() || !typeDef.GetFields(f => f.Name == "E" && f.IsConst).Any())
+					{
 						mathType = compilation.FindType(typeof(Math));
+					}
 				}
 
 				expr = TryExtractExpression(mathType, type, constantValue, "PI", isDouble)

ICSharpCode.Decompiler/TypeSystem/TypeSystemExtensions.cs

@@ -20,6 +20,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
@@ -540,5 +541,30 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		{
 			return type.ChangeNullability(Nullability.Oblivious);
 		}
+
+		public static bool IsDirectImportOf(this ITypeDefinition type, IModule module)
+		{
+			var moduleReference = type.ParentModule;
+			foreach (var asmRef in module.PEFile.AssemblyReferences) {
+				if (asmRef.FullName == moduleReference.FullAssemblyName)
+					return true;
+				if (asmRef.Name == "netstandard" && asmRef.GetPublicKeyToken() != null) {
+					var referencedModule = module.Compilation.FindModuleByReference(asmRef);
+					if (referencedModule != null && !referencedModule.PEFile.GetTypeForwarder(type.FullTypeName).IsNil)
+						return true;
+				}
+			}
+			return false;
+		}
+
+		public static IModule FindModuleByReference(this ICompilation compilation, IAssemblyReference assemblyName)
+		{
+			foreach (var module in compilation.Modules) {
+				if (module.FullAssemblyName == assemblyName.FullName) {
+					return module;
+				}
+			}
+			return null;
+		}
 	}
 }