fix issue 490

src/FastExpressionCompiler/FastExpressionCompiler.cs

@@ -4368,7 +4368,8 @@ private static bool TryEmitArithmeticAndOrAssign(
                                 // required for calling the method on the value type parameter
                                 var objType = objExpr.Type;
                                 objVarByAddress = !closure.LastEmitIsAddress && objType.IsValueType && // todo: @wip avoid ad-hocking with parameter here
-                                    (objExpr.NodeType != ExpressionType.Parameter || !((ParameterExpression)objExpr).IsByRef);
+                                    (objExpr.NodeType != ExpressionType.Parameter || !((ParameterExpression)objExpr).IsByRef) &&
+                                    !objType.IsByRefLike(); // ref struct types cannot be stored in local variables
                                 if (objVarByAddress)
                                     objVar = EmitStoreAndLoadLocalVariableAddress(il, objType);
                                 else
@@ -4938,7 +4939,7 @@ private static bool TryEmitMethodCall(Expression expr,
                     if (!TryEmit(objExpr, paramExprs, il, ref closure, setup, flags | ParentFlags.InstanceAccess))
                         return false;
                     objIsValueType = objExpr.Type.IsValueType;
-                    loadObjByAddress = objIsValueType && objExpr.NodeType != ExpressionType.Parameter && !closure.LastEmitIsAddress;
+                    loadObjByAddress = objIsValueType && objExpr.NodeType != ExpressionType.Parameter && !closure.LastEmitIsAddress && !objExpr.Type.IsByRefLike();
                 }
 
                 var parCount = methodParams.Length;
@@ -5047,7 +5048,10 @@ public static bool TryEmitMemberGet(MemberExpression expr,
                             // Value type special treatment to load address of value instance in order to call a method.
                             // For the parameters, we will skip the address loading because the `LastEmitIsAddress == true` for `Ldarga`, 
                             // so the condition here will be skipped
-                            if (!closure.LastEmitIsAddress && objExpr.Type.IsValueType)
+                            // Ref struct types cannot be stored in local variables, so we skip them here
+                            if (!closure.LastEmitIsAddress &&
+                            objExpr.Type.IsValueType &&
+                            !objExpr.Type.IsByRefLike())
                             EmitStoreAndLoadLocalVariableAddress(il, objExpr.Type);
                     }
 
@@ -5094,7 +5098,7 @@ public static bool TryEmitMemberGet(MemberExpression expr,
                         closure.LastEmitIsAddress = isByAddress;
                         if (!isByAddress)
                         {
-                            if (objExpr.Type.IsEnum)
+                            if (objExpr.Type.IsEnum && !objExpr.Type.IsByRefLike())
                                 EmitStoreAndLoadLocalVariableAddress(il, objExpr.Type);
                             il.Demit(OpCodes.Ldfld, field);
                         }
@@ -7816,6 +7820,33 @@ internal static bool IsFloatingPoint(this Type type) =>
             type == typeof(float) ||
             type == typeof(double);
 
+        /// <summary>Checks if the type is a ref struct (byref-like type). 
+        /// Compatible with older frameworks that don't have Type.IsByRefLike property.</summary>
+        [MethodImpl((MethodImplOptions)256)]
+        [RequiresUnreferencedCode(Trimming.Message)]
+        internal static bool IsByRefLike(this Type type)
+        {
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER || NET6_0_OR_GREATER
+            return type.IsByRefLike;
+#else
+            // For older frameworks (net472, netstandard2.0), check for IsByRefLikeAttribute via reflection
+            // IsByRefLike property was introduced in .NET Standard 2.1 / .NET Core 2.1
+            if (!type.IsValueType)
+                return false;
+
+            // Check if the type has IsByRefLikeAttribute
+            var attributes = type.GetCustomAttributes(inherit: false);
+            foreach (var attr in attributes)
+            {
+                var attrType = attr.GetType();
+                if (attrType.Name == "IsByRefLikeAttribute" || 
+                    attrType.FullName == "System.Runtime.CompilerServices.IsByRefLikeAttribute")
+                    return true;
+            }
+            return false;
+#endif
+        }
+
         internal static bool IsPrimitiveWithZeroDefaultExceptDecimal(this Type type)
         {
             switch (Type.GetTypeCode(type))

test/FastExpressionCompiler.IssueTests/Issue490_ref_struct_conditional.cs

@@ -0,0 +1,82 @@
+using System;
+
+#if LIGHT_EXPRESSION
+using static FastExpressionCompiler.LightExpression.Expression;
+namespace FastExpressionCompiler.LightExpression.IssueTests;
+#else
+using System.Linq.Expressions;
+using static System.Linq.Expressions.Expression;
+namespace FastExpressionCompiler.IssueTests;
+#endif
+
+public struct Issue490_ref_struct_conditional : ITest, ITestX
+{
+    public int Run()
+    {
+        Return_true_when_token_is_null();
+        Return_false_when_token_isnot_null();
+        return 1;
+    }
+
+    public void Run(TestRun t)
+    {
+        Return_true_when_token_is_null(t);
+        Return_false_when_token_isnot_null(t);
+    }
+
+    public delegate bool RefStructReaderDelegate(ref MyJsonReader reader);
+
+    public enum MyJsonTokenType : byte
+    {
+        None = 0,
+        StartObject = 1,
+        Null = 11
+    }
+
+    public ref struct MyJsonReader
+    {
+        public MyJsonTokenType TokenType { get; set; }
+    }
+
+    public void Return_true_when_token_is_null(TestContext t = default)
+    {
+        var readerParam = Parameter(typeof(MyJsonReader).MakeByRefType(), "reader");
+        var tokenType = Property(readerParam, nameof(MyJsonReader.TokenType));
+        var nullToken = Constant(MyJsonTokenType.Null);
+
+        var body = Condition(
+            Equal(tokenType, nullToken),
+            Constant(true),
+            Constant(false));
+
+        var lambda = Lambda<RefStructReaderDelegate>(body, readerParam);
+        var func = lambda.CompileFast();
+
+        var reader = new MyJsonReader();
+        reader.TokenType = MyJsonTokenType.Null;
+
+        var result = func(ref reader);
+        t.AreEqual(true, result);
+    }
+
+    public void Return_false_when_token_isnot_null(TestContext t = default)
+    {
+        var readerParam = Parameter(typeof(MyJsonReader).MakeByRefType(), "reader");
+        var tokenType = Property(readerParam, nameof(MyJsonReader.TokenType));
+        var nullToken = Constant(MyJsonTokenType.Null);
+
+        var body = Condition(
+            Equal(tokenType, nullToken),
+            Constant(true),
+            Constant(false));
+
+        var lambda = Lambda<RefStructReaderDelegate>(body, readerParam);
+        var func = lambda.CompileFast();
+
+        var reader = new MyJsonReader();
+        reader.TokenType = MyJsonTokenType.StartObject;
+
+        var result = func(ref reader);
+        t.AreEqual(false, result);
+    }
+}

test/FastExpressionCompiler.TestsRunner.Net472/Program.cs

@@ -404,6 +404,9 @@ void Run(Func<int> run, string name = null)
                 Run(new Issue461_InvalidProgramException_when_null_checking_type_by_ref().Run);
                 Run(new LightExpression.IssueTests.Issue461_InvalidProgramException_when_null_checking_type_by_ref().Run);
 
+                Run(new Issue490_ref_struct_conditional().Run);
+                Run(new LightExpression.IssueTests.Issue490_ref_struct_conditional().Run);
+
                 Console.WriteLine($"{Environment.NewLine}IssueTests are passing in {sw.ElapsedMilliseconds} ms.");
             });
 

test/FastExpressionCompiler.TestsRunner/Program.cs

@@ -389,6 +389,9 @@ void Run(Func<int> run, string name = null)
                 Run(new Issue461_InvalidProgramException_when_null_checking_type_by_ref().Run);
                 Run(new LightExpression.IssueTests.Issue461_InvalidProgramException_when_null_checking_type_by_ref().Run);
 
+                Run(new Issue490_ref_struct_conditional().Run);
+                Run(new LightExpression.IssueTests.Issue490_ref_struct_conditional().Run);
+
                 Console.WriteLine($"{Environment.NewLine}IssueTests are passing in {sw.ElapsedMilliseconds} ms.");
             });