fix regression in init only fields

Author: lofcz

FastCloner.Tests/SpecialCaseTests.cs

@@ -3157,4 +3157,26 @@ public void LargeCircular_Test()
         Assert.That(clonedNode.Data, Has.Count.EqualTo(2));
         Assert.That(originalNode.Data[0], Is.Not.EqualTo(clonedNode.Data[1]));
     }
+    
+    [Test]
+    public void SelfReferenced_WithInitOnlyField_Test()
+    {
+    	SelfReferencedWithInitOnlyField original = new SelfReferencedWithInitOnlyField
+    	{
+    		WithReadOnlyField = new ClassWithReadOnlyField()
+    	};
+    
+    	SelfReferencedWithInitOnlyField clone = original.DeepClone();
+    	
+    	Assert.That(clone, Is.Not.SameAs(original));
+    	Assert.That(clone.WithReadOnlyField, Is.Not.SameAs(original.WithReadOnlyField));
+    	Assert.That(clone.WithReadOnlyField.ReadOnlyValue, Is.EqualTo(original.WithReadOnlyField.ReadOnlyValue));
+    }
+    
+    private class SelfReferencedWithInitOnlyField
+    {
+    	public SelfReferencedWithInitOnlyField? Predecessor { get; set; }
+    
+    	public ClassWithReadOnlyField WithReadOnlyField { get; set; }
+    }
 }

FastCloner/Code/ClonerToExprGenerator.cs

@@ -91,7 +91,22 @@ private static object GenerateProcessMethod(Type type, bool isDeepClone)
             }
             else
             {
-                expressionList.Add(Expression.Assign(Expression.Field(toLocal, fieldInfo), Expression.Field(fromLocal, fieldInfo)));
+                Expression sourceValue = Expression.Field(fromLocal, fieldInfo);
+                if (fieldInfo.IsInitOnly)
+                {
+                    ConstantExpression setter = Expression.Constant(FieldAccessorGenerator.GetFieldSetter(fieldInfo));
+                    expressionList.Add(
+                        Expression.Invoke(
+                            setter,
+                            Expression.Convert(toLocal, typeof(object)),
+                            Expression.Convert(sourceValue, typeof(object))
+                        )
+                    );
+                }
+                else
+                {
+                    expressionList.Add(Expression.Assign(Expression.Field(toLocal, fieldInfo), sourceValue));
+                }
             }
         }
 

FastCloner/Code/FieldAccessorGenerator.cs

@@ -18,10 +18,20 @@ private static Action<object, object> CreateFieldSetter(FieldInfo field)
 
         UnaryExpression targetCast = Expression.Convert(targetParam, field.DeclaringType);
         UnaryExpression valueCast = Expression.Convert(valueParam, field.FieldType);
-        BinaryExpression assign = Expression.Assign(Expression.Field(targetCast, field), valueCast);
+        Expression body;
+        
+        if (field.IsInitOnly)
+        {
+            MethodInfo setValueMethod = typeof(FieldInfo).GetMethod(nameof(FieldInfo.SetValue), [typeof(object), typeof(object)])!;
+            body = Expression.Call(Expression.Constant(field), setValueMethod, targetCast, valueCast);
+        }
+        else
+        {
+            body = Expression.Assign(Expression.Field(targetCast, field), valueCast);
+        }
 
         Expression<Action<object, object>> lambda = Expression.Lambda<Action<object, object>>(
-            assign,
+            body,
             targetParam,
             valueParam
         );