diff --git a/src/NSubstitute/NSubstitute.csproj b/src/NSubstitute/NSubstitute.csproj
index 5f4111434..8b6dd3fd7 100644
--- a/src/NSubstitute/NSubstitute.csproj
+++ b/src/NSubstitute/NSubstitute.csproj
@@ -36,7 +36,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Castle.Core" Version="4.3.0-*" />
+    <PackageReference Include="Castle.Core" Version="4.3.1-*" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.3.0-*" />
   </ItemGroup>
   
diff --git a/src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs b/src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs
index b7c66a837..7dc3e3856 100644
--- a/src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs
+++ b/src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs
@@ -46,12 +46,19 @@ public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] add
         }
 
         /// <summary>
-        /// Allows to dynamically create a type in runtime. Returns an instance of <see cref="TypeBuilder"/>,
-        /// so type could be customized and built later.
+        /// Allows to dynamically create a type in runtime.
+        /// Use the <paramref name="typeBuildCallback"/> callback to define and build the type.
         /// </summary>
-        public TypeBuilder DefineDynamicType(string typeName, TypeAttributes flags)
+        /// <param name="typeBuildCallback">Callback used to construct the type.</param>
+        /// <returns>The result returned by <paramref name="typeBuildCallback"/> callback.</returns>
+        public Type DefineDynamicType(Func<ModuleBuilder, Type> typeBuildCallback)
         {
-            return _proxyGenerator.ProxyBuilder.ModuleScope.DefineType(true, typeName, flags);
+            var moduleBuilder = _proxyGenerator.ProxyBuilder.ModuleScope.ObtainDynamicModuleWithStrongName();
+
+            using (_proxyGenerator.ProxyBuilder.ModuleScope.Lock.ForWriting())
+            {
+                return typeBuildCallback.Invoke(moduleBuilder);
+            }
         }
 
         private object CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[] additionalInterfaces,
diff --git a/src/NSubstitute/Proxies/DelegateProxy/DelegateProxyFactory.cs b/src/NSubstitute/Proxies/DelegateProxy/DelegateProxyFactory.cs
index b4931bfea..03ebbc9e7 100644
--- a/src/NSubstitute/Proxies/DelegateProxy/DelegateProxyFactory.cs
+++ b/src/NSubstitute/Proxies/DelegateProxy/DelegateProxyFactory.cs
@@ -14,6 +14,7 @@ namespace NSubstitute.Proxies.DelegateProxy
     public class DelegateProxyFactory : IProxyFactory
     {
         private const string MethodNameInsideProxyContainer = "Invoke";
+        private const string IsReadOnlyAttributeFullTypeName = "System.Runtime.CompilerServices.IsReadOnlyAttribute";
         private readonly CastleDynamicProxyFactory _castleObjectProxyFactory;
         private readonly ConcurrentDictionary<Type, Type> _delegateContainerCache = new ConcurrentDictionary<Type, Type>();
         private long _typeSuffixCounter;
@@ -39,7 +40,7 @@ public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] add
             return DelegateProxy(typeToProxy, callRouter);
         }
 
-        private bool HasItems<T>(T[] array)
+        private static bool HasItems<T>(T[] array)
         {
             return array != null && array.Length > 0;
         }
@@ -67,31 +68,95 @@ private Type GenerateDelegateContainerInterface(Type delegateType)
                 delegateTypeName,
                 typeSuffixCounter.ToString(CultureInfo.InvariantCulture));
 
-            var typeBuilder = _castleObjectProxyFactory.DefineDynamicType(
-                typeName,
-                TypeAttributes.Abstract | TypeAttributes.Interface | TypeAttributes.Public);
+            return _castleObjectProxyFactory.DefineDynamicType(moduleBuilder =>
+            {
+                var typeBuilder = moduleBuilder.DefineType(
+                    typeName,
+                    TypeAttributes.Abstract | TypeAttributes.Interface | TypeAttributes.Public);
+
+                // Notice, we don't copy the custom modifiers here.
+                // That's absolutely fine, as custom modifiers are ignored when delegate is constructed.
+                // See the related discussion here: https://github.com/dotnet/coreclr/issues/18401
+                var methodBuilder = typeBuilder
+                    .DefineMethod(
+                        MethodNameInsideProxyContainer,
+                        MethodAttributes.Abstract | MethodAttributes.Virtual | MethodAttributes.Public,
+                        CallingConventions.Standard,
+                        delegateSignature.ReturnType,
+                        delegateSignature.GetParameters().Select(p => p.ParameterType).ToArray());
+
+                // Copy original method attributes, so "out" parameters are recognized later.
+                for (var i = 0; i < delegateParameters.Length; i++)
+                {
+                    var parameter = delegateParameters[i];
+
+                    // Increment position by 1 to skip the implicit "this" parameter.
+                    var paramBuilder = methodBuilder.DefineParameter(i + 1, parameter.Attributes, parameter.Name);
+
+                    // Read-only parameter ('in' keyword) is recognized by presence of the special attribute.
+                    // If source parameter contained that attribute, ensure to copy it to the generated method.
+                    // That helps Castle to understand that parameter is read-only and cannot be mutated.
+                    DefineIsReadOnlyAttributeIfNeeded(parameter, paramBuilder, moduleBuilder);
+                }
+
+                // Preserve the original delegate type in attribute, so it can be retrieved later in code.
+                methodBuilder.SetCustomAttribute(
+                    new CustomAttributeBuilder(
+                        typeof(ProxiedDelegateTypeAttribute).GetConstructors().Single(),
+                        new object[] {delegateType}));
+
+                return typeBuilder.CreateTypeInfo().AsType();
+            });
+        }
+
+        private static void DefineIsReadOnlyAttributeIfNeeded(
+            ParameterInfo sourceParameter, ParameterBuilder paramBuilder, ModuleBuilder dynamicModuleBuilder)
+        {
+            // Read-only parameter can be by-ref only.
+            if (!sourceParameter.ParameterType.IsByRef)
+            {
+                return;
+            }
+
+            // Lookup for the attribute using full type name.
+            // That's required because compiler can embed that type directly to the client's assembly
+            // as type identity doesn't matter - only full type attribute name is checked.
+            var isReadOnlyAttrType = sourceParameter.CustomAttributes
+                .Select(ca => ca.AttributeType)
+                .FirstOrDefault(t => t.FullName.Equals(IsReadOnlyAttributeFullTypeName, StringComparison.Ordinal));
 
-            var methodBuilder = typeBuilder
-                .DefineMethod(
-                    MethodNameInsideProxyContainer,
-                    MethodAttributes.Abstract | MethodAttributes.Virtual | MethodAttributes.Public,
-                    delegateSignature.ReturnType,
-                    delegateParameters.Select(p => p.ParameterType).ToArray());
+            // Parameter doesn't contain the IsReadOnly attribute.
+            if (isReadOnlyAttrType == null)
+            {
+                return;
+            }
 
-            // Copy original method attributes, so "out" parameters are recognized later.
-            for (var i = 0; i < delegateParameters.Length; i++)
+            // If the compiler generated attribute is used (e.g. runtime doesn't contain the attribute),
+            // the generated attribute type might be internal, so we cannot referecnce it in the dynamic assembly.
+            // In this case use the attribute type from the dynamic assembly.
+            if (!isReadOnlyAttrType.GetTypeInfo().IsVisible)
             {
-                // Increment position by 1 to skip the implicit "this" parameter.
-                methodBuilder.DefineParameter(i + 1, delegateParameters[i].Attributes, delegateParameters[i].Name);
+                isReadOnlyAttrType = GetIsReadOnlyAttributeInDynamicModule(dynamicModuleBuilder);
             }
 
-            // Preserve the original delegate type in attribute, so it can be retrieved later in code.
-            methodBuilder.SetCustomAttribute(
-                new CustomAttributeBuilder(
-                    typeof(ProxiedDelegateTypeAttribute).GetConstructors().Single(),
-                    new object[] {delegateType}));
+            paramBuilder.SetCustomAttribute(
+                new CustomAttributeBuilder(isReadOnlyAttrType.GetConstructor(Type.EmptyTypes), new object[0]));
+        }
+
+        private static Type GetIsReadOnlyAttributeInDynamicModule(ModuleBuilder moduleBuilder)
+        {
+            var existingType = moduleBuilder.Assembly.GetType(IsReadOnlyAttributeFullTypeName, throwOnError: false, ignoreCase: false);
+            if (existingType != null)
+            {
+                return existingType;
+            }
 
-            return typeBuilder.CreateTypeInfo().AsType();
+            return moduleBuilder
+                .DefineType(
+                    IsReadOnlyAttributeFullTypeName,
+                    TypeAttributes.Class | TypeAttributes.Sealed | TypeAttributes.NotPublic,
+                    typeof(Attribute))
+                .CreateTypeInfo().AsType();
         }
     }
 }
\ No newline at end of file
diff --git a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue378_InValueTypes.cs b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue378_InValueTypes.cs
index 3c4905e91..33e78aa1a 100644
--- a/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue378_InValueTypes.cs
+++ b/tests/NSubstitute.Acceptance.Specs/FieldReports/Issue378_InValueTypes.cs
@@ -1,5 +1,4 @@
-using System;
-using NUnit.Framework;
+using NUnit.Framework;
 
 namespace NSubstitute.Acceptance.Specs.FieldReports
 {
@@ -8,22 +7,91 @@ namespace NSubstitute.Acceptance.Specs.FieldReports
     /// </summary>
     public class Issue378_InValueTypes
     {
-        public readonly struct Struct { }
+        public readonly struct Struct
+        {
+            public Struct(int value)
+            {
+                Value = value;
+            }
+
+            public int Value { get; }
+        }
+
+        public interface IStructByReadOnlyRefConsumer { void Consume(in Struct value); }
+
+        public interface IStructByValueConsumer { void Consume(Struct value); }
 
-        public interface IStructByRefConsumer { void Consume(in Struct message); }
+        public delegate void DelegateStructByReadOnlyRefConsumer(in Struct value);
 
-        public interface IStructByValueConsumer { void Consume(Struct message); }
+        public delegate void DelegateStructByReadOnlyRefConsumerMultipleArgs(in Struct value1, in Struct value2);
 
         [Test]
-        public void IStructByRefConsumer_Test()
+        public void IStructByReadOnlyRefConsumer_Test()
         {
-            _ = Substitute.For<IStructByRefConsumer>();
+            var value = new Struct(42);
+
+            var subs = Substitute.For<IStructByReadOnlyRefConsumer>();
+            subs.Consume(in value);
         }
 
         [Test]
         public void IStructByValueConsumer_Test()
         {
-            _ = Substitute.For<IStructByValueConsumer>();
+            var value = new Struct(42);
+
+            var subs = Substitute.For<IStructByValueConsumer>();
+            subs.Consume(value);
+        }
+
+        [Test]
+        public void DelegateByReadOnlyRefConsumer_Test()
+        {
+            var value = new Struct(42);
+
+            var subs = Substitute.For<DelegateStructByReadOnlyRefConsumer>();
+            subs.Invoke(in value);
+        }
+
+        [Test]
+        public void InterfaceReadOnlyRefCannotBeModified()
+        {
+            var readOnlyValue = new Struct(42);
+
+            var subs = Substitute.For<IStructByReadOnlyRefConsumer>();
+            subs.When(x => x.Consume(Arg.Any<Struct>())).Do(c => { c[0] = new Struct(24); });
+
+            subs.Consume(in readOnlyValue);
+
+            Assert.That(readOnlyValue.Value, Is.EqualTo(42));
+        }
+
+        [Test]
+        public void DelegateReadOnlyRefCannotBeModified()
+        {
+            var readOnlyValue = new Struct(42);
+
+            var subs = Substitute.For<DelegateStructByReadOnlyRefConsumer>();
+            subs.When(x => x.Invoke(Arg.Any<Struct>())).Do(c => { c[0] = new Struct(24); });
+
+            subs.Invoke(in readOnlyValue);
+
+            Assert.That(readOnlyValue.Value, Is.EqualTo(42));
+        }
+
+        [Test]
+        public void DelegateMultipleReadOnlyRefCannotBeModified()
+        {
+            var readOnlyValue1 = new Struct(42);
+            var readOnlyValue2 = new Struct(42);
+
+            var subs = Substitute.For<DelegateStructByReadOnlyRefConsumerMultipleArgs>();
+            subs.When(x => x.Invoke(Arg.Any<Struct>(), Arg.Any<Struct>()))
+                .Do(c => { c[0] = new Struct(24); c[1] = new Struct(24); });
+
+            subs.Invoke(in readOnlyValue1, in readOnlyValue2);
+
+            Assert.That(readOnlyValue1.Value, Is.EqualTo(42));
+            Assert.That(readOnlyValue2.Value, Is.EqualTo(42));
         }
     }
 }
\ No newline at end of file