diff --git a/ArchUnitNET/ArchUnitNET.csproj b/ArchUnitNET/ArchUnitNET.csproj
index 0917fdcbc..c800c2a19 100644
--- a/ArchUnitNET/ArchUnitNET.csproj
+++ b/ArchUnitNET/ArchUnitNET.csproj
@@ -20,6 +20,7 @@
         <PackageReference Include="CycleDetection" Version="2.0.0" />
         <PackageReference Include="JetBrains.Annotations" Version="2020.3.0" />
         <PackageReference Include="Mono.Cecil" Version="0.11.3" />
+        <PackageReference Include="System.ValueTuple" Version="4.5.0" />
     </ItemGroup>
     
     <ItemGroup>
diff --git a/ArchUnitNET/Domain/Class.cs b/ArchUnitNET/Domain/Class.cs
index 823cac8ec..7ef4e08eb 100644
--- a/ArchUnitNET/Domain/Class.cs
+++ b/ArchUnitNET/Domain/Class.cs
@@ -60,7 +60,7 @@ public bool? IsStruct
 
         public IEnumerable<Class> InheritedClasses => BaseClass == null
             ? Enumerable.Empty<Class>()
-            : BaseClass.InheritedClasses.Append(BaseClass);
+            : BaseClass.InheritedClasses.Concat(new[] {BaseClass});
 
         public Visibility Visibility => Type.Visibility;
         public bool IsNested => Type.IsNested;
diff --git a/ArchUnitNET/Domain/Extensions/TypeExtensions.cs b/ArchUnitNET/Domain/Extensions/TypeExtensions.cs
index 2c9142302..3455b437d 100644
--- a/ArchUnitNET/Domain/Extensions/TypeExtensions.cs
+++ b/ArchUnitNET/Domain/Extensions/TypeExtensions.cs
@@ -28,9 +28,9 @@ public static IEnumerable<IType> GetAssignableTypes(this IType type)
             switch (type)
             {
                 case Interface intf:
-                    return intf.ImplementedInterfaces.Append(intf);
+                    return intf.ImplementedInterfaces.Concat(new[] {intf});
                 case Class cls:
-                    return cls.InheritedClasses.Append(cls).Concat(cls.ImplementedInterfaces);
+                    return cls.InheritedClasses.Concat(new[] {cls}).Concat(cls.ImplementedInterfaces);
                 default:
                     return Enumerable.Empty<IType>();
             }
diff --git a/ArchUnitNET/Domain/PropertyMember.cs b/ArchUnitNET/Domain/PropertyMember.cs
index 54b37a70b..1457da003 100644
--- a/ArchUnitNET/Domain/PropertyMember.cs
+++ b/ArchUnitNET/Domain/PropertyMember.cs
@@ -57,7 +57,7 @@ public List<IMemberTypeDependency> MemberDependencies
                 var setterDependencies = Setter?.MemberDependencies ?? Enumerable.Empty<IMemberTypeDependency>();
                 var getterDependencies = Getter?.MemberDependencies ?? Enumerable.Empty<IMemberTypeDependency>();
                 return setterDependencies.Concat(getterDependencies).Concat(AttributeDependencies)
-                    .Append(PropertyTypeDependency).ToList();
+                    .Concat(new[] {PropertyTypeDependency}).ToList();
             }
         }
 
diff --git a/ArchUnitNET/Fluent/Syntax/Elements/Members/MemberPredicatesDefinition.cs b/ArchUnitNET/Fluent/Syntax/Elements/Members/MemberPredicatesDefinition.cs
index 00249359b..7162f194b 100644
--- a/ArchUnitNET/Fluent/Syntax/Elements/Members/MemberPredicatesDefinition.cs
+++ b/ArchUnitNET/Fluent/Syntax/Elements/Members/MemberPredicatesDefinition.cs
@@ -51,7 +51,7 @@ public static IPredicate<T> AreDeclaredIn(IType firstType, params IType[] moreTy
         {
             IEnumerable<T> Condition(IEnumerable<T> members)
             {
-                var types = moreTypes.Append(firstType);
+                var types = moreTypes.Concat(new[] {firstType});
                 return members.Intersect(types.SelectMany(type => type.Members).OfType<T>());
             }
 
@@ -64,7 +64,7 @@ public static IPredicate<T> AreDeclaredIn(Type firstType, params Type[] moreType
         {
             IEnumerable<T> Condition(IEnumerable<T> members, Architecture architecture)
             {
-                var types = moreTypes.Append(firstType).Select(architecture.GetITypeOfType);
+                var types = moreTypes.Concat(new[] {firstType}).Select(architecture.GetITypeOfType);
                 return members.Intersect(types.SelectMany(type => type.Members).OfType<T>());
             }
 
@@ -176,7 +176,7 @@ public static IPredicate<T> AreNotDeclaredIn(IType firstType, params IType[] mor
         {
             IEnumerable<T> Condition(IEnumerable<T> members)
             {
-                var types = moreTypes.Append(firstType);
+                var types = moreTypes.Concat(new[] {firstType});
                 return members.Except(types.SelectMany(type => type.Members).OfType<T>());
             }
 
@@ -189,7 +189,7 @@ public static IPredicate<T> AreNotDeclaredIn(Type firstType, params Type[] moreT
         {
             IEnumerable<T> Condition(IEnumerable<T> members, Architecture architecture)
             {
-                var types = moreTypes.Append(firstType).Select(architecture.GetITypeOfType);
+                var types = moreTypes.Concat(new[] {firstType}).Select(architecture.GetITypeOfType);
                 return members.Except(types.SelectMany(type => type.Members).OfType<T>());
             }
 
diff --git a/ArchUnitNET/Fluent/Syntax/Elements/Types/TypePredicatesDefinition.cs b/ArchUnitNET/Fluent/Syntax/Elements/Types/TypePredicatesDefinition.cs
index 924f73baa..450990cdc 100644
--- a/ArchUnitNET/Fluent/Syntax/Elements/Types/TypePredicatesDefinition.cs
+++ b/ArchUnitNET/Fluent/Syntax/Elements/Types/TypePredicatesDefinition.cs
@@ -21,7 +21,7 @@ public static IPredicate<T> Are(Type firstType, params Type[] moreTypes)
             IEnumerable<T> Filter(IEnumerable<T> ruleTypes, Architecture architecture)
             {
                 var typeList = moreTypes.Select(architecture.GetITypeOfType)
-                    .Append(architecture.GetITypeOfType(firstType)).OfType<T>();
+                    .Concat(new[] {architecture.GetITypeOfType(firstType)}).OfType<T>();
                 return ruleTypes.Intersect(typeList);
             }
 
@@ -91,7 +91,7 @@ public static IPredicate<T> AreAssignableTo(IType firstType, params IType[] more
         {
             IEnumerable<T> Condition(IEnumerable<T> ruleTypes)
             {
-                var types = moreTypes.Append(firstType);
+                var types = moreTypes.Concat(new[] {firstType});
                 return ruleTypes.Where(type => type.GetAssignableTypes().Intersect(types).Any());
             }
 
@@ -104,7 +104,7 @@ public static IPredicate<T> AreAssignableTo(Type firstType, params Type[] moreTy
         {
             IEnumerable<T> Condition(IEnumerable<T> ruleTypes, Architecture architecture)
             {
-                var types = moreTypes.Append(firstType).Select(architecture.GetITypeOfType);
+                var types = moreTypes.Concat(new[] {firstType}).Select(architecture.GetITypeOfType);
                 return ruleTypes.Where(type => type.GetAssignableTypes().Intersect(types).Any());
             }
 
@@ -219,7 +219,7 @@ public static IPredicate<T> ResideInAssembly(Assembly assembly, params Assembly[
         {
             IEnumerable<T> Condition(IEnumerable<T> types, Architecture architecture)
             {
-                var assemblyList = moreAssemblies.Append(assembly).Select(architecture.GetAssemblyOfAssembly);
+                var assemblyList = moreAssemblies.Concat(new[] {assembly}).Select(architecture.GetAssemblyOfAssembly);
                 return types.Where(type => assemblyList.Contains(type.Assembly));
             }
 
@@ -267,7 +267,7 @@ public static IPredicate<T> AreNot(Type firstType, params Type[] moreTypes)
             IEnumerable<T> Filter(IEnumerable<T> ruleTypes, Architecture architecture)
             {
                 var typeList = moreTypes.Select(architecture.GetITypeOfType)
-                    .Append(architecture.GetITypeOfType(firstType)).OfType<T>();
+                    .Concat(new[] {architecture.GetITypeOfType(firstType)}).OfType<T>();
                 return ruleTypes.Except(typeList);
             }
 
@@ -339,7 +339,7 @@ public static IPredicate<T> AreNotAssignableTo(IType firstType, params IType[] m
         {
             IEnumerable<T> Condition(IEnumerable<T> ruleTypes)
             {
-                var types = moreTypes.Append(firstType);
+                var types = moreTypes.Concat(new[] {firstType});
                 return ruleTypes.Where(type => !type.GetAssignableTypes().Intersect(types).Any());
             }
 
@@ -352,7 +352,7 @@ public static IPredicate<T> AreNotAssignableTo(Type firstType, params Type[] mor
         {
             IEnumerable<T> Condition(IEnumerable<T> ruleTypes, Architecture architecture)
             {
-                var types = moreTypes.Append(firstType).Select(architecture.GetITypeOfType);
+                var types = moreTypes.Concat(new[] {firstType}).Select(architecture.GetITypeOfType);
                 return ruleTypes.Where(type => !type.GetAssignableTypes().Intersect(types).Any());
             }
 
@@ -468,7 +468,7 @@ public static IPredicate<T> DoNotResideInAssembly(Assembly assembly, params Asse
         {
             IEnumerable<T> Condition(IEnumerable<T> types, Architecture architecture)
             {
-                var assemblyList = moreAssemblies.Append(assembly).Select(architecture.GetAssemblyOfAssembly);
+                var assemblyList = moreAssemblies.Concat(new[] {assembly}).Select(architecture.GetAssemblyOfAssembly);
                 return types.Where(type => !assemblyList.Contains(type.Assembly));
             }