[release/8.0] Fix issue 14259: Deserialization of the CodeTypeReference to generic type is incorrect

src/System.Windows.Forms.Design/src/System/ComponentModel/Design/Serialization/CodeDomSerializerBase.cs

@@ -78,6 +78,7 @@ private static void GetTypeNameFromCodeTypeReferenceHelper(IDesignerSerializatio
         else
         {
             //create the MyGeneric`2[ part
+            typeName.Append(typeref.BaseType);
             if (!typeref.BaseType.Contains('`'))
             {
                 typeName.Append($"`{typeref.TypeArguments.Count}");

src/System.Windows.Forms.Design/tests/UnitTests/Serialization/CodeDomSerializerTests.cs

@@ -1,7 +1,10 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.CodeDom;
+using System.ComponentModel.Design;
 using System.ComponentModel.Design.Serialization;
+using Moq;
 
 namespace System.Windows.Forms.Design.Serialization.Tests;
 
@@ -13,4 +16,26 @@ public void CodeDomSerializer_Constructor()
         CodeDomSerializer underTest = new();
         Assert.NotNull(underTest);
     }
+
+    [Fact]
+    public void CodeDomSerializer_Deserialize_GenericTypeRef()
+    {
+        CodeDomSerializer underTest = new();
+        Type type = typeof(List<int?>);
+        CodeTypeOfExpression expression = new(new CodeTypeReference(type));
+
+        Mock<IDesignerSerializationManager> mockManager = new(MockBehavior.Strict);
+        mockManager
+            .Setup(s => s.GetService(typeof(TypeDescriptionProviderService)))
+            .Returns(null);
+        mockManager
+            .Setup(s => s.GetType("System.Int32"))
+            .Returns(typeof(int));
+        mockManager
+            .Setup(s => s.GetType($"System.Collections.Generic.List`1[[System.Nullable`1[[{typeof(int).AssemblyQualifiedName}]]]]"))
+            .Returns(type);
+
+        object result = underTest.Deserialize(mockManager.Object, expression);
+        Assert.Equal(type, result);
+    }
 }