XmlSerializer.Serialize doesn't work when using TrimMode=link

src/libraries/System.Private.Xml/src/ILLink/ILLink.Suppressions.xml

@@ -25,6 +25,12 @@
       <property name="Scope">member</property>
       <property name="Target">M:System.Xml.Serialization.XmlAttributes.get_IgnoreAttribute</property>
     </attribute>
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2057</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:System.Xml.Serialization.XmlSerializationWriterCodeGen.WriteCheckDefault(System.Xml.Serialization.TypeMapping,System.String,System.Object,System.Boolean)</property>
+    </attribute>
     <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
       <argument>ILLink</argument>
       <argument>IL2057</argument>
@@ -41,13 +47,25 @@
       <argument>ILLink</argument>
       <argument>IL2067</argument>
       <property name="Scope">member</property>
-      <property name="Target">M:System.Xml.Serialization.CodeGenerator.CreateTypeBuilder(System.Reflection.Emit.ModuleBuilder,System.String,System.Reflection.TypeAttributes,System.Type,System.Type[])</property>
+      <property name="Target">M:System.Xml.Serialization.ReflectionXmlSerializationReader.ReflectionCreateObject(System.Type)</property>
     </attribute>
     <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
       <argument>ILLink</argument>
-      <argument>IL2067</argument>
+      <argument>IL2070</argument>
       <property name="Scope">member</property>
-      <property name="Target">M:System.Xml.Serialization.ReflectionXmlSerializationReader.ReflectionCreateObject(System.Type)</property>
+      <property name="Target">M:System.Xml.Serialization.Compiler.AddImport(System.Type,System.Collections.Hashtable)</property>
+    </attribute>
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2070</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:System.Xml.Serialization.ReflectionAwareCodeGen.WriteMappingInfo(System.Xml.Serialization.TypeMapping,System.String,System.Type)</property>
+    </attribute>
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2070</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:System.Xml.Serialization.ReflectionAwareCodeGen.WriteMemberInfo(System.Type,System.String,System.String,System.String)</property>
     </attribute>
     <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
       <argument>ILLink</argument>
@@ -193,6 +211,12 @@
       <property name="Scope">member</property>
       <property name="Target">M:System.Xml.Serialization.TempAssembly.LoadGeneratedAssembly(System.Type,System.String,System.Xml.Serialization.XmlSerializerImplementation@)</property>
     </attribute>
+    <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
+      <argument>ILLink</argument>
+      <argument>IL2072</argument>
+      <property name="Scope">member</property>
+      <property name="Target">M:System.Xml.Serialization.XmlSerializationILGen.GenerateTypedSerializer(System.String,System.String,System.Xml.Serialization.XmlMapping,System.Xml.Serialization.CodeIdentifiers,System.String,System.String,System.String)</property>
+    </attribute>
     <attribute fullname="System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute">
       <argument>ILLink</argument>
       <argument>IL2072</argument>

src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeGenerator.cs

@@ -23,13 +23,13 @@ namespace System.Xml.Serialization
 {
     internal class CodeGenerator
     {
-        internal static BindingFlags InstancePublicBindingFlags = BindingFlags.Instance | BindingFlags.Public;
-        internal static BindingFlags InstanceBindingFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
-        internal static BindingFlags StaticBindingFlags = BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic;
-        internal static MethodAttributes PublicMethodAttributes = MethodAttributes.Public | MethodAttributes.HideBySig;
-        internal static MethodAttributes PublicOverrideMethodAttributes = MethodAttributes.Public | MethodAttributes.Virtual | MethodAttributes.HideBySig;
-        internal static MethodAttributes ProtectedOverrideMethodAttributes = MethodAttributes.Family | MethodAttributes.Virtual | MethodAttributes.HideBySig;
-        internal static MethodAttributes PrivateMethodAttributes = MethodAttributes.Private | MethodAttributes.HideBySig;
+        internal const BindingFlags InstancePublicBindingFlags = BindingFlags.Instance | BindingFlags.Public;
+        internal const BindingFlags InstanceBindingFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
+        internal const BindingFlags StaticBindingFlags = BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic;
+        internal const MethodAttributes PublicMethodAttributes = MethodAttributes.Public | MethodAttributes.HideBySig;
+        internal const MethodAttributes PublicOverrideMethodAttributes = MethodAttributes.Public | MethodAttributes.Virtual | MethodAttributes.HideBySig;
+        internal const MethodAttributes ProtectedOverrideMethodAttributes = MethodAttributes.Family | MethodAttributes.Virtual | MethodAttributes.HideBySig;
+        internal const MethodAttributes PrivateMethodAttributes = MethodAttributes.Private | MethodAttributes.HideBySig;
 
         private readonly TypeBuilder _typeBuilder;
         private MethodBuilder? _methodBuilder;
@@ -1242,7 +1242,13 @@ internal static ModuleBuilder CreateModuleBuilder(AssemblyBuilder assemblyBuilde
         {
             return assemblyBuilder.DefineDynamicModule(name);
         }
-        internal static TypeBuilder CreateTypeBuilder(ModuleBuilder moduleBuilder, string name, TypeAttributes attributes, Type parent, Type[] interfaces)
+
+        internal static TypeBuilder CreateTypeBuilder(
+            ModuleBuilder moduleBuilder,
+            string name,
+            TypeAttributes attributes,
+            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] Type parent,
+            Type[] interfaces)
         {
             // parent is nullable if no base class
             return moduleBuilder.DefineType(TempAssembly.GeneratedAssemblyNamespace + "." + name,

src/libraries/System.Private.Xml/tests/TrimmingTests/System.Xml.TrimmingTests.proj

@@ -0,0 +1,10 @@
+<Project DefaultTargets="Build">
+  <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props))" />
+
+  <ItemGroup>
+    <TestConsoleAppSourceFiles Include="XmlSerializer.Deserialize.cs" />
+    <TestConsoleAppSourceFiles Include="XmlSerializer.Serialize.cs" />
+  </ItemGroup>
+
+  <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.targets))" />
+</Project>

src/libraries/System.Private.Xml/tests/TrimmingTests/XmlSerializer.Deserialize.cs

@@ -0,0 +1,86 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Linq;
+using System.Xml.Schema;
+
+namespace System.Xml.Serialization.TrimmingTests
+{
+    internal class Program
+    {
+        // Preserve these types until XmlSerializer is fully trim-safe.
+        // see https://github.com/dotnet/runtime/issues/44768
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(Response))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(DataUpdates))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(DataUpdatesDataUpdateInfo))]
+        public static int Main()
+        {
+            using StringReader stringReader = new StringReader(@"<?xml version=""1.0"" encoding=""UTF-8""?>
+				<Response>
+				    <DataUpdates>
+				        <DataUpdateInfo DataDate=""2009-04-13T00:00:00"" DataType=""Data"" LastUpdatedDate=""2010-12-12T02:53:19.257"" />
+				        <DataUpdateInfo DataDate=""2009-04-14T00:00:00"" DataType=""Data"" LastUpdatedDate=""2010-12-12T02:53:19.257"" />
+				        <DataUpdateInfo DataDate=""2009-04-15T00:00:00"" DataType=""Data"" LastUpdatedDate=""2010-12-12T01:52:51.047"" />
+				    </DataUpdates>
+				</Response>");
+
+            Response obj = (Response)new XmlSerializer(typeof(Response)).Deserialize(stringReader);
+            if (obj.DataUpdates.DataUpdateInfo.Count == 3 &&
+                obj.DataUpdates.DataUpdateInfo.All(i => i.DataDate.Year == 2009 && i.LastUpdatedDate.Year == 2010))
+            {
+                return 100;
+            }
+
+            return -1;
+        }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    [XmlRoot(Namespace = "", IsNullable = false)]
+    public class Response
+    {
+        public Response()
+        {
+            this.DataUpdates = new DataUpdates();
+        }
+
+        [XmlElement(Order = 0)]
+        public DataUpdates DataUpdates { get; set; }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    [XmlRoot(Namespace = "", IsNullable = false)]
+    public class DataUpdates
+    {
+        public DataUpdates()
+        {
+            this.DataUpdateInfo = new List<DataUpdatesDataUpdateInfo>();
+        }
+
+        [XmlElement("DataUpdateInfo", Form = XmlSchemaForm.Unqualified, Order = 0)]
+        public List<DataUpdatesDataUpdateInfo> DataUpdateInfo { get; set; }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    public class DataUpdatesDataUpdateInfo
+    {
+        public DataUpdatesDataUpdateInfo()
+        {
+        }
+
+        [XmlAttribute]
+        public DateTime DataDate { get; set; }
+
+        [XmlAttribute]
+        public string DataType { get; set; }
+
+        [XmlAttribute]
+        public DateTime LastUpdatedDate { get; set; }
+    }
+}

src/libraries/System.Private.Xml/tests/TrimmingTests/XmlSerializer.Serialize.cs

@@ -0,0 +1,95 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Xml.Schema;
+
+namespace System.Xml.Serialization.TrimmingTests
+{
+    internal class Program
+    {
+        // Preserve these types until XmlSerializer is fully trim-safe.
+        // see https://github.com/dotnet/runtime/issues/44768
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(Response))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(DataUpdates))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(DataUpdatesDataUpdateInfo))]
+        public static int Main()
+        {
+            Response obj = new Response();
+            obj.DataUpdates.DataUpdateInfo.Add(new DataUpdatesDataUpdateInfo()
+            {
+                DataDate = new DateTime(2009, 4, 13),
+                DataType = "Data",
+                LastUpdatedDate = new DateTime(2010, 12, 12)
+            });
+            obj.DataUpdates.DataUpdateInfo.Add(new DataUpdatesDataUpdateInfo()
+            {
+                DataDate = new DateTime(2009, 4, 14),
+                DataType = "Data",
+                LastUpdatedDate = new DateTime(2010, 12, 12)
+            });
+
+            using StringWriter writer = new StringWriter();
+            new XmlSerializer(typeof(Response)).Serialize(writer, obj);
+            string serialized = writer.ToString();
+
+            if (serialized.Contains("<Response") &&
+                serialized.Contains("<DataUpdates>") &&
+                serialized.Contains(@"<DataUpdateInfo DataDate=""2009-04-13T00:00:00"" DataType=""Data"" LastUpdatedDate=""2010-12-12T00:00:00"" />") &&
+                serialized.Contains(@"<DataUpdateInfo DataDate=""2009-04-14T00:00:00"" DataType=""Data"" LastUpdatedDate=""2010-12-12T00:00:00"" />"))
+            {
+                return 100;
+            }
+
+            return -1;
+        }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    [XmlRoot(Namespace = "", IsNullable = false)]
+    public class Response
+    {
+        public Response()
+        {
+            this.DataUpdates = new DataUpdates();
+        }
+
+        [XmlElement(Order = 0)]
+        public DataUpdates DataUpdates { get; set; }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    [XmlRoot(Namespace = "", IsNullable = false)]
+    public class DataUpdates
+    {
+        public DataUpdates()
+        {
+            this.DataUpdateInfo = new List<DataUpdatesDataUpdateInfo>();
+        }
+
+        [XmlElement("DataUpdateInfo", Form = XmlSchemaForm.Unqualified, Order = 0)]
+        public List<DataUpdatesDataUpdateInfo> DataUpdateInfo { get; set; }
+    }
+
+    [Serializable]
+    [XmlType(AnonymousType = true)]
+    public class DataUpdatesDataUpdateInfo
+    {
+        public DataUpdatesDataUpdateInfo()
+        {
+        }
+
+        [XmlAttribute]
+        public DateTime DataDate { get; set; }
+
+        [XmlAttribute]
+        public string DataType { get; set; }
+
+        [XmlAttribute]
+        public DateTime LastUpdatedDate { get; set; }
+    }
+}