Fix NullReferenceException for inherited ShouldSerialize methods

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

@@ -661,9 +661,7 @@ private void WriteStructMethod(StructMapping mapping, string n, string? ns, obje
 
                     if (m.CheckShouldPersist)
                     {
-                        string methodInvoke = $"ShouldSerialize{m.Name}";
-                        MethodInfo method = o!.GetType().GetMethod(methodInvoke, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static | BindingFlags.DeclaredOnly)!;
-                        shouldPersist = (bool)method.Invoke(o, Array.Empty<object>())!;
+                        shouldPersist = (bool)m.CheckShouldPersistMethodInfo!.Invoke(o, Array.Empty<object>())!;
                     }
 
                     if (m.Attribute != null)
@@ -693,9 +691,7 @@ private void WriteStructMethod(StructMapping mapping, string n, string? ns, obje
 
                     if (m.CheckShouldPersist)
                     {
-                        string methodInvoke = $"ShouldSerialize{m.Name}";
-                        MethodInfo method = o!.GetType().GetMethod(methodInvoke, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static | BindingFlags.DeclaredOnly)!;
-                        shouldPersist = (bool)method.Invoke(o, Array.Empty<object>())!;
+                        shouldPersist = (bool)m.CheckShouldPersistMethodInfo!.Invoke(o, Array.Empty<object>())!;
                     }
 
                     bool checkShouldPersist = m.CheckShouldPersist && (m.Elements!.Length > 0 || m.Text != null);

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs

@@ -1676,6 +1676,30 @@ public static void Xml_TypeWithShouldSerializeMethod_WithNonDefaultValue()
         Assert.Equal(value.Foo, actual.Foo);
     }
 
+    [Fact]
+    public static void Xml_InheritedShouldSerializeMethod_WithDefaultValue()
+    {
+        var value = new DerivedTypeWithInheritedShouldSerialize();
+
+        var actual = SerializeAndDeserialize(value, WithXmlHeader("<DerivedTypeWithInheritedShouldSerialize xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" />"));
+
+        Assert.NotNull(actual);
+        Assert.Equal(value.Foo, actual.Foo);
+        Assert.Equal(value.Bar, actual.Bar);
+    }
+
+    [Fact]
+    public static void Xml_InheritedShouldSerializeMethod_WithNonDefaultValue()
+    {
+        var value = new DerivedTypeWithInheritedShouldSerialize() { Foo = "SomeValue", Bar = "SomeBar" };
+
+        var actual = SerializeAndDeserialize(value, WithXmlHeader("<DerivedTypeWithInheritedShouldSerialize Bar=\"SomeBar\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"><Foo>SomeValue</Foo></DerivedTypeWithInheritedShouldSerialize>"));
+
+        Assert.NotNull(actual);
+        Assert.Equal(value.Foo, actual.Foo);
+        Assert.Equal(value.Bar, actual.Bar);
+    }
+
     [Fact]
     public static void Xml_KnownTypesThroughConstructorWithArrayProperties()
     {

src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.cs

@@ -924,6 +924,28 @@ public bool ShouldSerializeFoo()
         }
     }
 
+    public class BaseTypeWithShouldSerializeMethod
+    {
+        public string Foo { get; set; } = "default";
+
+        [System.Xml.Serialization.XmlAttribute]
+        public string Bar { get; set; } = "default";
+
+        public bool ShouldSerializeFoo()
+        {
+            return Foo != "default";
+        }
+
+        public bool ShouldSerializeBar()
+        {
+            return Bar != "default";
+        }
+    }
+
+    public class DerivedTypeWithInheritedShouldSerialize : BaseTypeWithShouldSerializeMethod
+    {
+    }
+
     public class KnownTypesThroughConstructorWithArrayProperties
     {
         public object StringArrayValue;