Add guidance for indicating nullability on DTOs

docs/csharp/nullable-migration-strategies.md

@@ -86,3 +86,120 @@ This doesn't mean you can't use a nullable type (either value type or reference
 What it does mean is that you can't use `T?` in a generic class or method declaration without constraints. For example, <xref:System.Linq.Enumerable.FirstOrDefault%60%601(System.Collections.Generic.IEnumerable%7B%60%600%7D)?displayProperty=nameWithType> won't be changed to return `T?`. You can overcome this limitation by adding either the `struct` or `class` constraint. With either of those constraints, the compiler knows how to generate code for both `T` and `T?`.
 
 You may want to restrict the types used for a generic type argument to be non-nullable types. You can do that by adding the `notnull` constraint on that type argument. When that constraint is applied, the type argument must not be a nullable type.
+
+## Late-initialized properties, Data Transfer Objects and nullability
+
+Indicating the nullability of properties that are late-initialized, meaning set after construction, may require special consideration to ensure that your class continues to correctly express the original design intent.
+
+Types that contain late-initialized properties, such as Data Transfer Objects (DTOs), are often instantiated by an external library, like a database ORM (Object Relational Mapper), a deserializer, or some other component that automatically populates properties from another source.
+
+Consider the following DTO class, prior to enabling nullable reference types, that represents a student:
+
+```csharp
+class Student
+{
+    [Required]
+    public string FirstName { get; set; }
+
+    [Required]
+    public string LastName { get; set; }
+
+    public string VehicleRegistration { get; set; }
+}
+```
+
+The design intent (indicated in this case by the `Required` attribute) suggests that in this system, the `FirstName` and `LastName` properties are **mandatory**, and therefore not null.
+
+The `VehicleRegistration` property is **not mandatory**, so may be null.
+
+When you enable nullable reference types, you want to indicate on our DTO which of the properties may be nullable, consistent with your original intent:
+
+```csharp
+class Student
+{
+    [Required]
+    public string FirstName { get; set; }
+
+    [Required]
+    public string LastName { get; set; }
+
+    public string? VehicleRegistration { get; set; }
+}
+```
+
+For this DTO, the only property that may be null is ``VehicleRegistration``.
+
+However, the compiler raises `CS8618` warnings for both `FirstName` and `LastName`, indicating the non-nullable properties are uninitialized.
+
+There are three options available to you that resolve the compiler warnings in a way that maintains the original intent. Any of these options are valid; you should choose the one that best suits your coding style and design requirements.
+
+### Initialize in the constructor
+
+The ideal way to resolve the uninitialized warnings is to initialize the properties in the constructor:
+
+```csharp
+class Student
+{
+    public Student(string firstName, string lastName)
+    {
+        FirstName = firstName;
+        LastName = lastName;
+    }
+
+    [Required]
+    public string FirstName { get; set; }
+
+    [Required]
+    public string LastName { get; set; }
+
+    public string? VehicleRegistration { get; set; }
+}
+```
+
+This approach only works if the library that you use to instantiate the class supports passing parameters in the constructor.
+
+In addition, a library may support passing *some* properties in the constructor, but not all.
+For example, EF Core supports [constructor binding](/ef/core/modeling/constructors) for normal column properties, but not navigation properties.
+
+Check the documentation on the library that instantiates your class, to understand the extent to which it supports constructor binding.
+
+### Property with nullable backing field
+
+If constructor binding won't work for you, one way to deal with this problem is to have a non-nullable property with a nullable backing field:
+
+```csharp
+private string? _firstName;
+
+[Required]
+public string FirstName
+{
+    set => _firstName = value;
+    get => _firstName
+           ?? throw new InvalidOperationException("Uninitialized " + nameof(FirstName))
+}
+```
+
+In this scenario, if the `FirstName` property is accessed before it has been initialized, then the code throws an `InvalidOperationException`, because the API contract has been used incorrectly.
+
+You should consider that some libraries may have special considerations when using backing fields. For example, EF Core may need to be configured to use [backing fields](/ef/core/modeling/backing-field) correctly.
+
+### Initialize the property to null
+
+As a terser alternative to using a nullable backing field, or if the library that instantiates your class is not compatible with that approach, you can simply initialize the property to `null` directly, with the help of the null-forgiving operator (`!`):
+
+```csharp
+[Required]
+public string FirstName { get; set; } = null!;
+
+[Required]
+public string LastName { get; set; } = null!;
+
+public string? VehicleRegistration { get; set; }
+```
+
+You will never observe an actual null value at runtime except as a result of a programming bug, by accessing the property before it has been properly initialized.
+
+## See also
+
+- [Migrate an existing codebase to nullable references](tutorials/upgrade-to-nullable-references.md)
+- [Working with Nullable Reference Types in EF Core](/ef/core/miscellaneous/nullable-reference-types)