Add article about nullable value types

content/articles/dotnet/boxing-nullable-value-types.md

@@ -0,0 +1,39 @@
+---
+title: Boxing Nullable Value Types in C#
+topics:
+- csharp
+- dotnet
+---
+
+A nullable value type value (e.g. `int?`) loses its nullability type information when it's boxed:
+
+**Concrete value:**
+
+```c#
+int? valueType = 42;
+object? boxed = valueType;
+Console.WriteLine(boxed?.GetType()); // System.Int32
+```
+
+**Null value:**
+
+```c#
+int? valueType = null;
+object? boxed = valueType;
+Console.WriteLine(boxed is null); // true
+```
+
+This also means that using `Nullable.GetUnderlyingType()` is useless for boxed value types:
+
+```c# {hl_lines="9"}
+public void MyMethod(object? value)
+{
+    if (value is null)
+    {
+        return;
+    }
+
+    // Always false!
+    bool isNullable = Nullable.GetUnderlyingType(value.GetType()) != null;
+}
+```