Add article about separate nullability with C# properties

content/articles/csharp-dotnet/property-nullability.md

@@ -0,0 +1,119 @@
+---
+title: Separate nullability with C# properties
+description: "[C#] How to define a different nullability for getter and setter of a property."
+topics:
+- csharp
+- dotnet
+- nullability
+---
+
+This article shows how to define a different nullability for the getter and setter of a property in C#.
+
+## Summary
+
+If you don't want to read the article:
+
+```c#
+public class MyClass
+{
+    // getter: not null
+    // setter: can be null
+    [NotNull]
+    public string? SectionName
+    {
+        get => _sectionName ?? "DefaultSection";
+        set => _sectionName = value;
+    }
+
+    private string? _sectionName;
+
+    // getter: can be null
+    // setter: not null
+    [DisallowNull]
+    public string? ExplicitName
+    {
+        get => _explicitName;
+        set => _explicitName = value ?? throw new ArgumentNullException();
+    }
+
+    private string? _explicitName;
+}
+```
+
+## Details
+
+Normally, the getter and the setter of a property have identical nullability ("not null" or "can be null") - as defined by the property's type.
+
+But sometimes you may want to have a different nullability for getter or setter:
+
+1. The getter never returns `null` whereas the setter accepts `null` as value.
+1. The getter can return `null` but the setter doesn't accept `null`.
+
+### [NotNull]
+
+For **option 1**, add `[NotNull]` to the property:
+
+```c#
+// getter: not null
+// setter: can be null
+[NotNull]
+public string? SectionName
+{
+    get => _sectionName ?? "DefaultSection";
+    set => _sectionName = value;
+}
+
+private string? _sectionName;
+```
+
+The [description of `[NotNull]`](https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.notnullattribute) is:
+
+> Specifies that an output is not `null` even if the corresponding type allows it.
+
+### [DisallowNull]
+
+For **option 2**, add `[DisallowNull]` to the property:
+
+```c#
+// getter: can be null
+// setter: not null
+[DisallowNull]
+public string? ExplicitName
+{
+    get => _explicitName;
+    set => _explicitName = value ?? throw new ArgumentNullException();
+}
+
+private string? _explicitName;
+```
+
+The [description of `[DisallowNull]`](https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.disallownullattribute) is:
+
+> Specifies that `null` is disallowed as an input even if the corresponding type allows it.
+
+### Value Types
+
+These attributes also work for value types:
+
+```c#
+public class MyClass
+{
+    [NotNull]
+    public int? BufferSize
+    {
+        get => _bufferSize ?? 4096;
+        set => _bufferSize = value;
+    }
+
+    private int? _bufferSize;
+
+    [DisallowNull]
+    public int? ExplicitAge
+    {
+        get => _explicitAge;
+        set => _explicitAge = value ?? throw new ArgumentNullException();
+    }
+
+    private int? _explicitAge;
+}
+```