Updated C# and .NET docs supplement (markdown)

C#-and-.NET-docs-supplement.md

@@ -66,7 +66,53 @@ You can only have 1 `default:` branch (not to be confused with `case default:`),
 
 ## `MemoryMarshal.Cast` can't be used with reference types
 
-See [this SO answer](https://stackoverflow.com/a/79146967/7467292). tl;dr: It's possible in .NET Core only.
+See [this SO answer](https://stackoverflow.com/a/79146967). tl;dr: It's possible in .NET Core only.
+
+<details>
+<summary>full implementations of extension methods for later</summary>
+
+```csharp
+public static Span<TTo> Cast<TFrom, TTo>(Span<TFrom> span)
+	where TTo: class
+	where TFrom: class
+{
+	if (typeof(TTo).IsAssignableFrom(typeof(TFrom))) throw new InvalidCastException();
+	return UnsafeDowncast<TFrom, TTo>(span);
+}
+
+public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span)
+	where TTo: class
+	where TFrom: class
+{
+	if (typeof(TTo).IsAssignableFrom(typeof(TFrom))) throw new InvalidCastException();
+	return UnsafeDowncast<TFrom, TTo>(span);
+}
+
+public static Span<TBase> CastUp<TDerived, TBase>(Span<TDerived> span)
+	where TBase: class
+	where TDerived: class, TBase
+	=> UnsafeDowncast<TDerived, TBase>(span);
+
+public static ReadOnlySpan<TBase> CastUp<TDerived, TBase>(ReadOnlySpan<TDerived> span)
+	where TBase: class
+	where TDerived: class, TBase
+	=> UnsafeDowncast<TDerived, TBase>(span);
+
+public static Span<TTo> UnsafeDowncast<TFrom, TTo>(Span<TFrom> span)
+	where TTo: class
+	where TFrom: class
+	=> MemoryMarshal.CreateSpan<TTo>(
+		ref Unsafe.As<TFrom, TTo>(ref MemoryMarshal.GetReference(span)),
+		span.Length);
+
+public static ReadOnlySpan<TTo> UnsafeDowncast<TFrom, TTo>(ReadOnlySpan<TFrom> span)
+	where TTo: class
+	where TFrom: class
+	=> MemoryMarshal.CreateReadOnlySpan<TTo>(
+		ref Unsafe.As<TFrom, TTo>(ref MemoryMarshal.GetReference(span)),
+		span.Length);
+```
+</details>
 
 ## MSBuild `Condition` placement