Update & fix C# blocks under tutorials/scripting/

- `_Process` and `_PhysicsProcess` take their delta argument as a `double`
- Use string interpolation instead of concatenating strings
- Use `string` instead of `String` (`using System` was removed from everywhere in a previous PR)
- Use `System.Array.Empty<T>()` to init empty arrays
- Use `SignalName.*` instead of `nameof(*)` for signals
- Do not use `event` as an argument name (that's a keyword)
- Match node paths to screenshots when retrieving nodes from the tree
- Add a C# example for scene unique nodes
- Update the "Cross language scripting" page
- Add a note about using `is` against `null`

tutorials/scripting/c_sharp/c_sharp_basics.rst

@@ -218,7 +218,7 @@ Here's a blank C# script with some comments to demonstrate how it works.
             GD.Print("Hello from C# to Godot :)");
         }
 
-        public override void _Process(float delta)
+        public override void _Process(double delta)
         {
             // Called every frame. Delta is time since the last frame.
             // Update game logic here.

tutorials/scripting/c_sharp/c_sharp_differences.rst

@@ -466,7 +466,7 @@ GDScript                C#
 ``PackedByteArray``      ``byte[]``
 ``PackedFloat32Array``   ``float[]``
 ``PackedFloat64Array``   ``double[]``
-``PackedStringArray``    ``String[]``
+``PackedStringArray``    ``string[]``
 ``PackedColorArray``     ``Color[]``
 ``PackedVector2Array``   ``Vector2[]``
 ``PackedVector3Array``   ``Vector3[]``

tutorials/scripting/c_sharp/c_sharp_exports.rst

@@ -307,9 +307,9 @@ Exported arrays should be initialized empty.
 .. code-block:: csharp
 
     [Export]
-    private Vector3[] Vector3s = new Vector3[0];
+    private Vector3[] Vector3s = System.Array.Empty<Vector3>();
     [Export]
-    private String[] String = new String[0];
+    private string[] Strings = System.Array.Empty<string>();
 
 
 You can omit the default value, but then it would be null if not assigned.

tutorials/scripting/c_sharp/c_sharp_features.rst

@@ -67,7 +67,7 @@ Generic methods are also provided to make this type conversion transparent.
 
 To check if the node can be cast to Sprite2D, you can use the ``is`` operator.
 The ``is`` operator returns false if the node cannot be cast to Sprite2D,
-otherwise it returns true.
+otherwise it returns true. Note that using the ``is`` operator against ``null`` is always going to be falsy.
 
 .. code-block:: csharp
 
@@ -76,6 +76,11 @@ otherwise it returns true.
         // Yup, it's a Sprite2D!
     }
 
+    if (null is Sprite2D)
+    {
+        // This block can never happen.
+    }
+
 For more advanced type checking, you can look into `Pattern Matching <https://docs.microsoft.com/en-us/dotnet/csharp/pattern-matching>`_.
 
 .. _doc_c_sharp_signals:
@@ -107,7 +112,7 @@ If you want to connect a signal in the editor, you need to (re)build the project
 
     public void MyCallbackWithArguments(string foo, int bar)
     {
-        GD.Print("My callback with: ", foo, " and ", bar, "!");
+        GD.Print($"My callback with: {foo} and {bar}!");
     }
 
     public void SomeFunction()
@@ -122,11 +127,11 @@ Emitting signals is done with the ``EmitSignal`` method.
 
     public void SomeFunction()
     {
-        EmitSignal(nameof(MySignal));
-        EmitSignal(nameof(MySignalWithArguments), "hello there", 28);
+        EmitSignal(SignalName.MySignal);
+        EmitSignal(SignalName.MySignalWithArguments, "hello there", 28);
     }
 
-Notice that you can always reference a signal name with the ``nameof`` keyword (applied on the delegate itself).
+Notice that you can always reference a signal name with its generated ``SignalName``.
 
 It is possible to bind values when establishing a connection by passing a Godot array.