.. _doc_c_sharp_features:

C# features
===========

This page provides an overview of the commonly used features of both C# and Godot
and how they are used together.

.. _doc_c_sharp_features_type_conversion_and_casting:

Type conversion and casting
---------------------------

C# is a statically typed language. Therefore, you can't do the following:

.. code-block:: csharp

    var mySprite = GetNode("MySprite");
    mySprite.SetFrame(0);

The method ``GetNode()`` returns a ``Node`` instance.
You must explicitly convert it to the desired derived type, ``Sprite`` in this case.

For this, you have various options in C#.

**Casting and Type Checking**

Throws ``InvalidCastException`` if the returned node cannot be cast to Sprite.
You would use it instead of the ``as`` operator if you are pretty sure it won't fail.

.. code-block:: csharp

    Sprite mySprite = (Sprite)GetNode("MySprite");
    mySprite.SetFrame(0);

**Using the AS operator**

The ``as`` operator returns ``null`` if the node cannot be cast to Sprite,
and for that reason, it cannot be used with value types.

.. code-block:: csharp

    Sprite mySprite = GetNode("MySprite") as Sprite;
    // Only call SetFrame() if mySprite is not null
    mySprite?.SetFrame(0);

**Using the generic methods**

Generic methods are also provided to make this type conversion transparent.

``GetNode<T>()`` casts the node before returning it. It will throw an ``InvalidCastException`` if the node cannot be cast to the desired type.

.. code-block:: csharp

    Sprite mySprite = GetNode<Sprite>("MySprite");
    mySprite.SetFrame(0);

``GetNodeOrNull<T>()`` uses the ``as`` operator and will return ``null`` if the node cannot be cast to the desired type.

.. code-block:: csharp

    Sprite mySprite = GetNodeOrNull<Sprite>("MySprite");
    // Only call SetFrame() if mySprite is not null
    mySprite?.SetFrame(0);

**Type checking using the IS operator**

To check if the node can be cast to Sprite, you can use the ``is`` operator.
The ``is`` operator returns false if the node cannot be cast to Sprite,
otherwise it returns true.

.. code-block:: csharp

    if (GetNode("MySprite") is Sprite)
    {
        // Yup, it's a sprite!
    }

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:

C# signals
----------

For a complete C# example, see the **Handling a signal** section in the step by step :ref:`doc_scripting` tutorial.

Declaring a signal in C# is done with the ``[Signal]`` attribute on a delegate.

.. code-block:: csharp

    [Signal]
    delegate void MySignal();

    [Signal]
    delegate void MySignalWithArguments(string foo, int bar);

These signals can then be connected either in the editor or from code with ``Connect``.
If you want to connect a signal in the editor, you need to (re)build the project assemblies to see the new signal. This build can be manually triggered by clicking the “Build” button at the top right corner of the editor window.

.. code-block:: csharp

    public void MyCallback()
    {
        GD.Print("My callback!");
    }

    public void MyCallbackWithArguments(string foo, int bar)
    {
        GD.Print("My callback with: ", foo, " and ", bar, "!");
    }

    public void SomeFunction()
    {
        instance.Connect("MySignal", this, "MyCallback");
        instance.Connect(nameof(MySignalWithArguments), this, "MyCallbackWithArguments");
    }

Emitting signals is done with the ``EmitSignal`` method.

.. code-block:: csharp

    public void SomeFunction()
    {
        EmitSignal(nameof(MySignal));
        EmitSignal("MySignalWithArguments", "hello there", 28);
    }

Notice that you can always reference a signal name with the ``nameof`` keyword (applied on the delegate itself).

It is possible to bind values when establishing a connection by passing a Godot array.

.. code-block:: csharp

    public int Value { get; private set; } = 0;

    private void ModifyValue(int modifier)
    {
        Value += modifier;
    }

    public void SomeFunction()
    {
        var plusButton = (Button)GetNode("PlusButton");
        var minusButton = (Button)GetNode("MinusButton");

        plusButton.Connect("pressed", this, "ModifyValue", new Godot.Collections.Array { 1 });
        minusButton.Connect("pressed", this, "ModifyValue", new Godot.Collections.Array { -1 });
    }

Signals support parameters and bound values of all the `built-in types <https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/built-in-types-table>`_ and Classes derived from :ref:`Godot.Object <class_Object>`.
Consequently, any ``Node`` or ``Reference`` will be compatible automatically, but custom data objects will need to extend from `Godot.Object` or one of its subclasses.

.. code-block:: csharp

    public class DataObject : Godot.Object
    {
        public string Field1 { get; set; }
        public string Field2 { get; set; }
    }


Finally, signals can be created by calling ``AddUserSignal``, but be aware that it should be executed before any use of said signals (with ``Connect`` or ``EmitSignal``).

.. code-block:: csharp

    public void SomeFunction()
    {
        AddUserSignal("MyOtherSignal");
        EmitSignal("MyOtherSignal");
    }

Preprocessor defines
--------------------

Godot has a set of defines that allow you to change your C# code
depending on the environment you are compiling to.

.. note:: If you created your project before Godot 3.2, you have to modify
          or regenerate your `csproj` file to use this feature
          (compare ``<DefineConstants>`` with a new 3.2+ project).

Examples
~~~~~~~~

For example, you can change code based on the platform:

.. code-block:: csharp

        public override void _Ready()
        {
    #if GODOT_SERVER
            // Don't try to load meshes or anything, this is a server!
            LaunchServer();
    #elif GODOT_32 || GODOT_MOBILE || GODOT_WEB
            // Use simple objects when running on less powerful systems.
            SpawnSimpleObjects();
    #else
            SpawnComplexObjects();
    #endif
        }

Or you can detect which engine your code is in, useful for making cross-engine libraries:

.. code-block:: csharp

        public void MyPlatformPrinter()
        {
    #if GODOT
            GD.Print("This is Godot.");
    #elif UNITY_5_3_OR_NEWER
            print("This is Unity.");
    #else
            throw new InvalidWorkflowException("Only Godot and Unity are supported.");
    #endif
        }

Full list of defines
~~~~~~~~~~~~~~~~~~~~

* ``GODOT`` is always defined for Godot projects.

* One of ``GODOT_64`` or ``GODOT_32`` is defined depending on if the architecture is 64-bit or 32-bit.

* One of ``GODOT_X11``, ``GODOT_WINDOWS``, ``GODOT_OSX``,
  ``GODOT_ANDROID``, ``GODOT_IOS``, ``GODOT_HTML5``, or ``GODOT_SERVER``
  depending on the OS. These names may change in the future.
  These are created from the ``get_name()`` method of the
  :ref:`OS <class_OS>` singleton, but not every possible OS
  the method returns is an OS that Godot with Mono runs on.

When **exporting**, the following may also be defined depending on the export features:

* One of ``GODOT_PC``, ``GODOT_MOBILE``, or ``GODOT_WEB`` depending on the platform type.

* One of ``GODOT_ARM64_V8A`` or ``GODOT_ARMEABI_V7A`` on Android only depending on the architecture.

* One of ``GODOT_ARM64`` or ``GODOT_ARMV7`` on iOS only depending on the architecture.

* Any of ``GODOT_S3TC``, ``GODOT_ETC``, and ``GODOT_ETC2`` depending on the texture compression type.

* Any custom features added in the export menu will be capitalized and prefixed: ``foo`` -> ``GODOT_FOO``.

To see an example project, see the OS testing demo:
https://github.com/godotengine/godot-demo-projects/tree/master/misc/os_test