godot-docs/tutorials/scripting/c_sharp/diagnostics/GD0301.rst

GD0301: The generic type argument must be a Variant compatible type
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                                      Value
====================================  ======================================
**Rule ID**                           GD0301
**Category**                          Usage
**Fix is breaking or non-breaking**   Breaking
**Enabled by default**                Yes
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Cause
-----

An unsupported type is specified for a generic type argument when a
:ref:`Variant-compatible type <c_sharp_variant_compatible_types>` is expected.

Rule description
----------------

When a generic type parameter is annotated with the ``[MustBeVariant]`` attribute,
the generic type is required to be a Variant-compatible type. For example,
the generic ``Godot.Collections.Array<T>`` type only supports items of a type
that can be converted to Variant.

.. code-block:: csharp

    class SomeType { }

    // SomeType is not a valid type because it doesn't derive from GodotObject,
    // so it's not compatible with Variant.
    var invalidArray = new Godot.Collections.Array<SomeType>();

    // System.Int32 is a valid type because it's compatible with Variant.
    var validArray = new Godot.Collections.Array<int>();

How to fix violations
---------------------

To fix a violation of this rule, change the generic type argument to be a
Variant-compatible type or use a different API that doesn't require the generic
type argument to be a Variant-compatible type.

When to suppress warnings
-------------------------

Do not suppress a warning from this rule. API that contains generic type arguments
annotated with the ``[MustBeVariant]`` attribute usually has this requirement
because the values will be passed to the engine, if the type can't be marshalled
it will result in runtime errors.