GD0302: The generic type parameter must be annotated with the MustBeVariant attribute

Rule ID	GD0302
Category	Usage
Fix is breaking or non-breaking	Breaking
Enabled by default	Yes

Cause

A generic type is specified for a generic type argument when a Variant-compatible type is expected, but the specified generic type is not annotated with the [MustBeVariant] attribute.

Rule description

When a generic type parameter is annotated with the [MustBeVariant] attribute, the generic type is required to be a Variant-compatible type. When the type used is also a generic type, this generic type must be annoted with the [MustBeVariant] attribute as well. For example, the generic Godot.Collections.Array<T> type only supports items of a type that can be converted to Variant, a generic type can be specified if it's properly annotated.

public void Method1<T>()
{
    // T is not valid here because it may not a Variant-compatible type.
    var invalidArray = new Godot.Collections.Array<T>();
}

public void Method2<[MustBeVariant] T>()
{
    // T is guaranteed to be a Variant-compatible type because it's annotated
    // with the [MustBeVariant] attribute, so it can be used here.
    var validArray = new Godot.Collections.Array<T>();
}

How to fix violations

To fix a violation of this rule, add the [MustBeVariant] attribute to the generic type that is used as a generic type argument that must be Variant-compatible.

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.