Get a float variable given a generic array c#

You can define an interface and then have the classes exposing a spawn chance implementing it. It is then easy to test whether an object implements it or not.

public interface ISpawnable
{
    float SpawnChance { get; }
}

SpawnChance must be a property. It is not necessary to declare the setter for this use case (because you only need reading it); however, implementations can still provide one.

Then you can test

if (obj is ISpawnable spawnable) {
    float w = spawnable.SpawnChance;
    ...
}

Or with LINQ if you have a list of mixed objects

var weights = mixedTypeObjects
    .OfType<ISpawnable>()         // Filters objects implementing it and casts to it.
    .Select(x => x.SpawnChance);

If you have generic types or methods, you can use this interface as generic type constraint.

public class SpawnService<T>
    where T : ISpawnable
{ ... }

This restricts T to be a type implementing ISpawnable and allows you to access SpawnChance without any casting within this class.

Another option is to derive spawnable objects from a common base class exposing a spawn chance. The interface is more flexible since it works for classes deriving from different base classes. This is an advantage if you already have a class hierarchy and you want classes implement it independently from this hierarchy.

You can also simply pass game objects to the method and filter and cast inside:

public static int GetRandomWeightedIndex(this IEnumerable<GameObjects> gameObjects)
{
    var weights = gameObjects
        .OfType<ISpawnable>()
        .Select(g => g.SpawnChance);
    ...
}