contravariance.cs

public interface IProcessor<in T>
{
    void Process(T input);
}

public class StringProcessor : IProcessor<string>
{
    public void Process(string input)
    {
        Console.WriteLine($"Processing string: {input}");
    }
}

public class ObjectProcessor : IProcessor<object>
{
    public void Process(object input)
    {
        Console.WriteLine($"Processing object: {input}");
    }
}

class Program
{
    static void Main()
    {
        IProcessor<object> objectProcessor = new ObjectProcessor();
        IProcessor<string> stringProcessor = new StringProcessor();

        Process("Hello world!", stringProcessor); // Processing string: Hello world!
        Process("Hello world!", objectProcessor); // Processing object: Hello world!
    }

    static void Process<T>(T input, IProcessor<T> processor)
    {
        processor.Process(input);
    }
}