Implementing a Generic Singleton Factory in C#?

Hello fellow developers,

I am currently working on a project where I have to create several singleton instances of different types. I know that creating singletons for each type can be a bit tedious and error-prone, so I wanted to implement a generic singleton factory in C#. However, I am a bit unsure about the best way to accomplish this task.

The objective is to have a method which can create a singleton instance of any given type. Here is a simple code that I have so far:

public class SingletonFactory
{
    private static readonly Dictionary<Type, object> instances = new Dictionary<Type, object>();

    public static T GetInstance<T>() where T : class, new()
    {
        if (!instances.ContainsKey(typeof(T)))
        {
            instances[typeof(T)] = new T();
        }
        return (T)instances[typeof(T)];
    }
}

This is a minimalist version, and I understand that it does not take into account things like thread safety and disposing.

So, my questions are:

1. How can I enhance the above code to make it thread-safe?
2. Is there any better approach to implement this pattern which would also take into account the disposal of instances when they are no longer needed?
3. Is there a way to make this pattern work if the class that needs to be instantiated has some parameters in its constructor?

I appreciate any help or advice that you can provide. Thanks!