Why are static methods in .Net framework classes always thread-safe?

Question

I have noticed the following statement in most places of .Net framework documentation.

Question: What is the secret to this? I don't think a static class is always thread-safe. My question relates to standard classes that are available in .Net framework, and not the custom classes created by developers.

Would the method 'GetString' in static class below be thread-safe just because the method is a static method?

public static class MyClass
{
    static int x = 0;

    static MyClass()
    {
        x = 23;
    }

    public static string GetString()
    {
        x++;
        return x.ToString();
    }
}

Answer 1

The framework methods you mention are not thread-safe just from the fact they are static, but because they have been specifically designed to be thread-safe. Thread-safety is often hard to achieve, but it's usually necessary for static methods, since any state they mutate is shared between threads.

The sample method you posted isn't thread-safe, because it mutates state that is shared between threads, without any synchronization mechanism.