Where to set default values

I believe Microsoft's guidelines for static members of a class are to initialize members in-line rather than the static constructor. Not sure if that extends to non-static members. I'll be honest, my preference is the former. Constructors "do things" while initial values are kind of "window dressing" to the action of the class, as it were.

I think it depends. There is a reason why both ways are possible.

What I would strive for is to have relevant code together. So, if one field has more complicated initialization, which means it has to be set in constructor, then all related fields should be set in the constructor too.

But if the initialization is simple, it's probably better to have the fields and their default values together.

Also, don't forget that if those are not just default values, and you won't need to change them later, you should use the readonly modifier.

In this case you should probably use a const:

public class MyClass 
{
    private const int SomeInt = 300;
}

Set default values in the Constructor due to Order

public class MyClass {
    public int CopyOfSomeInt = someInt; //Value of zero
    public int someInt = 300;
    // etc. etc.
}

or within the constructor

public class MyClass {
    public int someInt;

    public MyClass() {
        CopyOfSomeInt = someInt;
        someInt = 300;
        CopyOfSomeInt = someInt; //Value of 300
    }
}

I would set them in the constructor, since I would also recommend the use of properties over fields. The differences are subtle, but they do come up.

I would also capitalize the property name, or the public field if you left it like that:

public class MyClass
{
    public int SomeInt { get; set; }

    public MyClass()
    {
        SomeInt = 300;
    }
}