.Net Coding Standards Using a prefix "Is" or "Has" on Method Names

I would be using

bool IsActivePage
{
  get
  {
    // some logic
  }
}

if the method has no side effects and is inexpensive.

I see no need to have both a method and a property for the same thing.

I think both are defensible. The key is really to think about how standardized a convention like this should be. In general, you should either decide at your team or company level about how to handle cases like this, and then be consistent after that. As long as code you and your company produce is clear to everyone involved, that's what matters.

I vote for your solution: so YES, for methods , I personally think, it's better to have Get..Bla(), cause method intuitively, at least for me, is, not only something that returns a value to me, but also performs some calculations or calls other methods inside it, properties instead, just return value.

"Get" word, to me personally, seems DO SOMETHIGN+RETURN,

instead "Is" : check if this exists.

The Framework Design Guidelines state that you should "give methods names that are verbs or verb phrases" since "typically methods act on data". Properties, on the other hand, should be named "using a noun, noun phrase, or an adjective" and "you can also prefix Boolean properties with Is, Can, or Has, but only where it adds value".

In this case, you are using a method rather than a property, probably since it is either expensive or has some side effects. I suggest you choose the name that provides the most clarity of what the returned value represents. The important part is that you're being consistent and that you're not confusing other developers with your convention.