Fast Blur in C#

I need a way to get a fast blur effect. I'm working on a C# windows form application and I have a picturebox and I want to blur the picture of that. I tried a lot of ways but the speed of applying is not good. Its too slow. The code I tried is something like this :

I am converting the picturebox's image to bitmap then getting 3 neighborhoods pixels then getting an average from those colors (using get method) then applying the average color to the pixels using set method.but this code doesn't work correctly.

Please show me a way to apply a fast blur.