convert and save image with 300 dpi c#?

It depends on the type if image, but I must say that DPI is just a meta-data item which affects almost nothing. Some applications is it to render the image in "default resolution", which is mostly applied to print function.

Normally, it does not effect rendering of the image at all. If a pixel-to-pixel rendering is needed, the application simply uses pixel dimension of an image and ignore DPI (which can contradict the task as actual DPI is apparently a function of this dimension and the physical pixel size of the device which is usually not even known to the rendering function, it is not needed); in other cases the application resamples image to any size.

—SA