I've found the problem of the demo .net project. some line code should be modified properlly to enable the kernel sending result.

mathKernel.ResultFormat = Wolfram.NETLink.MathKernel.ResultFormatType.TraditionalForm;

and

//resultBox.Text = (string)mathKernel.Result;
this.graphicsBox.Image = (Image)mathKernel.Result;

Then re-compile the demo project and run it.Tthe MathLink source for NETLink .net side should be read through to make best use of it.

C:\Program Files\Wolfram Research\Mathematica\11.2\SystemFiles\Links\NETLink\Source

The code CurrentImage[] works too, but CurrentImage[]//Dynamic not.

I've found the problem of the demo .net project. some lines' codes should be modified properlly to enable the kernel sending result.

mathKernel.ResultFormat = Wolfram.NETLink.MathKernel.ResultFormatType.TraditionalForm;

and

//resultBox.Text = (string)mathKernel.Result;
this.graphicsBox.Image = (Image)mathKernel.Result;

Then re-compile the demo project and run it.The MathLink source for NETLink .net side should be read through to make best use of it.

C:\Program Files\Wolfram Research\Mathematica\11.2\SystemFiles\Links\NETLink\Source

The code CurrentImage[] works too, but CurrentImage[]//Dynamic not.