The Custom Vision service is one of the Azure Cognitive Services that is provided by Microsoft. With custom vision, you can train a model with your own image data. The portal provides a Prediction API for the trained model to classify an image you supply. There can be reasons why you might not want to use the online REST endpoint and you could prefer an offline model. To work with machine learning in .NET, we can use ML.NET. There are several tutorials how to use this framework. But none of them combine the use of the Bitmap class with a downloaded ONNX model from the custom vision portal. In this article I will walk through the steps to set up an application to classify bitmaps using a custom vision model with ML.NET.

When trying to have an application work with an ONNX model I downloaded from the Microsoft Custom Vision portal, I got the following exception: TypeInitializationException: EntryPointNotFoundException: Unable to find an entry point named 'OrtGetApiBase' in DLL 'onnxruntime'. Searching for the error online did not yield any solutions. After solving the problem, I wanted to share the solution for anyone else running into the same exception.