Media foundation is the recommended technology by Microsoft and it has really supported it by a lot of samples and explanation but all are with the native
The Media Foundation .NET C# wrapper can work well for all those tasks you mentioned. I've used it personally to implement the capture, resizing, and encoding, and I imagine decoding shouldn't be too hard either.
The project does have some examples you can download but I find them rather lacking. The wrapper is so close to the C++ implementation that it's not too hard to use C++ examples and just port the logic to C#. I used the following as a reference and found it to be very valuable: https://github.com/sipsorcery/mediafoundationsamples