Being an nopCommerce MVP (AKA nopmvp) myself, if often see people asking, repeatedly, about the similar questions. These questions are related to nopCommerce / C# exceptions, and can actually be solved relatively easilly if you know what the error messages mean. Unfortunately, many developers are not trained to properly read C# and ASP.Net exceptions and make sense of them.
So, in this blog post, I'll try to lay out a few common exceptions that nopCommerce developers face, and provide insight on each of them.
1) AutoFac issue - None of the constructors found with...
When implementing your own custom services, you'll often find issues similar to the following. It is taken from a forum thread that I've replied.
None of the constructors found with 'Autofac.Core.Activators.Reflection.DefaultConstructorFinder' on type 'Nop.Plugin.Widgets.DodSlider.Controllers.WidgetDodSliderController' can be invoked with the available services and parameters:
Cannot resolve parameter 'Nop.Services.Localization.LocalizationService localizationService' of constructor 'Void .ctor(Nop.Plugin.Widgets.DodSlider.Services.IDodSliderService, Nop.Services.Localization.LocalizationService, Nop.Core.Plugins.IPluginFinder)'.
The issue is quite obvious - AutoFac, the IOC container / dependency injection manager, was not able to locate and instantiate the required class. It can be due to your custom service not properly registered in DependencyRegistrar, or you have supplied the actual Class (instead of the C# Interface) in the constructor.
2) Object Reference Not Set To Instance Of Object
If there is a "hall of fame" for the common issues .Net developers will face, this will easily take the Number 1 spot. Sadly, as common as this issue appear, many .Net developers doesn't seem to know what it means exactly.
I've replied many times on the forum regarding this issue (such as here), and people seem to keep asking the same question - never endingly!
So that does this error message mean? It means you are trying to use a variable that has not been initialized. Codes like the following will trigger the issue:
MyCustomClass myClass = null;
myClass.ThisProperty = true; // this will throw error, myClass has not been initialized
And the fix is easy (and self explanatory): initialize your variable!
There are many other common problems that are being asked repeatedly on nopCommerce forums. When I get a chance, I'll continue on this series. Stay tuned!