Unit testing : How to Mock Public Methods in C# with NSubstitute

With NSubstitute, a public method can only be mocked if it is virtual (or abstract).
If the method is public but not virtual, NSubstitute cannot mock it.

The options to enables to mock it are :

1. Make the method virtual

This is the Minimal change and fully supported by NSubstitute

public class ProductService
{
    public virtual int GetPrice()
    {
        return 1;
    }
}

var service = Substitute.For<MyService>();
service.GetValue().Returns(42);

2. Extract an interface (cleanest design)

Refactor for Dependency Injection. Refactor your code so that the logic in the method is moved to a dependency (e.g., a helper/service class) that can be mocked. This is the best option.

public interface IProductService
{
    int GetPrice();
}

Inject the interface and mock it instead.

var service = Substitute.For<IProductService>();
service.GetPrice().Returns(42);

3. Use a wrapper / adapter (for third-party or legacy code)

public interface IProductServiceWrapper
{
    int GetPrice();
}

public class ProductServiceWrapper : IProductServiceWrapper
{
    private readonly ProductService _service;
    public int GetPrice() => _service.GetValue();
}

Mock ProductServiceWrapper