Mocking or faking DbEntityEntry or creating a new DbEntityEntry

Just like the other case, what you need is to add an additional level of indirection:

interface ISalesContext
{
    IDbSet<T> GetIDbSet<T>();
    void SetModified(object entity)
}

class SalesContext : DbContext, ISalesContext
{
    public IDbSet<T> GetIDbSet<T>()
    {
        return Set<T>();
    }

    public void SetModified(object entity)
    {
        Entry(entity).State = EntityState.Modified;
    }
}

So, instead of calling the implementation, you just call SetModified.

Found this question when I needed to unit test with Moq, no need for your own interface. I wanted to set specific fields to not modified but the method SetModified can be used with object as well.

DbContext:

public class AppDbContext : DbContext
{   
    ...
    public virtual void SetModified(GuidEntityBase entity)
    {
        Entry(entity).State = EntityState.Modified;
        Entry(entity).Property(x => x.CreatedDate).IsModified = false;
        Entry(entity).Property(x => x.CreatedBy).IsModified = false;
    }
    ...
}

Test:

var mockContext = new Mock<AppDbContext>();
mockContext.Setup(c => c.MyDbSet).Returns(mockMyDbSet.Object);
mockContext.Setup(c => c.SetModified(It.IsAny<GuidEntityBase>()));