Value cannot be null. Parameter name: entitySet

Question

I have a fairly standard setup with simply POCO classes

public class Project
{

    public int ProjectId { get; set; }
    public string Name { get; set; }
          


        

Answer 1

For anyone not finding a resolution in the other answers, I got this error when I created a derived class from a class that had an instance in some model. The exception occurred on the first usage of my context in a request.

This is a stripped-down example that will reproduce the behaviour. Model is a DbSet in my context.

public class Model
{
    public int Id { get; set; }
    public Duration ExposureDuration { get; set; }
}

public class Duration
{
    public int Value { get; set; }
    public string Unit { get; set; }
}

//Adding this will cause the exception to occur.
public class DurationExtended : Duration
{ }

This happened during work in progress. When I changed the model property ExposureDuration to type DurationExtended, all was working again.