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

I got this error when I declared a variable of type Type - which is probably because is a complex type not supported by the DB.

When I changed it to string, the error went away

public class Sample
{
  public int SampleID {get;set;}
  public Type TypeInfo {get; set;} //This caused the error, 
                                   //because Type is not directly convertible 
                                  //in to a SQL datatype
}

Answer 2

I had the same issue and the cause was a POCO class that had a property of type Type.

Answer 3

This problem can occur if one of the POCO classes was not declared in the DbContext.

I added them and the error went away

I had changed the name of the Task POCO class because of its association with a built in .NET name System.Threading.Tasks. However I had not changed this in the "TaskTimeLog" POCO where there was a relation. When going through the code the "Task" property in the "TaskTimeLog" POCO was not showing an error because it was now attached to that threading keyword and the reason I had changed the name in the first place.

Answer 4

In my case I had to reference another model class called IanaTimeZone, but instead of

public virtual IanaTimeZone Timezone { get; set; }

out of rush I typed this:

public virtual TimeZone Timezone { get; set; }

and it compiled fine because VS thought it was System.TimeZone but EF6 was throwing the error. Stupid thing but took me a while to figure out, so maybe this will help someone.

Answer 5

Remove the line <Generator>EntityModelCodeGenerator</Generator> from your project file.

Check out this https://www.c-sharpcorner.com/UploadFile/5d065a/poco-classes-in-entity-framework/

Answer 6

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.