Entity Framework Code First Exception: Value cannot be null. Parameter name: source

Question

I am working on a Windows Forms Application and I have several sql tables with the same structure but with different names. I wanted to use LINQ so I added Entity Framework to the project using NuGet and I am trying to do this with Entity Framework Code First.

First I setup a class for Timer data. Each property has a matching column in each of the tables.

public class Timer
{
    public int id { get; set; }
    public DateTime Time { get; set; }
    public string Task { get; set; }
    public string ElapsedTime { get; set; }
    public string MachineName { get; set; }
    public int MachineId { get; set; }
    public string RunGuid { get; set; }
    public string Domain { get; set; }
    public string Area { get; set; }
    public double TotalMilliseconds { get; set; }
    public string ReleaseVersion { get; set; }
    public int NavigationProfileId { get; set; }

}
class Timers : IEnumerable<Timer>
{
    public List<Timer> TimerList { get; set; }

    public IEnumerator<Timer> GetEnumerator()
    {
        return TimerList.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return TimerList.GetEnumerator();
    }
}

Then I created a class where I could pass in the name of the table so I could return the table as a collection.

public class TimerContext : DbContext
{
    private readonly string _tableName;

    public TimerContext(string tableName) : base("name=fooDb")
    {
        _tableName = tableName;
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Timer>().ToTable(_tableName);

        base.OnModelCreating(modelBuilder);
    }

    public IEnumerable<Timer> Timers { get; set; }
}

Then I want to be able to create the collections like this.

var currTimers = new TimerContext(currentReleaseTimerTableName).Timers.ToList();
var prevTimers = new TimerContext(previousReleaseTimerTableName).Timers.ToList();

I setup the app.Config file like this.

<connectionStrings>
    <add name="fooDb" providerName="System.Data.sqlclient" connectionString="Data Source=10.0.0.25;Initial Catalog=foo;User ID=foouser;Password=foopass;" />
  </connectionStrings>

I know this connection string works because I've been using it with other SQL commands, but when I try to use Entity Framework I keep getting the error Value cannot be null. Parameter name: source Value cannot be null. Parameter name: source when I call var currTimers = new TimerContext(currentReleaseTimerTableName).Timers.ToList(); .

I know the error says the source is null, but what does it mean by source ? Where am I going wrong?

Answer 1

Timers is null because it wasn't set. It should be of type DbSet(of Timer).