C# Where clause won't return the null values from a nullable bool

Question

I have a nullable bool which contains true / false / null on a SQL server database. I am trying to query the server to return a dataset which contains false FlagForDelete records and another to return null FlagForDelete records. I've tried several things and cannot isolate the two data sets I need for my target condition.

Can someone provide a hint on how I can accomplish this? Thanks very much.

Target condition

Dataset1 to contain only false records.  
Dataset2 to contain only null records.  

I've tried the below code and can't get it to work. I get all records regardless of true/false/null.

Dataset

MaterialID  |   StockQty    |   FlagForDelete
----------------------------------------------
MAT1        |   10          |   true
MAT2        |   20          |   false
MAT3        |   30          |   null

Model

public class Material
{
    public int MaterialID { get; set; } 
    public decimal? StockQty { get; set; }      
    public bool? FlagForDelete { get; set; }     
}

Controller

IQueryable<Material> Dataset1 = _context.Material.AsNoTracking().Where(ffd => ffd.FlagForDelete == false);
IQueryable<Material> Dataset2 = _context.Material.AsNoTracking().Where(ffd => ffd.FlagForDelete == null);

Answer 1

Reproduced this with SQL server.
Added your Material class. Correction to you keys were made.
The HasValue as stated was fine. Also Convert.ToBoolean helped with translation. 
Some options for various results have been provided.
 

       [HttpGet]  
       public IActionResult GetNulls()
       {
           List<Material> mats = new List<Material>
           {
               new Material { MaterialID = 1, StockQty = 10, FlagForDelete = true },
               new Material { MaterialID = 2, StockQty = 20, FlagForDelete = false },
               new Material { MaterialID = 3, StockQty = 30 }
           };
           string Response = "SUCCESS:";
           try
           {
               List<Material> NotFalse = mats.Where(m => m.FlagForDelete != Convert.ToBoolean(false)).ToList();
               List<Material> NotTrue = mats.Where(m => m.FlagForDelete != Convert.ToBoolean(true)).ToList();
               List<Material> NotEither = mats.Where(m => m.FlagForDelete != Convert.ToBoolean(true) &&  m.FlagForDelete != Convert.ToBoolean(false)).ToList();
               List<Material> Lakemann = mats.Where(m => !m.FlagForDelete.HasValue).ToList();
               var combine = new { NotFalse, NotTrue, NotEither , Lakemann };
               var options = new JsonSerializerOptions { WriteIndented = true };
               string json = JsonSerializer.Serialize(combine, options);
               return Ok(json);
           }
           catch (Exception ex)
           {Response = "ERROR: **" + ex.Message;}
           finally
           {};
           return NoContent();
       }

JSON
{
  "NotFalse": [
    {
      "MaterialID": 1,
      "StockQty": 10,
      "FlagForDelete": true
    },
    {
      "MaterialID": 3,
      "StockQty": 30,
      "FlagForDelete": null
    }
  ],
  "NotTrue": [
    {
      "MaterialID": 2,
      "StockQty": 20,
      "FlagForDelete": false
    },
    {
      "MaterialID": 3,
      "StockQty": 30,
      "FlagForDelete": null
    }
  ],
  "NotEither": [
    {
      "MaterialID": 3,
      "StockQty": 30,
      "FlagForDelete": null
    }
  ],
  "Lakemann": [
    {
      "MaterialID": 3,
      "StockQty": 30,
      "FlagForDelete": null
    }
  ]
}

Answer 2

In C# a boolean column that can been null would map to a bool?;

...Where(ffd => ffd.FlagForDelete == false);
...Where(ffd => !ffd.FlagForDelete.HasValue);