populate a data transfer object from a related model

I have these two models,

public class PatReg
    {
        public Int64 FileId { get; set; }
        [Required, Display(Name = "First Name")]
        public string FName { get; set; }
        [Required, Display(Name = "Middle Name")]
        public string MName { get; set; }
        [Required, Display(Name = "Last Name")]
        public string LName { get; set; }
        [Display(Name = "Full Name"), NotMapped]
        public string fullname
        {
            get { return FName + " " + MName + " " + LName; }
        }
        [Required, Display(Name = "Date of Birth")]
        [DataType(DataType.Date)]
        public DateTime Dob { get; set; }
        public ICollection<PatPar> PatPar { get; set; }
    }

    public class PatPar

    {
        [Key]
        public Int64 RecId { get; set; }
        [Display(Name = "Patient File Id"), Required]
        public Int64 FileId { set; get; }
        [Display(Name = "Partner File Id"), Required]
        public Int64 ParFileId { set; get; }
        [Display(Name = "Start Date"), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true), Required]
        public DateTime SDate { set; get; }
        [Display(Name = "End Date"), DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
        public DateTime? EDate { set; get; }
    }

Their JSON output looks like,

{"sdata": {"fileId": 1708010001,"fName": "Json","mName": "S","lName": "Makenzi","fullname": "Json S Abu Makenzi","dob": "1984-04-26T00:00:00","patPar": [{"recId": 2,"fileId": 1708010001,"parFileId": 1708010002,"sDate": "1999-12-12T00:00:00","eDate": null,
        }, {"recId": 3,"fileId": 1708010001,"parFileId": 1708010003,"sDate": "1955-12-14T00:00:00","eDate": null,
        }]
    }
}

I m trying to get the output to include the fullname in PatPar. The property exists in PatReg and can be acquired  parFileId - FileId

so I have this DTO

public class PatParDto

        {
            public long RecId { get; set; }
            public long FileId { get; set; }
            public long ParFileId { get; set; }
            public DateTime SDate { get; set; }
            public DateTime? EDate { get; set; }
            public string FullName { get; set; }
        }

and inside my controller I tried all the following but failed,

 public async Task<IActionResult> GetPatReg([FromRoute] long id)
        {

            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }
            var PatientInfo = await _context.PatReg.Where(i => i.FileId == id).ToListAsync();


            var PartnerInfo = await _context.PatPar.Select(m => new PatParDto {
                RecId = m.RecId,
                FileId = m.FileId,
                ParFileId = m.ParFileId,
                SDate = m.SDate,
                EDate = m.EDate,
                FullName = _context.PatReg.Where(z=>z.FileId==m.ParFileId).Select(x=>x.fullname).ToString()
            }).ToListAsync();

            if (PatientInfo == null)
            {
                return NotFound();
            }

            var DataRes = new {
                sdata = PatientInfo,
            };

            return Ok(DataRes);
        }

how can I do it?

my desired JSON should look like 

{"sdata": {"fileId": 1708010001,"fName": "Json","mName": "S","lName": "Makenzi","fullname": "Json S Abu Makenzi","dob": "1984-04-26T00:00:00","patPar": [{"recId": 2,"fullname": "Sarah S Maz","fileId": 1708010001,"parFileId": 1708010002,"sDate": "1999-12-12T00:00:00","eDate": null,
        }, {"recId": 3,"fullname": "Mosi Maz","fileId": 1708010001,"parFileId": 1708010003,"sDate": "1955-12-14T00:00:00","eDate": null,
        }]
    }
}