C# Convert Typed list to Generics

Question

C# Convert Typed list to Generics

Born2Achieve 21

Hello,

I am using vs2022 and C# language and oracle as backend. oracle procedure gets input as facility type and based on type it returns the list of data and each facility type have common columns with few additional columns per type. on the below code, MaptoList is an extension class method which maps the reader column and value to class. the below logic is not working and says cannot convert from non generic list to generic. please do the needful to solve the compile error.

public class Common  
{  
  
        public string Name { get; set; }  
        public string Address { get; set; }  
        public string State{ get; set; }  
  
}  
  
  
 public class Employee : Common  
    {  
        public string SSN{ get; set; }  
    }  
  
public class Vendor : Common  
    {  
          
	public string TaxNumber{ get; set; }  
    }  
  
  
  
 public async Task<List<T>> GetFacility(string Facility_Type)  
        {  
            using (EMPDB dbConnect = new EMPDB())  
            {  
                DbCommand dbCommand = dbConnect.Database.GetDbConnection().CreateCommand();  
                dbCommand.CommandType = CommandType.StoredProcedure;  
                dbCommand.CommandText = "GetFacilityReport";  
                dbCommand.BuildSqlParameter("Facility_Type", OracleDbType.Varchar2, Facility_Type, ParameterDirection.Input);   
                List<T> lstFacility_Response= new List<T>();  
                if (dbCommand.Connection.State != ConnectionState.Open)  
                {  
                    dbCommand.Connection.Open();  
                }  
                using (var reader = await dbCommand.ExecuteReaderAsync())  
                {  
		   switch (Facility_Type)  
                    {	  
			case "Employee":  
			lstFacility_Response= reader.MapToList<Employee>();  
			break;  
  
			case "Vendor":  
			lstFacility_Response= reader.MapToList<Vendor>();  
			break;  
                      
		   }  
                }  
                 
            }  
 return lstFacility_Response  
        }

Karen Payne MVP 35,591 Reputation points Volunteer Moderator

2022-12-20T10:03:00.153+00:00

Thought I would share what was asked on Stackoverflow

https://stackoverflow.com/questions/74858037/c-sharp-convert-list-to-generic-list

2 answers

Your answer

Karen Payne MVP 35,591 Reputation points Volunteer Moderator

2022-12-20T10:03:00.153+00:00

Thought I would share what was asked on Stackoverflow

https://stackoverflow.com/questions/74858037/c-sharp-convert-list-to-generic-list

Answer 1

Karen Payne MVP 35,591 Volunteer Moderator

Seem like you can do this which is slimmed down and using my classes GetFacility<Customers>();

public static List<T> GetFacility<T>()  
{  
    Type type = typeof(T);  
  
    if (type == typeof(Contacts))  
    {  
        Console.WriteLine("Do work for Contact");   
    }  
    else if (type == typeof(Customers))  
    {  
        Console.WriteLine("Do work for Customer");  
    }  
    else if (type == typeof(Products))  
    {  
        Console.WriteLine("Do work for Product");  
    }  
  
  
    return Array.Empty<T>().ToList();  
  
}

Born2Achieve 21 Reputation points

2022-12-19T22:18:57.377+00:00

Thanks karen for the reply and on your example, you are suggesting based on the "type of" add the logic. but when i return, that's where i am getting the issue explained in my original thread. Also, line 19 on your example, you are returning empty list. i am confused about it. please help me to solve the issue.
Karen Payne MVP 35,591 Reputation points Volunteer Moderator

2022-12-20T10:02:11.62+00:00

The empty list is so the code compiles, its simply a rough idea, incomplete. The original idea came from a similar method I did with Entity Framework Core which I was going to rework for you but lack the time.

Answer 2

Bruce (SqlWork.com) 81,976 Volunteer Moderator

just like your mapper, you should use the type rather string value:

    public async Task<List<T>> GetFacility<T>() where T: Common  
    {  
         ...  
  
           
         return reader.MapToList<T>();  
    }

Born2Achieve 21 Reputation points

2022-12-19T22:21:34.737+00:00

thank you bruce for the reply and how can use T:common. the return class would be List<Employee> or List<Vendor>. i don't want to create two separate methods.
Bruce (SqlWork.com) 81,976 Reputation points Volunteer Moderator

2022-12-19T23:49:45.003+00:00
you could have GetFacility return List<Common> but then you would need to cast the elements, as you can not cast a generic list

public async Task<List<Common>> GetFacility<T>() where T: Common { ... var list = new List<Common>() list.AddRange(reader.MapToList<T>()); return list; }

which has little benefit, as the caller must cast each element, rather than getting a list of known type, so you could pass the string instead.
Born2Achieve 21 Reputation points

2022-12-20T02:44:01.303+00:00

Hi Bruce,

Thanks for the reply and i cannot use List<Common> as return type. i need to return as generic list because the return type could be List<Employee> or List<Vendor> based on the Facility_Type.

Born2Achieve 21

Tried as below and not working

    public async Task<List<T>> GetFacility<T>(string Facility_Type)  
             {  
                 using (EMPDB dbConnect = new EMPDB())  
                 {  
                     DbCommand dbCommand = dbConnect.Database.GetDbConnection().CreateCommand();  
                     dbCommand.CommandType = CommandType.StoredProcedure;  
                     dbCommand.CommandText = "GetFacilityReport";  
                     dbCommand.BuildSqlParameter("Facility_Type", OracleDbType.Varchar2, Facility_Type, ParameterDirection.Input);   
                     List<T> lstFacility_Response= new List<T>();  
                     if (dbCommand.Connection.State != ConnectionState.Open)  
                     {  
                         dbCommand.Connection.Open();  
                     }  
                     using (var reader = await dbCommand.ExecuteReaderAsync())  
                     {  
                 lstFacility_Response= reader.MapToList<T>();          
                     }  
                         
                 }  
      		return lstFacility_Response;  
             }

Born2Achieve 21

       public static List < T > MapToList < T > (this DbDataReader dr) where T: new() {  
         List < T > RetVal = null;  
         var Entity = typeof (T);  
         var PropDict = new Dictionary < string,  
           PropertyInfo > ();  
         try {  
           if (dr != null && dr.HasRows) {  
             RetVal = new List < T > ();  
             var Props = Entity.GetProperties(BindingFlags.Instance | BindingFlags.Public);  
             PropDict = Props.ToDictionary(p => p.Name.ToUpper(), p => p);  
             while (dr.Read()) {  
               T newObject = new T();  
               for (int Index = 0; Index < dr.FieldCount; Index++) {  
                 if (PropDict.ContainsKey(dr.GetName(Index).ToUpper())) {  
                   var Info = PropDict[dr.GetName(Index).ToUpper()];  
                   if ((Info != null) && Info.CanWrite) {  
                     var Val = dr.GetValue(Index);  
                     Info.SetValue(newObject, (Val == DBNull.Value) ? null : Val, null);  
                   }  
                 }  
               }  
               RetVal.Add(newObject);  
             }  
           }  
         } catch (Exception) {  
           throw;  
         }  
         return RetVal;  
       }  
       }

Getting error as : The type 'typename' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'parameter' in the generic type or method 'generic'

Viorel 125.7K Reputation points

2022-12-20T10:44:18.097+00:00

Did you try public async Task<List<T>> GetFacility<T>(string Facility_Type) where T : new()?

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C# Convert Typed list to Generics

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