Using a generic type inside another generic in C# class that uses SQLite db in Xamarin

Question

I have a simple data base control handler class.

It uses a SQLiteConnection. I have several tables in my SQLite db.

Now I want to write a generic simple accessor function for all tables of a specific type that use an int Id as primary key. Therefore I have a base class TableWithIntId that always has an int Id as primary key.

My simplified code is:

private SQLiteConnection sqliteConnection;

public T LoadRecord<T>(int id) where T : Database.TableWithIntId
{
    try
    {
        return (from objTable in sqliteConnection.Table<T>()
            where objTable.Id == id
            select objTable).First();
    }
    catch (Exception ex)
    {
        ErrorMessage = ex.Message;
        return null;
    }
}

The problem is, that I get the following error:

error CS0310: 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'SQLiteConnection.Table()'

This is confusing for me, because T is non abstract and has an automatic default constructor.

public class TableWithIntId
{
    [PrimaryKey]
    public int Id { get; set; }
}

You need to specify new() constraint for T in where clause. — Alessandro D'Andria
– Alessandro D'Andria, Commented Dec 18, 2017 at 9:55

René Vogt · Accepted Answer · 2017-12-18 10:05:55Z

3

The constraint where T : Database.TableWithIntId only states that T must derive from Database.TableWithIntId. But derived classes don't necessarily have a parameterless constructor (if you give them only constructors with parameters).

Add a new() constraint to solve that:

public T LoadRecord<T>(int id) where T : Database.TableWithIntId, new()

That restricts T to classes with parameterless constructors.

edited Dec 18, 2017 at 10:05

answered Dec 18, 2017 at 9:57

René Vogt

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Comments

TheGeneral · Accepted Answer · 2017-12-18 10:02:30Z

1

Take a look at new Constraint (C# Reference)

  public T LoadRecord<T>(int id) where T : TableWithIntId, new()
    {
       try
       {
           return (from objTable in sqliteConnection.Table<T>()
                   where objTable.Id == id
                   select objTable).First();
       }
       catch (Exception ex)
       {
           ErrorMessage = ex.Message;
           return null;
       }
   }

answered Dec 18, 2017 at 10:02

TheGeneral

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