在方法中使用非抽象类型作为泛型类型的问题

Question

I have this code that I am trying to use in my application:

public partial class DataManager 
{

    public DataManager()
    {
        db2 = DependencyService.Get<ISQLiteDB2>().GetConnection();
    }

    T RunQuery<T>(string qry)
    {
        lock (l)
        {
            try
            {
                T data = db2.Query<T>(qry);
                return data;
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
                Console.WriteLine(qry);
                throw;
            }
        }
    }

However the code is showing this 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.Query(string, params object[])'

Can anyone give me advice on what this means and possibly how I can solve the problem.

Answer 1

You must use a Generic Constraint to satisfy the requirement and let the compiler know that the type passed in will always have a parameterless constructor, add where T: new() to the method definition. Read more on generic constraints here https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters

public partial class DataManager 
{
    public DataManager()
    {
        db2 = DependencyService.Get<ISQLiteDB2>().GetConnection();
    }

    List<T> RunQuery<T>(string qry) where T: new()
    {
        lock (l)
        {
            try
            {
                List<T> data = db2.Query<T>(qry);
                return data;
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
                Console.WriteLine(qry);
                throw;
            }
        }
    }
}