Question

I’ve got a method I wish to pass an SqlExpression and connection string to load data for a given type. Problem is I can’t seem to nail down the syntax. My thought is to be able to call a static method like:

OrmLiteConfig.DialectProvider = ServiceStack.OrmLite.MySqlDialect.Provider;
SqlExpressionVisitor<SampleItem> ev = OrmLiteConfig.DialectProvider.ExpressionVisitor<SampleItem>();
var result = SomeClass.Query<SampleItem>(ev, connectionString);

With a method declared in SomeClass as:

public static List<T> Query<T>(SqlExpressionVisitor<T> ev, string connectionString)
{
    IDbConnection conn = connectionString.OpenDbConnection();
    var result = conn.Select<T>(ev);
    return result;
}

However, the way I am calling the Select method gives me the syntax error:

“Error 10 ‘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 “

I am new to generics (obviously).

Answer 1

‘T’ must be a non-abstract type with a public parameterless constructor

So, you need to ensure that T has a parameterless constructor. That simply means applying the following generic constraint:

public static List<T> Query<T>(SqlExpressionVisitor<T> ev, string connectionString)
  where T : new()
{ ... }

Your class likely already meets the conditions, you just need to ensure that someone else can’t pass in a non-constructable type.

The reason for the error is that the Select method you’re calling adds these same constraints. You are essentially maintaining the same requirements for your method.