通用返回类型和非空对象

Question

I have a Sql Utility class which contains a lot of handy methods around Sql queries. This class contains the following method:

public static T ExecuteScalar<T>(
    string query, 
    SqlConnection connection, 
    params SqlParameter[] parameters) 
    where T : class, new()
{
    SqlCommand command =
        CreateCommand(query, connection, parameters);

    return command.ExecuteScalar() as T;
}

Is it possible to return for example Guid objects or other non-nullable classes.

Something like this:

Guid result = 
    SqlUtils.ExecuteScalar<Guid>(
        @"SELECT [Id] FROM [dbo].[MyTable]
            WHERE [Column1] = @Param1", 
        connection, 
        new SqlParameter("@Param1", "someValue"));

Answer 1

You can use default(T) (and you should remove the generic type constraints):

SqlCommand command = 
    CreateCommand(query, connection, parameters);

object value = command.ExecuteScalar();

if (value == null || value is DbNull)
{
    return default(T)'
}

return (T)value;

Answer 2

Not the way you've written it. The as operator may return null if the cast fails, therefore T has to be of a reference type.

For value types, you'll need to use a conventional cast operator (T) . You'll also need to remove the constraint that T is a reference type on the method definition.

public static T ExecuteScalar<T>(string query, 
                             SqlConnection connection, 
                             params SqlParameter[] parameters) 
{
    SqlCommand command = CreateCommand(query, connection, parameters);
    var result = command.ExecuteScalar();
    if (result is T) return (T)result;
    return default(T);
}

Answer 3

Because the constraints of this method return a class type, you'd be able to return any object that match a class type (non-primitive). Given the nature of your method though, I don't see any need for the constraint. You should be able to remove this constraint and return any type that you can generate.