在 LINQ to SQL 中查找或创建对象的通用方法？

很多时候在我的LINQ 到 SQL http://en.wikipedia.org/wiki/Language_Integrated_Query#LINQ_to_SQL_.28formerly_called_DLINQ.29代码中，我需要“查找或创建”一个实体，如下所示：

var invoiceDb = ctx.Invoices.FirstOrDefault(a => a.InvoicerId == InvoicerId &&
                                                 a.Number == invoiceNumber);
if (invoiceDb == null)
{
    invoiceDb = new Invoice();
    invoiceDb.Number = invoiceNumber;
    ctx.Invoices.InsertOnSubmit(invoiceDb);
}

我希望使其成为一种通用方法......有什么好的想法吗？

我想出了这些似乎对我很有效的扩展方法。

    public static T FindOrCreate<T>(this Table<T> table, Func<T, bool> find, Action<T> create) where T : class, new()
    {
        T val = table.FirstOrDefault(find);
        if (val == null)
        {
            val = new T();
            create(val);
            table.InsertOnSubmit(val);
        }
        return val;
    }

    public static T FindOrCreate<T>(this Table<T> table, Func<T, bool> find) where T : class, new()
    {
        return FindOrCreate(table, find, a => { });
    }

它的用法如下：

    var invoiceDb = ctx.Invoices.FindOrCreate(a => a.InvoicerId == InvoicerId &&
                                                     a.Number == invoiceNumber);
    invoiceDb.Number = invoiceNumber;

Or

    var invoiceDb = ctx.Invoices.FindOrCreate(a => a.InvoicerId == InvoicerId &&
                                                     a.Number == invoiceNumber,
                                              a => a.Number = invoiceNumber);