Linq 2 SQL - 通用 where 子句

问题描述

有没有办法做到这一点

public T GetItemById(int id)
{
    Table<T> table = _db.GetTable<T>();
    table.Where(t => t.Id == id);
}

请注意，上下文中不存在 i.Id，因为 linq 不知道它正在使用什么对象，而 Id 是表的主键

Note that i.Id does not exist in the context as linq does not know what object it is working with, and Id is the primary key of the table

推荐答案

(移除了绑定到属性的方法)

(removed approach bound to attributes)

这是元模型方式(因此它适用于映射文件以及属性对象):

edit: and here's the meta-model way (so it works with mapping files as well as attributed objects):

static TEntity Get<TEntity>(this DataContext ctx, int key) where TEntity : class
{
    return Get<TEntity, int>(ctx, key);
}
static TEntity Get<TEntity, TKey>(this DataContext ctx, TKey key) where TEntity : class
{
    var table = ctx.GetTable<TEntity>();
    var pkProp = (from member in ctx.Mapping.GetMetaType(typeof(TEntity)).DataMembers
                  where member.IsPrimaryKey
                  select member.Member).Single();
    ParameterExpression param = Expression.Parameter(typeof(TEntity), "x");
    MemberExpression memberExp;
    switch (pkProp.MemberType)
    {
        case MemberTypes.Field: memberExp = Expression.Field(param, (FieldInfo)pkProp); break;
        case MemberTypes.Property: memberExp = Expression.Property(param, (PropertyInfo)pkProp); break;
        default: throw new NotSupportedException("Invalid primary key member: " + pkProp.Name);
    }
    Expression body = Expression.Equal(
        memberExp, Expression.Constant(key, typeof(TKey)));
    var predicate = Expression.Lambda<Func<TEntity, bool>>(body, param);
    return table.Single(predicate);
}