dynamicquery - 以 System.Linq.Dynamic 的方式动态调用 SelectMany

Question

在 System.Linq.Dynamic 中，有几种方法可以动态形成 Select、Where 和其他 Linq 语句。但是 SelectMany 没有。

Select的方法如下:

    public static IQueryable Select(this IQueryable source, string selector, params object[] values)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (selector == null) throw new ArgumentNullException("selector");
        LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, null, selector, values);
        IQueryable result = source.Provider.CreateQuery(
            Expression.Call(
                typeof(Queryable), "Select",
                new Type[] { source.ElementType, lambda.Body.Type },
                source.Expression, Expression.Quote(lambda)));

        return result;
    }

我试图修改上面的代码，经过几个小时的工作，我找不到出路。

欢迎提出任何建议。

莹

Answer 1

已经为我们的项目实现了这个，让我知道它是否适合你!

public static IQueryable SelectMany(this IQueryable source, string selector, params object[] values)
{
    if (source == null) 
        throw new ArgumentNullException("source");
    if (selector == null) 
        throw new ArgumentNullException("selector");

    // Parse the lambda
    LambdaExpression lambda = 
        DynamicExpression.ParseLambda(source.ElementType, null, selector, values);

    // Fix lambda by recreating to be of correct Func<> type in case 
    // the expression parsed to something other than IEnumerable<T>.
    // For instance, a expression evaluating to List<T> would result 
    // in a lambda of type Func<T, List<T>> when we need one of type
    // an Func<T, IEnumerable<T> in order to call SelectMany().
    Type inputType = source.Expression.Type.GetGenericArguments()[0];
    Type resultType = lambda.Body.Type.GetGenericArguments()[0];
    Type enumerableType = typeof(IEnumerable<>).MakeGenericType(resultType);
    Type delegateType = typeof(Func<,>).MakeGenericType(inputType, enumerableType);
    lambda = Expression.Lambda(delegateType, lambda.Body, lambda.Parameters);

    // Create the new query
    return source.Provider.CreateQuery(
        Expression.Call(
            typeof(Queryable), "SelectMany",
            new Type[] { source.ElementType, resultType },
            source.Expression, Expression.Quote(lambda)));
}