以System.Linq.Dynamic的方式动态调用SelectMany [英] To call SelectMany dynamically in the way of System.Linq.Dynamic

问题描述

在System.Linq.Dynamic中，有一些方法可以动态形成Select，Where和其他Linq语句.但是SelectMany没有.

In System.Linq.Dynamic, there are a few methods to form Select, Where and other Linq statements dynamically. But there is no for SelectMany.

Select的方法如下:

The method for Select is as the following:

    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，在工作了几个小时之后，我找不到出路.

I tried to modify the above code, after hours working, I couldn't find a way out.

欢迎提出任何建议.

Y

推荐答案

已经为我们的项目实现了这一功能，请让我知道它是否对您有用！

Already implemented this one for our project, let me know if it works for you!

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)));
}

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