To call SelectMany dynamically in the way of System.Linq.Dynamic

问题

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

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;
    }

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

Any suggestions are welcome.

Ying

回答1:

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)));
}

回答2:

I have added another SelectMany that retuns an AnonymousType.

 public static IQueryable SelectMany(this IQueryable source, string selector, string resultsSelector, 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);

        ParameterExpression[] parameters = new ParameterExpression[] { 
        Expression.Parameter(source.ElementType, "outer"), Expression.Parameter(resultType, "inner") };
        LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, resultsSelector, values);

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

I still need to figure out how to do the following using Dynamic, the goal is to return a new result object.

        var customerandorderflat = db.Customers
            .SelectMany(c => c.Orders.SelectMany(o => o.Order_Details,
                    (ord, orddetail) => new
                        {
                            OrderID = ord.OrderID,
                            UnitPrice = orddetail.UnitPrice
                        }).DefaultIfEmpty(),
                (cus, ord) => new
                {
                    CustomerId = cus.CustomerID,
                    CompanyName = cus.CompanyName,
                    OrderId = ord.OrderID == null ? -1 : ord.OrderID,
                    UnitPrice = ord.UnitPrice
                });

回答3:

I am using the NWDB when I try:

var customerandorderquery = db.Customers .SelectMany(c => c.Orders.DefaultIfEmpty()).Select("new(CustomerId, CompanyName, OrderId)"); 

I get an error because CompanyName is in Customers not Orders. So it is not seeing the combination of the two objects. When I do:

.SelectMany(c => c.Orders.DefaultIfEmpty(), (cus, ord) => new { CustomerId = cus.CustomerID, OrderId = ord.OrderID == null ? -1 : ord.OrderID }); 

It returns the desired result.

来源：https://stackoverflow.com/questions/2983134/to-call-selectmany-dynamically-in-the-way-of-system-linq-dynamic