将IQueryable <>类型对象转换为List <T>类型？

Question

I have IQueryable<> object.

I want to Convert it into List<> with selected columns like new { ID = s.ID, Name = s.Name } .

Edited

Marc you are absolutely right!

but I have only access to FindByAll() Method (because of my architecture).

And it gives me whole object in IQueryable<>

And I have strict requirement( for creating json object for select tag) to have only list<> type with two fields.

Answer 1

Then just Select :

var list = source.Select(s=>new { ID = s.ID, Name = s.Name }).ToList();

(edit) Actually - the names could be inferred in this case, so you could use:

var list = source.Select(s=>new { s.ID, s.Name }).ToList();

which saves a few electrons...

Answer 2

Add the following:

using System.Linq

...and call ToList() on the IQueryable<> .

Answer 3

The List class's constructor can convert an IQueryable for you:

public static List<TResult> ToList<TResult>(this IQueryable source)
{
    return new List<TResult>(source);
}

or you can just convert it without the extension method, of course:

var list = new List<T>(queryable);

Answer 4

System.Linq has ToList() on IQueryable<> and IEnumerable<>. It will cause a full pass through the data to put it into a list, though. You loose your deferred invoke when you do this. Not a big deal if it is the consumer of the data.

Answer 5

Here's a couple of extension methods I've jury-rigged together to convert IQueryables and IEnumerables from one type to another (ie DTO). It's mainly used to convert from a larger type (ie the type of the row in the database that has unneeded fields) to a smaller one.

The positive sides of this approach are:

• it requires almost no code to use - a simple call to .Transform <DtoType> () is all you need
• it works just like .Select(s=>new{...}) ie when used with IQueryable it produces the optimal SQL code, excluding Type1 fields that DtoType doesn't have.

LinqHelper.cs:

public static IQueryable<TResult> Transform<TResult>(this IQueryable source)
{
    var resultType = typeof(TResult);
    var resultProperties = resultType.GetProperties().Where(p => p.CanWrite);

    ParameterExpression s = Expression.Parameter(source.ElementType, "s");

    var memberBindings =
        resultProperties.Select(p =>
            Expression.Bind(typeof(TResult).GetMember(p.Name)[0], Expression.Property(s, p.Name))).OfType<MemberBinding>();

    Expression memberInit = Expression.MemberInit(
        Expression.New(typeof(TResult)),
        memberBindings
        );

    var memberInitLambda = Expression.Lambda(memberInit, s);

    var typeArgs = new[]
        {
            source.ElementType, 
            memberInit.Type
        };

    var mc = Expression.Call(typeof(Queryable), "Select", typeArgs, source.Expression, memberInitLambda);

    var query = source.Provider.CreateQuery<TResult>(mc);

    return query;
}

public static IEnumerable<TResult> Transform<TResult>(this IEnumerable source)
{
    return source.AsQueryable().Transform<TResult>();
}