How to do a Sum using Dynamic LINQ

Question

I have the following which works brilliantly with the dynamic linq library:

string where = \"Price < 5\";
string orderby = \"BookID ASC\";
IQueryable

        

Answer 1

I needed to expand Caramiriel's answer to handle fields as well:

    public static object Sum(this IQueryable source, string member)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (member == null) throw new ArgumentNullException("member");

        // Properties
        PropertyInfo property = source.ElementType.GetProperty(member);
        FieldInfo field = source.ElementType.GetField(member);

        ParameterExpression parameter = Expression.Parameter(source.ElementType, "s");
        Expression selector = Expression.Lambda(Expression.MakeMemberAccess(parameter, (MemberInfo)property ?? field), parameter);
        // We've tried to find an expression of the type Expression>,
        // which is expressed as ( (TSource s) => s.Price );

        // Method
        MethodInfo sumMethod = typeof(Queryable).GetMethods().First(
            m => m.Name == "Sum"
                && (property != null || field != null)
                && (property == null || m.ReturnType == property.PropertyType) // should match the type of the property
                && (field == null || m.ReturnType == field.FieldType) // should match the type of the property
                && m.IsGenericMethod);

        return source.Provider.Execute(
            Expression.Call(
                null,
                sumMethod.MakeGenericMethod(new[] { source.ElementType }),
                new[] { source.Expression, Expression.Quote(selector) }));
    }