Dynamic LINQ aggregates on IQueryable as a single query

Question

I\'m building a datagrid library that works on generic IQueryable data sources. At the bottom selected columns will have aggregates: sum, average, count etc.

I can c

Answer 1

here is my solution for sum, average and min, max .. this is what i have used in one of the projects.

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

        // Properties
        PropertyInfo property = source.ElementType.GetProperty(member);
        ParameterExpression parameter = Expression.Parameter(source.ElementType, "s");

        // We've tried to find an expression of the type Expression>,
        // which is expressed as ( (TSource s) => s.Price );

        Type propertyType = property.PropertyType;
        Type convertPropType = property.PropertyType;
        if (function == "Sum")//convert int to bigint
        {
            if (propertyType == typeof(Int32))
                convertPropType = typeof(Int64);
            else if (propertyType == typeof(Int32?))
                convertPropType = typeof(Int64?);
        }
        Expression selector = Expression.Lambda(Expression.Convert(Expression.MakeMemberAccess(parameter, property), convertPropType), parameter);
        //var methods = typeof(Queryable).GetMethods().Where(x => x.Name == function);
        // Method
        MethodInfo aggregateMethod = typeof(Queryable).GetMethods().SingleOrDefault(
            m => m.Name == function
                && m.IsGenericMethod
                && m.GetParameters().Length == 2 && m.GetParameters()[1].ParameterType.GenericTypeArguments[0].GenericTypeArguments[1] == convertPropType);// very hacky but works :)

        MethodCallExpression callExpr;
        // Sum, Average
        if (aggregateMethod != null)
        {
            callExpr = Expression.Call(
                    null,
                    aggregateMethod.MakeGenericMethod(new[] { source.ElementType }),
                    new[] { source.Expression, Expression.Quote(selector) });
            return source.Provider.Execute(callExpr);
        }
        // Min, Max
        else
        {
            aggregateMethod = typeof(Queryable).GetMethods().SingleOrDefault(
                m => m.Name == function
                    && m.GetGenericArguments().Length == 2
                    && m.IsGenericMethod);
            if (aggregateMethod != null)
            {
                callExpr = Expression.Call(
                    null,
                    aggregateMethod.MakeGenericMethod(new[] { source.ElementType, propertyType }),
                    new[] { source.Expression, Expression.Quote(selector) });

                return source.Provider.Execute(callExpr);
            }                
        }
        return null;
    }