Call an Enumerated Average through an expression

Question

I am trying to write dynamic code that performs some aggregations of Average, Sum, Max, etc.

For the im code to execute:

PropertyInfo sortProperty = typeof(T).GetProperty("PropertyName");

ParameterExpression parameter = Expression.Parameter(typeof(T), "p");


MemberExpression propertyAccess = Expression.MakeMemberAccess(parameter, sortProperty);
LambdaExpression orderByExp = Expression.Lambda(propertyAccess, parameter);

var exp = Expression.Lambda<Func<T, int>>(propertyAccess, parameter);

var call = Expression.Call(typeof(Enumerable), "Average", new[] { typeof(IEnumerable<T>) , typeof(Func<T, int>) }, parameter);

and I always get this exception:

There is no universal "Average" method of type "System.Linq.Enumerable" compatible with the arguments and arguments of the supplied type. No type arguments should be provided unless the method is general.

Answer 1

Let's look at this line. Here you callCall

var call = Expression.Call(typeof(Enumerable), "Average", new[] { typeof(IEnumerable<T>) , typeof(Func<T, int>) }, parameter);

: " , ". IEnumerable<T> Func<T, int>, Average (TSource).

" Expression, ". , T, Average IEnumerable<TSource> Func<TSource, decimal> ( , , decimal).

, , , , :

PropertyInfo sortProperty = typeof(T).GetProperty("Prop");
ParameterExpression parameter = Expression.Parameter(typeof(T), "p");
MemberExpression propertyAccess = Expression.MakeMemberAccess(parameter, sortProperty);

// parameter for the source collection
ParameterExpression source = Expression.Parameter(typeof(IEnumerable<T>), "source");

var exp = Expression.Lambda<Func<T, decimal>>(propertyAccess, parameter);
var call = Expression.Call(typeof(Enumerable), "Average", new[] {typeof(T)}, source, exp);

( ):

// assuming a class called T with a decimal property called Prop 
// because I'm a lazy and terrible person
var method = Expression.Lambda<Func<IEnumerable<T>, decimal>>(call, source).Compile();
var result = method(new List<T> {new T { Prop=10}, new T { Prop=20}});
// result is now 15