Question

I’m creating a dynamic expression, which will order items in a list by some rule (lambda exp.). This is the code:

Expression<Func<String, String>> exp = o => o;  MethodCallExpression orderByExp = Expression.Call(typeof(Enumerable), 'OrderBy',     new Type[] { typeof(String), exp.Body.Type }, Expression.Parameter(typeof(IEnumerable<String>), 'list'), exp); 

Now I want to execute previously created expression on specific data to sort it, but it fails because of some strange exceptions like ‘Lambda Parameter not in scope’ or ‘Argument expression is not valid’.

var data = new String[] { 'asdasdasd', 'asdads', '123', 'xcvxcvs', 'ASDSD' };  // one of attempts: doesn't work var result = data.AsQueryable().Provider.CreateQuery<String>(orderByExp); 

Can somebody help me with this?

Answer 1

This is the working code:

Expression<Func<String, String>> exp = o => o; var list = Expression.Parameter(typeof(IEnumerable<String>), 'list');  MethodCallExpression orderByExp = Expression.Call(typeof(Enumerable), 'OrderBy',     new Type[] { typeof(String), exp.Body.Type }, list, exp);  var lambda = Expression.Lambda<Func<IEnumerable<String>, IEnumerable<String>>>(orderByExp, list); var data = new String[] { 'asdasdasd', 'asdads', '123', 'xcvxcvs', 'ASDSD' }; var result = lambda.Compile()(data); 

1. To execute the MethodCallExpression you should wrap it in lambda expression.
2. Be sure you use the same instance of parameter expression (‘list’), when creating a MethodCallExpression and LambdaExpression, and not two separate instances even with the same name, otherwise you will get the: ‘Lambda Parameter not in scope’ exception without much explanation.

thanks experts