LINQ Expression Tree

c# expression-trees linq

Question

I want to generate expression trees using the API for the following:

var managers = dataContext.Employees.Where(e => e.Subordinates.Any());

Additionally, how do I then generate the expression tree to do this:

var managedEmployees = managers.ToDictionary(key => key.Manager, value => value.Subordinates.Select(s => s.FullName));

I've come up with the following so far for the .Where(), but it errors because it doesn't like the type parameters in new Type[] { typeof(Func<Employee, IEnumerable<Employee>>) }.

ParameterExpression employeesParameter = Expression.Parameter(typeof(Employee), "e");
MemberExpression subordinatesProperty = Expression.Property(employeesParameter, typeof(Employee).GetProperty("Subordinates"));
MethodCallExpression hasSubordinates = Expression.Call(typeof(Enumerable),
  "Any",
  new Type[] { typeof(Employee) },
  subordinatesProperty);
LambdaExpression whereLambda = Expression.Lambda(hasSubordinates, employeesParameter);
MethodCallExpression whereExpression = Expression.Call(typeof(Queryable),
  "Where",
  new Type[] { typeof(Func<Employee, IEnumerable<Employee>>) },
  dataContext.Employees.AsQueryable(),
  whereLambda);

Accepted Answer

I got this. The type parameters on Any and Where need to be Employee, not IQueryable<Employee> or IEnumerable<Employee> because it's just looking for the type parameters, not the actual types. I believe you also need an Expression.Constant(dataContext.Employees) instead of straight dataContext.Employees.

ParameterExpression employeesParameter = Expression.Parameter(typeof(Employee), "e");
MemberExpression subordinatesProperty = Expression.Property(employeesParameter, typeof(Employee).GetProperty("Subordinates"));

MethodCallExpression hasSubordinates = Expression.Call(typeof(Enumerable),
    "Any",
    new Type[] { typeof(Employee) },
    subordinatesProperty);
LambdaExpression whereLambda = Expression.Lambda(hasSubordinates, employeesParameter);
MethodCallExpression whereExpression = Expression.Call(typeof(Queryable),
    "Where",
    new Type[] { typeof(Employee) },
    Expression.Constant(dataContext.Employees),
    whereLambda);

Popular Answer

To call Any for your MemberExpression you should do this

ParameterExpression employeesParameter = Expression.Parameter(typeof(Employee), "e");
MemberExpression subordinatesProperty = Expression.Property(employeesParameter, typeof(Employee).GetProperty("Subordinates"));

var mi = typeof(Enumerable).GetMethods().First(x => x.Name == "Any" && x.GetParameters().Length == 1);
mi = mi.MakeGenericMethod(typeof (bool));
var hasSubordinates = Expression.Call(mi, subordinatesProperty);

and Where

var lambda = Expression.Lambda<Func<Employee, bool>>(hasSubordinates, employeesParameter);
var res = i.Where(lambda.Compile());


Licensed under: CC-BY-SA with attribution
Not affiliated with Stack Overflow
Is this KB legal? Yes, learn why
Licensed under: CC-BY-SA with attribution
Not affiliated with Stack Overflow
Is this KB legal? Yes, learn why