Question
For the following, I want to create expression trees using the API:
var managers = dataContext.Employees.Where(e => e.Subordinates.Any());
How can I create the expression tree to do this as well?
var managedEmployees = managers.ToDictionary(key => key.Manager, value => value.Subordinates.Select(s => s.FullName));
So far, I've thought of the following for the. However, Where() fails because it dislikes the type arguments innew 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 understand. The parameters of type onAny
and Where
should beEmployee
, notIQueryable<Employee>
or IEnumerable<Employee>
due to the fact that it only searches for type parameters and not real types. I think you also need anExpression.Constant(dataContext.Employees)
as opposed to straightdataContext.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 dialAny
in yourMemberExpression
Why you ought to 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());
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