Question

我想使用以下API生成表达式树：

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

此外，如何生成表达式树来执行此操作：

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

到目前为止，我已经为.Where（）提出了以下内容，但它错误，因为它不喜欢new Type[] { typeof(Func<Employee, IEnumerable<Employee>>) }中的类型参数。

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);

Answer 1

我得到了这个。 Any和Where上的类型参数需要为Employee，而不是IQueryable<Employee>或IEnumerable<Employee>，因为它只是查找类型参数，而不是实际类型。我相信您还需要Expression.Constant(dataContext.Employees)而不是直接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);

Answer 2

要为Any致电MemberExpression，您应该这样做

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);

和Where

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

LINQ表达式树

2 个答案: