我知道如何构建一个简单的Lambda表达式,例如 x => x > 5:
int[] nbs = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
IEnumerable<int> result1 = nbs.Where(x => x > 5);
ParameterExpression parameter = Expression.Parameter(typeof(int), "x");
ConstantExpression constant = Expression.Constant(5);
BinaryExpression expressionBody = Expression.GreaterThan(parameter, constant);
Expression<Func<int, bool>> expression = Expression.Lambda<Func<int, bool>>(expressionBody, parameter);
IEnumerable<int> result2 = nbs.Where(expression.Compile());
但是,我该如何像上面那样创建一个更复杂的Lambda表达式,比如 p=>p.FindAttribute("Gender")?.Value == "Female"?
public class Person
{
public bool POI { get; set; } = false;
public string Name { get; set; }
public List<Car> Cars { get; set; }
public List<Attribute> Attributes { get; set; }
public bool PersonOfInterest()
{
return POI;
}
public Attribute FindAttribute(string name)
{
return Attributes.FirstOrDefault(a => a.Name == name);
}
}
public class Attribute
{
public string Name { get; set; }
public string Value { get; set; }
public Attribute(string name, string value) { Name = name; Value = value; }
}
public class Car
{
public string Make { get; set; }
public int Horsepowers { get; set; }
public string Fuel { get; set; }
}
Person p1 = new Person();
p1.Name = "Thom";
p1.POI = true;
p1.Attributes = new List<Attribute>() {new Attribute("Length", "Tall"), new Attribute("Hair", "Long hair")};
p1.Cars = new List<Car>()
{
new Car(){Horsepowers = 100, Make = "Toyota", Fuel = "Diesel"},
new Car(){Horsepowers = 200, Make = "Fiat", Fuel = "Diesel"},
new Car(){Horsepowers = 300, Make = "Audi", Fuel = "Diesel"},
new Car(){Horsepowers = 150, Make = "Ferrari", Fuel = "Petrol"}
};
Person p2 = new Person();
p2.POI = false;
p2.Attributes = new List<Attribute>() { new Attribute("Nationality", "English"), new Attribute("Gender", "Female") };
p2.Name = "Sophie";
p2.Cars = new List<Car>()
{
new Car(){Horsepowers = 500, Make = "McLaren", Fuel = "Diesel"},
new Car(){Horsepowers = 200, Make = "Volvo", Fuel = "Diesel"},
new Car(){Horsepowers = 300, Make = "Audi", Fuel = "Diesel"},
new Car(){Horsepowers = 400, Make = "Ferrari", Fuel = "Diesel"}
};
IEnumerable<Person> res = persons.Where(p=>p.FindAttribute("Gender")?.Value == "Female");
当然,我总是可以在Person类上创建一个新方法,比如:
public bool HasAttributeWithValue(string name, string value)
{
return FindAttribute(name)?.Value == value;
}
但如果能够动态构建复杂的 lambda 表达式,对我来说仍然非常有趣!