Enumerable.Where中的参数

由闹心*发布于 2020-05-09 23:20:32

我想搜索所有为空或不为空的记录。我可以这样做（示例代码）：

public ICollection<Employees> SearchForPhoneNumberNull()
{
    return collection.Where(x => x.PhoneNumber == null);
}

public ICollection<Employees> SearchForPhoneNumberNotNull()
{
    return collection.Where(x => x.PhoneNumber != null);
}

Can i somehow connect these two methods, and for example send some variable that will decide is it != or ==?

Ida

2020-05-09 23:20:32

当然，像这样：

public ICollection<Employees> SearchForPhoneNumber(bool nullOnly)
{
    return collection.Where(x => nullOnly ? x.PhoneNumber == null : x.PhoneNumber != null);
}

Burne

2020-05-09 23:20:32

对于处理空电话号码和非空电话号码的单个功能，我将实现扩展功能。

static class EmployeesExtensions
{
    public static ICollection<Employees> SearchForPhoneNumber(
        this ICollection<Employees> collection, 
        Func<Employees, bool> predicate)
    {
        return collection.Where(predicate).ToList();
    }
}

这里有一些重要的事情要注意。

Extension functions must be in a static class (i.e., static class EmployeesExtensions
this ICollection<Employees> collection
- Defines the type of object on which the extension function can be called.
- this enables the function to be used as an extension function.
Func<Employees, bool> predicate
- Allows passing a lambda expression to the extension function.
  - Takes an Employees object as a parameter
  - Returns a bool (i.e., True/False).
- Example: e => e.PhoneNumber == null
- passed to .Where() by name, predicate
.ToList() is needed to convert the IEnumerable<Employees> to ICollection<Employees>. However, this forces the evaluation of the LINQ expression tree that you are building. So, rather than returning a composable expression tree, it returns the actual Employees objects.

因此，既然有了扩展方法，就可以在程序中使用它了。

class Program
{
    static void Main(string[] args)
    {
        var employees = CreateEmployeesCollection();

        Console.WriteLine("Employees with Phone Number");

        foreach (var e in employees.SearchForPhoneNumber(e => e.PhoneNumber != null))
        {
            Console.WriteLine($"ID: {e.Id}; Name: {e.Name}; Phone: {e.PhoneNumber}");
        }

        Console.WriteLine();
        Console.WriteLine("Employees with No Phone Number");

        foreach (var e in employees.SearchForPhoneNumber(e => e.PhoneNumber == null))
        {
            Console.WriteLine($"ID: {e.Id}; Name: {e.Name}; Phone: null");
        }
    }

    private static ICollection<Employees> CreateEmployeesCollection()
    {
        var employees = new Collection<Employees>();

        employees.Add(new Employees { Id = 1, Name = "John Doe", PhoneNumber = null });
        employees.Add(new Employees { Id = 2, Name = "Jane Doe", PhoneNumber = "212-555-1212" });
        employees.Add(new Employees { Id = 3, Name = "Mike Smith", PhoneNumber = "312-555-1213" });
        employees.Add(new Employees { Id = 4, Name = "Mary Smith", PhoneNumber = "312-555-1214" });
        employees.Add(new Employees { Id = 5, Name = "Bob Jones", PhoneNumber = "214-555-1215" });
        employees.Add(new Employees { Id = 5, Name = "Beth Jones", PhoneNumber = null });

        return employees;
    }
}

哦，同样，在这个例子中，我假设Employees类的以下定义

// File: Employees.cs
class Employees
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string PhoneNumber { get; set; }
}

心痛算什么

2020-05-09 23:20:32

You can use bool argument to verify if the phone number should be null or not.

Also the Where method returns IEnumerable, there is no explicit converstion to ICollection, thus I changed the method's declaration

        public IEnumerable<Employees> SearchForPhoneNumber(bool isNull = true)
        {
            return collection.Where(x => isNull ? x.PhoneNumber == null : x.PhoneNumber != null);
        }

闹心*

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