构造函数限制

Question

我想知道是否有办法限制构造中的值。这是我的代码：

class Student : Human 
{
    private double Grade;

    public Student(string FirstName, string LastName, double Grade)
        : base(FirstName, LastName)
    {
        this.FirstName = FirstName;
        this.LastName = LastName;
        this.Grade = Grade;
    }
}

当我创建一个新学生时，我想将 Grade 限制在 >= 2.00 和

Answer 1

你可以检查它并在运行时抛出一个异常，像这样：

if (grade < 2.00 || grade > 6.00)
    throw new ArgumentOutOfRangeException("grade");

始终将此类条件放在方法或构造函数的开头。我什至把它们放在自己的#region 中（但这是我个人的偏好）：

public Student(string firstName, string lastName, double grade)
    : base(firstName, lastName)
{
    #region Contract
    if (grade < 2.00 || grade > 6.00)
        throw new ArgumentOutOfRangeException("grade");
    #endregion
    this.FirstName = firstName;
    this.LastName = lastName;
    this.Grade = grade;
}

但是，有一种方法可以使用代码合同获得此类事情的编译时警告。您可以下载Code Contracts on MSDN 并在那里您还可以找到文档。它仅与非 Express 版本的 Visual Studio 集成，由 Microsoft 编写。它将检查方法调用是否可能遵守您指定的合同。您的代码将变为：

using System.Diagnotistics.Contracts;

public Student(string firstName, string lastName, double grade)
    : base(firstName, lastName)
{
    #region Contract
    Contract.Requires<ArgumentOutOfRangeException>(grade >= 2.00);
    Contract.Requires<ArgumentOutOfRangeException>(grade <= 6.00);
    #endregion
    this.FirstName = firstName;
    this.LastName = lastName;
    this.Grade = grade;
}

Answer 2

class Student : Human 
{
    private double Grade;

    public Student(string FirstName, string LastName, double Grade)
        : base(FirstName, LastName)
    {
        this.FirstName = FirstName;
        this.LastName = LastName;
        this.Grade = Grade;
        if(Grade >= 2 and Grade <= 6){
            throw new Exception("Incorrect grade input");
    }
}

或者抛出你想要的相关异常，例如ArgumentOutOfException 等...

Answer 3

class Student : Human 
{
    private double Grade;

    public Student(string FirstName, string LastName, double Grade)
        : base(FirstName, LastName)
    {
        if (Grade < 2 || Grade > 6)
            throw new ArgumentOutOfRangeException("Grade must be between 2 and 6");

        this.FirstName = FirstName;
        this.LastName = LastName;
        this.Grade = Grade;
    }
}

Answer 4

public Student(string FirstName, string LastName, double Grade)
    : base(FirstName, LastName)
    {
     if(Grade >= 2.0 || Grade <= 6.00)
      throw  new ArgumentException("your message");     
    }

Answer 5

你可以抛出异常：

class Student : Human 
{
    private double Grade;

    public Student(string FirstName, string LastName, double Grade)
        : base(FirstName, LastName)
    {
        if (Grade >= 2 && Grade <= 6) { 
          throw new ArgumentOutOfRangeException();
        }

        this.FirstName = FirstName;
        this.LastName = LastName;
        this.Grade = Grade;
    }
}

如果您使用Microsoft Code Contracts，您也可以这样做：

class Student : Human  {
    private double Grade;

    public Student(string FirstName, string LastName, double Grade)
        : base(FirstName, LastName)
    {
        System.Diagnotistics.Contracts.Contract.Requires<ArgumentOutOfRangeException>(Grade >= 2 && Grade <= 6);

        this.FirstName = FirstName;
        this.LastName = LastName;
        this.Grade = Grade;
    }
}

Answer 6

如果您希望有任何编译时间检查的机会，您必须使用代码合同。它们本质上是运行时的异常，因此相当于带有异常的参数验证。

public Student(string firstName, string lastName, decimal grade)
{
    Contract.Requires(grade >= 2);
    Contract.Requires(grade <= 6);

    FirstName = firstName;
    LastName = lastName;
    Grade = grade;     
}

看到这个问题

Design by contracts and constructors

Answer 7

还有另一个选项，可能有点复杂，但会确保给定的范围，根本不使用double，而是创建一个名为Grade 的自定义类。您的 Grade 类将包含来自和到 double 的强制转换，并将强制验证。 这意味着所有 Grade 验证逻辑都在 Grade 结构中，而 Student 类只接收一个 Grade 对象而不必担心。

public struct Grade
{
    public static readonly double MinValue = 2.0;
    public static readonly double MaxValue = 6.0;

    private double value;

    public static explicit operator double(Grade grade)
    {
        return grade.value + MinValue;
    }

    public static explicit operator Grade(double gradeValue)
    {
        if (gradeValue < MinValue || gradeValue > MaxValue)
            throw new ArgumentOutOfRangeException("gradeValue", "Grade must be between 2.0 and 6.0");

        return new Grade{ value = gradeValue - MinValue };
    }
}

你会这样使用它：

double userInput = GetUserInputForGrade();
Grade grade = (Grade)userInput; // perform explicit cast.
Student student = new Student(firstName, lastName, grade);

编辑：我已根据@EricLippert 的建议更新了代码，以使该类向开发人员公开其最小值/最大值。

编辑 2： 再次更新 @JeppeStigNielsen 的建议。 value 字段现在存储 MinValue 的偏移量，因此对 default(Grade) 的调用将返回一个有效值（等于 MinValue），无论 0 是否在有效范围内。