C#属性(Attribute)用法实例解析

[AttributeUsage(AttributeTargets.All)]
  public class TestAttribute : Attribute
  {
  }
  [TestAttribute]//结构
  public struct TestStruct { }
  
  [TestAttribute]//枚举
  public enum TestEnum { }


  [TestAttribute]//类上
  public class TestClass
  {
    [TestAttribute]
    public TestClass() { }
    
    [TestAttribute]//字段
    private string _testField;

    [TestAttribute]//属性
    public string TestProperty { get; set; }

    [TestAttribute]//方法上
    [return: TestAttribute]//定义返回值的写法
    public string TestMethod([TestAttribute] string testParam)//参数上
    {
      throw new NotImplementedException();
    }
  }

[AttributeUsage(AttributeTargets.Property)]
public class StringLengthAttribute : Attribute
{
  private int _maximumLength;
  public StringLengthAttribute(int maximumLength)
  {
    _maximumLength = maximumLength;
  }

  public int MaximumLength
  {
    get { return _maximumLength; }
  }
}

public class People
{
  [StringLength(8)]
  public string Name { get; set; }

  [StringLength(15)]
  public string Description { get; set; }
}

public class ValidationModel
{

  public void Validate(object obj)
  {
    var t = obj.GetType();

    //由于我们只在Property设置了Attibute,所以先获取Property
    var properties = t.GetProperties();
    foreach (var property in properties)
    {

      //这里只做一个stringlength的验证，这里如果要做很多验证，需要好好设计一下,千万不要用if elseif去链接
      //会非常难于维护，类似这样的开源项目很多，有兴趣可以去看源码。
      if (!property.IsDefined(typeof(StringLengthAttribute), false)) continue;

      var attributes = property.GetCustomAttributes();
      foreach (var attribute in attributes)
      {
        //这里的MaximumLength 最好用常量去做
        var maxinumLength = (int)attribute.GetType().
          GetProperty("MaximumLength").
          GetValue(attribute);

        //获取属性的值
        var propertyValue = property.GetValue(obj) as string;
        if (propertyValue == null)
          throw new Exception("exception info");//这里可以自定义，也可以用具体系统异常类

        if (propertyValue.Length > maxinumLength)
          throw new Exception(string.Format("属性{0}的值{1}的长度超过了{2}", property.Name, propertyValue, maxinumLength));
      }
    }

  }
}

private static void Main(string[] args)
{
    var people = new People()
    {
      Name = "qweasdzxcasdqweasdzxc",
      Description = "description"
    };
    try
    {
      new ValidationModel().Validate(people);
    }
    catch (Exception ex)
    {
      Console.WriteLine(ex.Message);
    }
    Console.ReadLine();
}