编写CompareTo DataAnnotation属性 [英] Writing a CompareTo DataAnnotation Attribute

问题描述

我遇到一种情况需要与字段进行比较（例如，确保开始时间早于结束时间）。我正在使用 System.ComponentModel.DataAnnotations 属性进行验证。

I have a situation where I want to compare to fields (example, ensuring the start time is before the end time). I'm using the System.ComponentModel.DataAnnotations attributes for my validation.

我的第一个想法是这样的：

My first thought was something like this:

public enum CompareToOperation
{
    EqualTo,
    LessThan,
    GreaterThan
}

public class CompareToAttribute : ValidationAttribute
{
    CompareToOperation _Operation;
    IComparable _Comparision;

    public CompareToAttribute(CompareToOperation operation, Func<IComparable> comparison)
    {
       _Operation = operation;
       _Comparision = comparison();
    }

    public override bool IsValid(object value)
    {
    if (!(value is IComparable))
        return false;

    switch (_Operation)
    {
        case CompareToOperation.EqualTo: return _Comparision.Equals(value);
        case CompareToOperation.GreaterThan: return _Comparision.CompareTo(value) == 1;
        case CompareToOperation.LessThan: return _Comparision.CompareTo(value) == -1;
    }

    return false;
    }
}

public class SimpleClass
{
   public DateTime Start {get;set;}
   [CompareTo(CompareToOperation.GreaterThan, () => this.Start)] // error here
   public DateTime End {get;set;}
}

但是这不起作用，在编译器错误中标记了属性：

This doesn't work however, there's a compiler error where the attribute is marked:

Expression cannot contain anonymous methods or lambda expressions

有人对此有解决方案吗？还是与另一字段的值相比，用于验证一个字段的另一种方法？

Does anyone have a solution for this? Or a different approach for validating one field compared to the value of another?

推荐答案

一个非常丑陋不那么灵活的方法是将其放在类上并使用反射。我还没有测试过，所以我实际上不确定它是否可以工作，但是它确实可以编译：）

A very ugly way that's not nearly as flexible is to put it on the class and use reflection. I haven't tested this, so I'm not actually sure it works, but it does compile :)

public enum CompareToOperation
{
    EqualTo,
    LessThan,
    GreaterThan
}

public class CompareToAttribute : ValidationAttribute
{
    CompareToOperation _Operation;
    string _ComparisionPropertyName1;
    string _ComparisionPropertyName2;

    public CompareToAttribute(CompareToOperation operation, string comparisonPropertyName1, string comparisonPropertyName2)
    {
        _Operation = operation;
        _ComparisionPropertyName1 = comparisonPropertyName1;
        _ComparisionPropertyName2 = comparisonPropertyName2;
    }

    private static IComparable GetComparablePropertyValue(object obj, string propertyName)
    {
        if (obj == null) return null;
        var type = obj.GetType();
        var propertyInfo = type.GetProperty(propertyName);
        if (propertyInfo == null) return null;
        return propertyInfo.GetValue(obj, null) as IComparable;
    }

    public override bool IsValid(object value)
    {
        var comp1 = GetComparablePropertyValue(value, _ComparisionPropertyName1);
        var comp2 = GetComparablePropertyValue(value, _ComparisionPropertyName2);

        if (comp1 == null && comp2 == null)
            return true;

        if (comp1 == null || comp2 == null)
            return false;

        var result = comp1.CompareTo(comp2);

        switch (_Operation)
        {
            case CompareToOperation.LessThan: return result == -1;
            case CompareToOperation.EqualTo: return result == 0;
            case CompareToOperation.GreaterThan: return result == 1;
            default: return false;
        }
    }
}

[CompareTo(CompareToOperation.LessThan, "Start", "End")]
public class SimpleClass
{
    public DateTime Start { get; set; }
    public DateTime End { get; set; }
}

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