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

问题描述

我想与字段进行比较(例如，确保开始时间在结束时间之前).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，在标记属性的地方有一个编译器错误:

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

Expression cannot contain anonymous methods or lambda expressions

有没有人有解决方案?或者验证一个字段与另一个字段的值相比的不同方法?

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

一个非常丑陋的方法是将它放在类上并使用反射.我还没有测试过这个，所以我不确定它是否有效，但它确实可以编译 :)

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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