从 Lambda 属性表达式获取自定义属性

问题描述

我正在使用 ASP.NET MVC 2 Preview 2 并编写了自定义 HtmlHelper 扩展方法来使用表达式创建标签.TModel 来自一个具有属性的简单类，并且这些属性可能具有定义验证要求的属性.我试图找出表达式在我的标签方法中表示的属性上是否存在某个属性.

I am using ASP.NET MVC 2 Preview 2 and have written a custom HtmlHelper extension method to create a label using an expression. The TModel is from a simple class with properties and the properties may have attributes to define validation requirements. I am trying to find out if a certain attribute exists on the property the expression represents in my label method.

类和标签的代码是:

public class MyViewModel
{
    [Required]
    public string MyProperty { get; set; }
}

public static MvcHtmlString Label<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string label)
{
    return MvcHtmlString.Create(string.Concat("<label for="", expression.GetInputName(), "">", label, "</label>"));
}

public static string GetInputName<TModel, TProperty>(this Expression<Func<TModel, TProperty>> expression)
{
    return expression.Body.ToString().Substring(expression.Parameters[0].Name.Length + 1);
}

然后我会这样称呼标签:

Then I would call the label like this:

Html.Label(x => x.MyProperty, "My Label")

有没有办法判断传递给Label方法的表达式值中的属性是否有Required属性?

Is there a way to find out if the property in the expression value passed to the Label method has the Required attribute?

我发现执行以下操作确实可以获取属性(如果存在)，但我希望有一种更简洁的方法来完成此操作.

I figured out that doing the following does get me the attribute if it exists, but I am hopeful there is a cleaner way to accomplish this.

public static MvcHtmlString Label<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, string label)
{
    System.Attribute.GetCustomAttribute(Expression.Property(Expression.Parameter(expression.Parameters[0].Type, expression.GetInputName()), expression.GetInputName()).Member, typeof(RequiredAttribute))

    return MvcHtmlString.Create(string.Concat("<label for="", expression.GetInputName(), "">", label, "</label>"));
}

推荐答案

您的表达式解析逻辑可能需要做一些工作.您不是处理实际类型，而是转换为字符串.

Your expression parsing logic could use some work. Rather than deal with the actual types, you are converting to strings.

这里有一组您可以使用的扩展方法.第一个获取成员的名称.第二个/第三个组合检查属性是否在成员上.GetAttribute 将返回请求的属性或 null，而 IsRequired 仅检查该特定属性.

Here is a set of extension methods that you might use instead. The first gets the name of the member. The second/third combine to check if the attribute is on the member. GetAttribute will return the requested attribute or null, and the IsRequired just checks for that specific attribute.

public static class ExpressionHelpers
{
    public static string MemberName<T, V>(this Expression<Func<T, V>> expression)
    {
        var memberExpression = expression.Body as MemberExpression;
        if (memberExpression == null)
            throw new InvalidOperationException("Expression must be a member expression");

        return memberExpression.Member.Name;
    }

    public static T GetAttribute<T>(this ICustomAttributeProvider provider) 
        where T : Attribute
    {
        var attributes = provider.GetCustomAttributes(typeof(T), true);
        return attributes.Length > 0 ? attributes[0] as T : null;
    }

    public static bool IsRequired<T, V>(this Expression<Func<T, V>> expression)
    {
        var memberExpression = expression.Body as MemberExpression;
        if (memberExpression == null)
            throw new InvalidOperationException("Expression must be a member expression");

        return memberExpression.Member.GetAttribute<RequiredAttribute>() != null;
    }
}

希望这对您有所帮助.

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