无法用 lambda 表达式替换字符串 propertyName 参数

Question

我使用以下代码从 DataAnnotations 中获取属性：

public static T GetAttributeFrom<T>(this object instance, string propertyName) where T : Attribute
{
    var attrType = typeof(T);
    var property = instance.GetType().GetProperty(propertyName);
    return (T)property .GetCustomAttributes(attrType, false).First();
}

感谢用户：jgauffin

我想通过传递 lambda 表达式而不是字符串来改进他的解决方案：

public static T GetAttributeFrom<T>(this object instance, Expression<Func<T>> propertyExpression) where T : Attribute
{
    var attrType = typeof(T);
    var property = instance.GetType().GetProperty(GetPropertyNameFromLambdaExpression(propertyExpression));
    return (T)property.GetCustomAttributes(attrType, false).First();
}

以及我从这个表达式中获取真实属性名称的方法：

public static string GetPropertyNameFromLambdaExpression<T>(Expression<Func<T>> propertyAsExpression)
{
    var memberExpression = propertyAsExpression.Body as MemberExpression;
    var propertyInfo = memberExpression.Member as PropertyInfo;
    if (memberExpression != null && propertyInfo != null)
    {
        return memberExpression.Member.Name;
    }
    throw new ArgumentException(propertyAsExpression.ToString());
}

如果我尝试这样的事情：

var id = this.GetAttributeFrom<RangeAttribute>(()=> Id).Maximum;

我收到此错误：

无法将表达式类型“int”转换为返回类型“RangeAttribute”

我该如何解决这个问题？

Answer 1

您同时使用泛型类型 T 作为 attribute 和 property 的类型。尝试从：

public static T GetAttributeFrom<T>(this object instance, string propertyName) where T : Attribute {
    var attrType = typeof(T);
    var property = instance.GetType().GetProperty(propertyName);
    return (T)property.GetCustomAttributes(attrType, false).First();
}

public static T GetAttributeFrom<T, U>(this object instance, Expression<Func<U>> propertyExpression) where T : Attribute {
    var attrType = typeof(T);
    var property = instance.GetType().GetProperty(GetPropertyNameFromLambdaExpression<U>(propertyExpression));
    return (T)property.GetCustomAttributes(attrType, false).First();
}

public static string GetPropertyNameFromLambdaExpression<U>(Expression<Func<U>> propertyAsExpression) {
    var memberExpression = propertyAsExpression.Body as MemberExpression;
    var propertyInfo = memberExpression.Member as PropertyInfo;
    if (memberExpression != null && propertyInfo != null) {
        return memberExpression.Member.Name;
    }
    throw new ArgumentException(propertyAsExpression.ToString());
}

那么你可以这样调用：

var max = this.GetAttributeFrom<RangeAttribute, int>(() => Id).Maximum;

您不能省略第二种类型 (GetAttributeFrom<RangeAttribute>(() => Id))：C# generics can't infer second parameter?。