我可以创建一个采用值类型或引用类型但总是返回可为空类型的通用方法吗

Question

这是我的方法。请注意，我将为泛型参数返回等效的可空类型R：

    public static Nullable<R> GetValue<T, R>(this T a, Expression<Func<T, R>> expression)
        where T : Attribute
        where R : struct
    {
        if (a == null)
            return null;

        PropertyInfo p = GetProperty(expression);
        if (p == null)
            return null;

        return (R)p.GetValue(a, null);
    }

我可以在调用中使用它来获取如下属性的值：

//I don't throw exceptions for invalid or missing calls 
//because I want to chain the calls together:
int maximumLength4 = instance.GetProperty(x => x.ToString())
                            .GetAttribute<StringLengthAttribute>()
                            .GetValue(x => x.MaximumLength)
                            .GetValueOrDefault(50);

我想对字符串使用相同的通用方法：

//I'd like to use the GetValue generic method with strings as well as integers 
string erroMessage = instance.GetProperty(x => x.ToString())
                            .GetAttribute<StringLengthAttribute>()
                            .GetValue(x => x.ErrorMessage);

但它不会编译：

为了在通用类型或方法“ System.Nullable”中将其用作参数“ T”，类型“ R”必须为非空值类型

无法隐式转换类型“字符串”？'串'

我是否可以使用任何技巧在此处获得相同的方法签名，但又获得泛型以将返回类型推断为可以为null的返回类型？

这是一些测试代码，表明它适用于整数值：

//[StringLength(256)]
//public string Name { get; set; }
PropertyInfo info = ReflectionAPI.GetProperty<Organisation, String>(x => x.Name);//not null
StringLengthAttribute attr = info.GetAttribute<StringLengthAttribute>();//not null
int? maximumLength = attr.GetValue(x => x.MaximumLength);//256
int? minimumLength = attr.GetValue(x => x.MinimumLength);//0

PropertyInfo info2 = ReflectionAPI.GetProperty<Organisation, int>(x => x.ID);//not null
StringLengthAttribute attr2 = info2.GetAttribute<StringLengthAttribute>();//null because ID doesn't have the attribute
int? maximumLength2 = attr2.GetValue(x => x.MaximumLength);//null
int? minimumLength2 = attr2.GetValue(x => x.MinimumLength);//null

//I can use the GetProperty extension method on an instance
Organisation instance = (Organisation)null;
PropertyInfo info3 = instance.GetProperty(x => x.ToString());//null because its a method call not a property
StringLengthAttribute attr3 = info3.GetAttribute<StringLengthAttribute>();//null
int? maximumLength3 = attr3.GetValue(x => x.MaximumLength);//null
int? minimumLength3 = attr3.GetValue(x => x.MinimumLength);//null

这是我剩下的ReflectionAPI：

public static class ReflectionAPI
{

    public static Nullable<R> GetValue<T, R>(this T a, Expression<Func<T, R>> expression)
        where T : Attribute
    {
        if (a == null)
            return null;

        PropertyInfo p = GetProperty(expression);
        if (p == null)
            return null;

        return (R)p.GetValue(a, null);
    }

    public static T GetAttribute<T>(this PropertyInfo p) where T : Attribute
    {
        if (p == null)
            return null;

        return p.GetCustomAttributes(false).OfType<T>().LastOrDefault();
    }

    public static PropertyInfo GetProperty<T, R>(Expression<Func<T, R>> expression)
    {
        if (expression == null)
            return null;

        MemberExpression memberExpression = expression.Body as MemberExpression;
        if (memberExpression == null)
            return null;

        return memberExpression.Member as PropertyInfo;
    }
}

Answer 1

不，没有单独的签名可以做到这一点-无法说“的可为null类型R，它R本身就是引用类型，或者Nullable<R>是非可为空值类型”。

您可以有不同的方法，每一个不同的约束上R-但你要么必须提供不同的名称，或使用可怕的黑客来有效的类型参数约束超载。

为了简单起见，我怀疑您基本上应该在这里使用两个不同的方法名称。所以签名：

public static Nullable<R> GetValue<T, R>(this T a, Expression<Func<T, R>> expression)
    where T : Attribute
    where R : struct

public static R GetReference<T, R>(this T a, Expression<Func<T, R>> expression)
    where T : Attribute
    where R : class

两个方面：

• 常规类型参数以开头T，例如TInput和TResult
• 为什么要使用表达式树GetValue呢？为什么不只Func<T, R>执行a 并执行呢？

作为第二个要点的示例：

public static Nullable<R> GetValue<T, R>(this T a, Func<T, R> func)
    where T : Attribute
    where R : struct
{
    if (a == null)
        return null;

    return func(a);
}