Can I create a generic method that takes a value type or a reference type but always returns a nullable type

问题

This is my method. Note that I am returning the equivalent nullable type for the generic parameter 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);
    }

I can use it in a call to get the value of an attribute like this:

//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);

I'd like to use the same generic method with strings:

//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);

but it won't compile:

The type 'R' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

Cannot implicitly convert type 'string?' to 'string'

Is there any trick I can use to get the same method signature here and yet get generics to infer the return type as one that can be null?

This is some test code to show that it works for integer values:

//[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

And this is the rest of my 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;
    }
}

回答1:

No, there's no individual signature that can do this - there's no way of saying "the nullable type of R, which is either R itself for a reference type, or Nullable<R> for a non-nullable value type".

You can have different methods, each with a different constraint on R - but then you either have to provide different names, or use horrible hacks to effectively overload by type parameter constraints.

I suspect you should basically have two different method names here, for simplicity. So signatures of:

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

Two asides:

• Conventionally type parameters start with T, e.g. TInput and TResult
• Why are you using expression trees at all for GetValue? Why not just take a Func<T, R> and execute it?

As an example of the second bullet point:

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);
}

来源：https://stackoverflow.com/questions/19571213/can-i-create-a-generic-method-that-takes-a-value-type-or-a-reference-type-but-al