类型转换与通用扩展方法不匹配

Question

I am attempting to hide a whole set of legacy methods behind a single extension method using generics. These legacy methods are all called GetValidXXX and have a similar signature (and yes they should really be out not ref ). The old GetValidXXX need to be kept for backwards compatibility.

    public static T GetAttributeValue<T>(this DbElement element, DbAttribute attribute, T defaultValue)
    {
        T result = default(T);
        if (typeof(T) == typeof(DbAttribute))
        {
            if (element.GetValidAttribute(attribute, ref result)) return result;
        }
        else if (typeof(T) == typeof(bool))
        {
            if (element.GetValidBool(attribute, ref result)) return result;
        }

        return defaultValue;
    }

This will not compile because result does not match the type in the specific GetValidXXX signature (return values is success/fail).

bool GetValidAttribute(DbAttribute attribute, ref DbAttribute result)
bool GetValidBool(DbAttribute attribute, ref bool result)
etc

How can I write this to achieve what I am aiming for ie to be able to write code that looks like this:

string description = element.GetAttributeValue(DbAttributeInstance.DESC, "unset");
bool isWritable = !element.GetAttributeValue(DbAttributeInstance.READONLY, true);

Answer 1

You can't use T for your ref parameters because the compiler cannot always guarantee it will be of those types. You would have to do something like this:

public static T GetAttributeValue<T>(this DbElement element, DbAttribute attribute, T defaultValue)
{
    if (typeof(T) == typeof(DbAttribute))
    {
        var dbAttribute = default(DbAttribute);
        if (element.GetValidAttribute(attribute, ref dbAttribute)) return (T)(object)dbAttribute;
    }
    else if (typeof(T) == typeof(bool))
    {
        var boolResult = default(bool);
        if (element.GetValidBool(attribute, ref boolResult)) return (T)(object)boolResult;
    }

    return defaultValue;
}

Answer 2

Convert.ChangeType() might be useful in your case.

Possible usage:

    public static T ConvertTypeOrGetDefault<T>(this object value, T defaultValue)
    {
        try
        {
            return (T)Convert.ChangeType(value, typeof(T));
        }
        catch (Exception ex)
        {
            return default(T);
        }
    }

It depends on how "hacky" you're willing to be. You also could consider re-factoring so that you don't have to hide the legacy methods.