Error Message: "Cannot convert type 'string?' to 'string' "

Question

I have this code:

//Return null if the extension doesn't have the value, returns the value if it does.
private T? getValue<T>(IEnumerable<Extension> extension, string attributeName)
{
    IEnumerable<Extension> ext = extension.Where(e => e.attributeName == attributeName);
    if (ext.Count() > 0)
    {
        return (T)ext.First().Attribute;
    }
    return null;
}

I'm calling it like:

//This works:
retVal.byteValue= getValueFromExtension<byte>(u, "ByteToGet") ?? 0;
//This doesn't work:
getValueFromExtension<string>(u, "Text") ?? "";

I get the compile error: "Error Message: "Cannot convert type 'string?' to 'string' "

How can I do effectively the idea in the code above without creating a new method?

I feel like I'm checking if it's null with the ?? operator, so, if the string is null, it will always be set to an empty string. It is handled how I expect for byte and int, why not for string?

FYI, the byteValue above, is of type byte, not byte?.

Answer 1

It seems you want null if it is a reference type and 0 if it is a number or similar value type. You can simply use the default keyword to get such a value from T. Also, you might want to add the this keyword to the first argument so that it can be used as an extension method.

private T getValue<T>(this IEnumerable<Extension> extension, string attributeName)  
{  
    Extension ext = extension.SingleOrDefault(e => e.attributeName == attributeName);

    if (ext != null)
        return (T)ext.Attribute;
    else
        return default(T);
}