从字典返回通用类型（否则返回默认通用值）

Question

I'm trying to pass a key to a dictionary (of objects) and get the value out (which could be various types) or fallback to a default value I supplied.

For example

// Called from some other method
// It should look in the dictionary for that key and return the value
// else return the int 365
GetValueFromSetting("numDaysInYear", 365)

public static T GetValueFromSettings<T>(string key, T defaultValue)
{
    // Settings is a dictionary, which I get in json form
    Dictionary<string, object> settingsDictionary = (Dictionary<string, object>)ParseConfig.CurrentConfig.Get<Dictionary<string, object>>("settings");

    if(settingsDictionary.ContainsKey(key))
    {
        return settingsDictionary[key];
    }

    return defaultValue;
}   

Firstly I was getting. Cannot implicitly convert type object to T. An explicit conversion exists (are you missing a cast?)

So I cast the key return with

return (T)settingsDictionary[key];

This got rid of the compile error but I have InvalidCastExpections. For instance in the json numbers are stored as 35.0 (which would be a double) and if I call:

GetValueFromSettings("someOffset", 32.0f);

I'll get an InvalidCastExpection when it finds the key in the json as 32.0 and tries to convert to a float.

I also tried using a generic instead of object:

public static T GetValueFromSettings<T>(string key, T defaultValue)
{
    // Settings is a dictionary, which I get in json form
    Dictionary<string, T> settingsDictionary = (Dictionary<string, T>)ParseConfig.CurrentConfig.Get<Dictionary<string, T>>("settings");

    if(settingsDictionary.ContainsKey(key))
    {
        return settingsDictionary[key];
    }

    return defaultValue;
}   

In the hope that it would fix it but that also results in an invalid cast exception. This time it's on the dictionary though as the json expects a type of Dictionary.

I've also seen System.Convert.ChangeType() but again no luck.

Any help would be appreciated.

Answer 1

What you're seeing (in the first case) is that you can't unbox from int to float . What you're seeing when casting the dictionary itself is that a Dictionary<string, object> isn't a Dictionary<string, float> , which seems entirely reasonable to me.

You might want to use:

// I don't *expect* that you need a cast herem, given the type argument
var settingsDictionary = ParseConfig.CurrentConfig.Get<Dictionary<string, object>>("settings");
object value;
if (!settingsDictionary.TryGetValue(key, out value))
{
    return defaultValue;
}
object converted = Convert.ChangeType(value, typeof(T));
return (T) converted;

That will handle far more conversions - but it will throw an exception if there's no suitable conversion available.