How do I make the return type of a method generic?

187
votes

Is there a way to make this method generic so I can return a string, bool, int, or double? Right now, it's returning a string, but if it's able find "true" or "false" as the configuration value, I'd like to return a bool for example.

    public static string ConfigSetting(string settingName)
    {  
         return ConfigurationManager.AppSettings[settingName];
    }
7
c#.netgenericsreturn-type
Is there a way for you to know what type each settings is? – thecoshman
I think the question you really want to ask is "How do I make my application config strongly-typed?" It's been too long since I worked with that to write a proper answer, though. – Simon
Yah, ideally I don't want to have to pass the type into the method. I'm only going to have the 4 types I mentioned. So if "true"/"false" is set, I want this function to return a boolean (without needing to pass it into the method), I can probably combine int and double into just double, and everything else should be a string. What is answered already will work fine, but I need to pass the type every time, which is probably fine. – MacGyver
Your comment sounds like you're asking for a method that will return a strongly-typed bool (or string, or int, or what have you) at runtime based on the actual data retrieved for the setting name key. C# won't do that for you; there's no way you can know the type of that value at compile time. In other words, that's dynamic typing, not static typing. C# can do that for you if you use the dynamic keyword. There's a performance cost for that, but for reading a config file, the performance cost is almost certainly insignificant. – phoog

7 Answers

395
votes

You need to make it a generic method, like this:

public static T ConfigSetting<T>(string settingName)
{  
    return /* code to convert the setting to T... */
}

But the caller will have to specify the type they expect. You could then potentially use Convert.ChangeType, assuming that all the relevant types are supported:

public static T ConfigSetting<T>(string settingName)
{  
    object value = ConfigurationManager.AppSettings[settingName];
    return (T) Convert.ChangeType(value, typeof(T));
}

I'm not entirely convinced that all this is a good idea, mind you...

31
votes

You could use Convert.ChangeType():

public static T ConfigSetting<T>(string settingName)
{
    return (T)Convert.ChangeType(ConfigurationManager.AppSettings[settingName], typeof(T));
}
18
votes

There are many ways of doing this(listed by priority, specific to the OP's problem)

  1. Option 1: Straight approach - Create multiple functions for each type you expect rather than having one generic function.

    public static bool ConfigSettingInt(string settingName)
{  
     return Convert.ToBoolean(ConfigurationManager.AppSettings[settingName]);
}

  2. Option 2: When you don't want to use fancy methods of conversion - Cast the value to object and then to generic type.

    public static T ConfigSetting<T>(string settingName)
{  
     return (T)(object)ConfigurationManager.AppSettings[settingName];
}

    Note - This will throw an error if the cast is not valid(your case). I would not recommend doing this if you are not sure about the type casting, rather go for option 3.

  3. Option 3: Generic with type safety - Create a generic function to handle type conversion.

    public static T ConvertValue<T,U>(U value) where U : IConvertible
{
    return (T)Convert.ChangeType(value, typeof(T));
}

    Note - T is the expected type, note the where constraint here(type of U must be IConvertible to save us from the errors)

7
votes

You have to convert the type of your return value of the method to the Generic type which you pass to the method during calling.

    public static T values<T>()
    {
        Random random = new Random();
        int number = random.Next(1, 4);
        return (T)Convert.ChangeType(number, typeof(T));
    }

You need pass a type that is type casteable for the value you return through that method.

If you would want to return a value which is not type casteable to the generic type you pass, you might have to alter the code or make sure you pass a type that is casteable for the return value of method. So, this approach is not reccomended.

3
votes

Create a function and pass out put parameter as of generic type.

 public static T some_function<T>(T out_put_object /*declare as Output object*/)
    {
        return out_put_object;
    }
0
votes

Please try below code :

public T? GetParsedOrDefaultValue<T>(string valueToParse) where T : struct, IComparable
{
 if(string.EmptyOrNull(valueToParse))return null;
  try
  {
     // return parsed value
     return (T) Convert.ChangeType(valueToParse, typeof(T));
  }
  catch(Exception)
  {
   //default as null value
   return null;
  }
 return null;
}
-1
votes
 private static T[] prepareArray<T>(T[] arrayToCopy, T value)
    {
        Array.Copy(arrayToCopy, 1, arrayToCopy, 0, arrayToCopy.Length - 1);
        arrayToCopy[arrayToCopy.Length - 1] = value;
        return (T[])arrayToCopy;
    }

I was performing this throughout my code and wanted a way to put it into a method. I wanted to share this here because I didn't have to use the Convert.ChangeType for my return value. This may not be a best practice but it worked for me. This method takes in an array of generic type and a value to add to the end of the array. The array is then copied with the first value stripped and the value taken into the method is added to the end of the array. The last thing is that I return the generic array.