Generic C# Class: Set "Generic" Property

Question

I'm quite new to C#, so I might have a problem that C# has a simple solution for. I have a generic class with a property of "generic" type. I want to have a function to set that property, but I need to convert it to do so.

public class BIWebServiceResult<T>
{
    public T Data;

    public delegate StatusCode StringToStatusCode(string Input);

    public void SetData(string Input, StringToStatusCode StringToError)
    {
        if (StringToError(Input) == 0)
        {
            if (Data is string[])
            {
                Data = new string[1];
                Data[0] = Input;
            }
            else if (Data is string)
            {
                Data = Input;
            }
            else if (Data is bool)
            {
                Data = DetectBool(Input);
            }
        }
    }

    private bool DetectBool(string Compare)
    {
        return Compare == "true";
    }
}

The problem with that approach is, that it does not work :)

(No that's not all code, just a snippet to show what my problem is)

It doesn't even compile, because "Data = new string[]" can't work if Data is - for example - boolean.

How do I implement a function that behaves differently depending on the type of my generic property?

Answer 1

You could try using the Convert.ChangeType() method:

Convert.ChangeType( input, typeof(T) );

but this will only work for the types that the Convert class is aware of. Conversions to most custom types just will fail with a InvalidCastException.

As a general pratice, this is not a good way to structure a generic class. Generics are meant to unify types based on a common interface. In your case, that common interface is that you expect a conversion from a string representation to the generic type.

If you really need to support conversion of arbitrary input from string to some type T you should provide a separate generic function as a parameter to the type that can perform the conversion. Here's an example:

class BIWebServiceResult<T>
{
    private readonly Func<string,T> m_ValueParser;  

    public BIWebServiceResult( Func<string,T> valueParser )
    {
        m_ValueParser = valueParser;
    }

    public void SetData(string Input, StringToStatusCode StringToError) 
    {
        Data = m_ValueParser( Input );   // use supplied conversion func
        //...
    }
}

Answer 2

An approach that will work for simple types is to use a TypeConverter.

T value = default(T);
TypeConverter converter = TypeDescriptor.GetConverter(typeof(T));
if (converter != null)
{
    if (converter.CanConvertFrom(typeof(string))
    {
        value = (T)converter.ConvertFrom(myString);
    }
}

Answer 3

You want a generic class, but you're changing its behavior based on its generic type argument.

Since this behavior is specialized according to T, you should really make your generic class an abstract base from which to derive specialized subclasses:

public abstract class BIWebServiceResult<T>
{
    public T Data { get; set; }

    public delegate StatusCode StringToStatusCode(string Input);

    public abstract void SetData(string Input, StringToStatusCode StringToError);
}

Then you might have, for example:

public class BIWebServiceStrArrayResult : BIWebServiceResult<string[]>
{
    public override void SetData(string Input, StringToStatusCode StringToError)
    {
        if (StringToError(Input) == 0)
        {
            Data = new string[1];
            Data[0] = Input;
        }
    }
}

Personally, though, I'd be inclined to do away with all this manual string manipulation altogether and leave the job of parsing input to whatever code is calling this method:

// This is the same signature used by, e.g., int.TryParse, double.TryParse, etc.
public delegate bool Parser<T>(string input, out T output);

public void SetData(string Input, Parser<T> parser)
{
    T value;
    if (parser(Input, out value))
        Data = value;
}

By the way, typically it's not really necessary to define your own delegates when the same signature is already available in the form of an Action* or Func*. In the case of your StringToStatusCode, this could simply be defined as a Func<string, StatusCode>. (But I would still personally recommend something like the last bit of code I posted instead.)

Answer 4

Hard to say if this would make much sense in your scenario, but you could perhaps use a child class for each of the possible data types, somewhat like:

public abstract class BIWebServiceResult<T>
    {
        public T Data;

        public delegate void StringToStatusCode(string Input);

        public abstract void SetData(string Input, StringToStatusCode StringToError);
    }


    public class StringBIServiceResult : BIWebServiceResult<string[]>
    {
        public override void SetData(string Input, StringToStatusCode StringToError)
        {

                Data = new string[1];
                Data[0] = Input;
        }

        private bool DetectBool(string Compare)
        {
            return Compare == "true";
        }
    }

this would avoid the casting and using type converters, but might be make your class inheritance chain unduly complex...