通用C＃类：设置“通用”属性

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 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 2

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 3

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 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...