How can I convert to a specific type in a generic version of TryParse()?

By : vdh_ant
Source: Stackoverflow.com
Question!

I have the following scenario where I want to pass in string and a generic type:

public class Worker {
    public void DoSomeWork<T>(string value) 
        where T : struct, IComparable<T>, IEquatable<T> { ... }
}

At some point along the way I need to convert the string value to its T value. But I don't want to do a straight convert as I need to perform some logic if the string cannot be converted to type T.

I was thinking that I could try using Convert.ChangeType() but this has the problem that if it doesn't convert it will throw an exception and I will be running the DoSomeWork() method often enough to not have to rely on a try/catch to determine whether the convert is valid.

So this got me thinking, I know that I will be working with numeric types, hence T will be any of the following: int, uint, short, ushort, long, ulong, byte, sbyte, decimal, float, double. Knowing this I thought that it might be possible to come up with a faster solution working with the fact that I know I will be using numeric types (note if T isn't a numeric type I throw an exception)...

public class NumericWorker {
    public void DoSomeWork<T>(string value) 
        where T : struct, IComparable<T>, IEquatable<T> 
    { 
        ParseDelegate<T> tryConverter = 
           SafeConvert.RetreiveNumericTryParseDelegate<T>();
        ... 
    }
}


public class SafeConvert
{
    public delegate bool ParseDelegate<T>(string value, out T result);

    public static ParseDelegate<T> RetreiveNumericTryParseDelegate<T>()
        where T : struct, IComparable<T>, IEquatable<T>
    {
        ParseDelegate<T> tryParseDelegate = null;

        if (typeof(T) == typeof(int))
        {
           tryParseDelegate = (string v, out T t) =>
              {
                 int typedValue; 
                 bool result = int.TryParse(v, out typedValue);
                 t = result ? (T)typedValue : default(T); 
                 //(T)Convert.ChangeType(typedValue, typeof(T)) : default(T);
                 return result;
              }; 
        }
        else if (typeof(T) == typeof(uint)) { ... }
        else if (typeof(T) == typeof(short)) { ... }
        else if (typeof(T) == typeof(ushort)) { ... }
        else if (typeof(T) == typeof(long)) { ... }
        else if (typeof(T) == typeof(ulong)) { ... }
        else if (typeof(T) == typeof(byte)) { ... }
        else if (typeof(T) == typeof(sbyte)) { ... }
        else if (typeof(T) == typeof(decimal)) { ... }
        else if (typeof(T) == typeof(float)) { ... }
        else if (typeof(T) == typeof(double)) { ... }

        return tryParseDelegate;
    }
}

But the above has the problem that I can't write t = result ? (T)typedValue : default(T); as the casting of typedValue to T causes issues and the only way I have been able to get around it thus far is by writing (T)Convert.ChangeType(typedValue, typeof(T)). But if I do this I am just doing another convert.

Hence I was wondering if anyone knows how I could fix this problem (if you think doing the ChangeType() is a problem) or if there is a better solution altogether that I haven't considered.

By : vdh_ant


Answers

Why not just use reflection and use the built in TryParse methods? Pretty much one for every native type with the exception of Guid.

public static Parser


You can try something simple

    public static T ConvertValue


ToType being the generic parameter here. This works for nullable types, just in case you needed it. You can extract your main method to be a generic converter, that will convert to any type, including nullables.

    ToType result = default(ToType);    

    result = ChangeType
By : Stan R.


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