使用反射将字符串转换为 Nullable

如何使用反射将字符串转换为可空值？

我有以下代码可以将几乎任何值转换为几乎任何值类型。上面有相当多的代码使用 IsAssignableFrom 等，所以这是最后的手段。

MethodInfo parse = t.GetMethod("Parse", new Type[] { typeof(string) });

if (parse != null)
{
    object parsed = parse.Invoke(null, new object[] { value.ToString() });
    return (T)parsed;
}
else
{
    throw new InvalidOperationException("The value you specified is not a valid " + typeof(T).ToString());
}

当我想转换为可空类型（如 long?）时，问题就出现了。

显然，长？类没有解析方法。

如何从可空值中提取解析方法template type?

EDIT:

这是我试图通过的一系列简短测试：

[Test]
public void ConverterTNullable()
{
    Assert.That((int?)1, Is.EqualTo(Converter<int?>.Convert(1)));
    Assert.That((int?)2, Is.EqualTo(Converter<int?>.Convert(2.0d)));
    Assert.That(3, Is.EqualTo(Converter<long>.Convert(3)));

    Assert.That((object)null, Is.EqualTo(Converter<long?>.Convert("")));
    Assert.That((object)null, Is.EqualTo(Converter<long?>.Convert(null)));
    Assert.That((object)null, Is.EqualTo(Converter<long?>.Convert(DBNull.Value)));

    Assert.That((long)1, Is.EqualTo(Converter<long?>.Convert("1")));
    Assert.That((long)2, Is.EqualTo(Converter<long?>.Convert(2.0)));
    Assert.That((long?)3, Is.EqualTo(Converter<long>.Convert(3)));
}

以及整个功能：

/// <summary>
/// Converts any compatible object to an instance of T.
/// </summary>
/// <param name="value">The value to convert.</param>
/// <returns>The converted value.</returns>
public static T Convert(object value)
{
    if (value is T)
    {
        return (T)value;
    }

    Type t = typeof(T);

    if (t == typeof(string))
    {
        if (value is DBNull || value == null)
        {
            return (T)(object)null;
        }
        else
        {
            return (T)(object)(value.ToString());
        }
    }
    else
    {
        if (value is DBNull || value == null)
        {
            return default(T);
        }

        if (value is string && string.IsNullOrEmpty((string)value))
        {
            return default(T);
        }

        try
        {
            return (T)value;
        }
        catch (InvalidCastException)
        {
        }

        if (Nullable.GetUnderlyingType(t) != null)
        {
            t = Nullable.GetUnderlyingType(t);
        }

        MethodInfo parse = t.GetMethod("Parse", new Type[] { typeof(string) });

        if (parse != null)
        {
            object parsed = parse.Invoke(null, new object[] { value.ToString() });
            return (T)parsed;
        }
        else
        {
            throw new InvalidOperationException("The value you specified is not a valid " + typeof(T).ToString());
        }
    }
}

I would probably use the TypeConverter in this case, and Nullable.GetUnderlyingType(); example on the way...

    static void Main()
    {
        long? val1 = Parse<long?>("123");
        long? val2 = Parse<long?>(null);
    }

    static T Parse<T>(string value)
    {
        return (T) TypeDescriptor.GetConverter(typeof(T))
            .ConvertFrom(value);
    }