Returning null for generic type extension in c#

Question

I have created an extension function, which takes the string as input and check for the value and based on the Generic Type, cast it to the destination type and return it, it's working well.

Now the problem is if i pass the input value as empty it should return null, for nullable types, but it simply throws exception.

For e.g.: if i want to cast it to datetime, it throws below exception:

String was not recognized as a valid DateTime

Below is my code:

public static class Extension
    {
        public static T ToNull<T>(this string value)
        {
            var stringType = "System.Nullable`1[[System.{0}, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]";

            if (typeof(T) == typeof(String))
                stringType = string.Format(stringType, "String");
            if (typeof(T) == typeof(Int32?) || typeof(T) == typeof(Int32))
                stringType = string.Format(stringType, "Int32");
            if (typeof(T) == typeof(DateTime?) || typeof(T) == typeof(DateTime))
                stringType = string.Format(stringType, "DateTime");
            if (typeof(T) == typeof(Int64?) || typeof(T) == typeof(Int64))
                stringType = string.Format(stringType, "Int64");


            Type originalType = Type.GetType(stringType);
            var underlyingType = Nullable.GetUnderlyingType(originalType);
            return (T)Convert.ChangeType(value, underlyingType ?? originalType);
        }
    }

and here how i am accessing it:

string s = "";
DateTime? t = s.ToNull<DateTime?>();
Console.WriteLine(t);

For above case i want to return null.

Answer 1

Then just return default value

//Check for empty or null first
if(string.IsNullOrEmpty(value)) return default(T);

//Then your code
var stringType = "System.Nullable`1[[System.{0}, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]";
...

Answer 2

To answer @mjwills reply about creating separate methods, i found a solution to keep it single method and still handle any types you throw to it, dynamically.

public static class Extension
{
    public static T ToNull<T>(this string value)
    {
        if (String.IsNullOrEmpty(value)) return default(T);
        Type originalType = typeof(T);
        var underlyingType = Nullable.GetUnderlyingType(originalType);
        return (T)Convert.ChangeType(value, underlyingType ?? originalType);
    }
}