if ((int)str[i] > 47 && (int)str[i] < 58)

while this works, it rather difficult to read (and depends on ASCII encoding). Better to compare the chars as chars:

if (str[i] >= '0' && str[i] <= '9')

or even better, let the framework do the work:

if (Char.IsDigit(str[i]))

Also, C# knows how to add a char to a string:

num = num + str[i];

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

This solution originally passed when it was submitted but the kata's test cases updated
and this solution now no longer pass, request to you Resubmit the solution.
Thank You.

can I Translation in C#?

https://www.codewars.com/kata/5300901726d12b80e8000498

Yours is actually a simpler version of FizzBuzz (so the dificulty should be ajusted)

(btw you can search for keywords in the kata pages to look for puzzles that might be similar to yours or to find more FizzBuzz puzzles)

Can You send me link of FizzBuzz Kata which you found same ?

Thank You

This is no different from FizzBuzz.