c# infix to postfix using stack method

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C#

I have managed to solve the problem in converting infix to postfix (using stack) but
it can only handle one digit at a time. so it means i am using character to scan it
from left to right.

Please help me or give me some tricks on how i can make my code handle two digits or more.
Example:

This has only one digit every operand
input infix is : 2+3*6/7

This has two or more digit every operand.
input infix is : 23+345*6/32

In my code:
I convert the input string into an array of characters
input.toCharArray();
and scan the the array from left to right.

string input = txtInput.Text;
char[] toArray = new char[input.Length];
toArray = input.ToCharArray();

Creating method to evaluate each character[Example code]:

public void Evaluate(char[] array)
{

for (int i = 0; i < array.Length; i++)
{

char current = array[i];
switch (current)
{
case '+':
case '-':
Process(1, current);
break;
case '*':
case '/':
Process(2, current);
break;
case '(':
st.Push(current);
break;
case ')':
while (!this.isEmpty())
{
char pop =(char) st.Pop();
if (pop == '(')

break;
else
postString += pop;
}
break;
default:
if (char.IsLetterOrDigit(current))
{
postString += current.ToString();
}
break;
}

}
try
{
while (!this.isEmpty())
{
char pop = (char)st.Pop();
MessageBox.Show("Popped left: " + pop);
postString += pop;
}
}
catch
{
}



==========================Thank You==================================