Evaluate postfix expression using stack in c#

Csharp program for Evaluate postfix expression using stack. Here more information.

// Include namespace system
using System;
/* 
    Csharp program for 
    Evaluate postfix expression
*/
// Stack Node
public class StackNode
{
	public int operand;
	public StackNode next;
	public StackNode(int operand, StackNode top)
	{
		this.operand = operand;
		this.next = top;
	}
}
public class MyStack
{
	public StackNode top;
	public int count;
	public MyStack()
	{
		this.top = null;
		this.count = 0;
	}
	// Returns the number of element in stack
	public int isSize()
	{
		return this.count;
	}
	public bool isEmpty()
	{
		if (this.isSize() > 0)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Add a new element in stack
	public void push(int value)
	{
		// Make a new stack node
		// And set as top
		this.top = new StackNode(value, this.top);
		// Increase node value
		this.count++;
	}
	// Remove a top element 
	public int pop()
	{
		var value = this.peek();
		if (this.isEmpty() == false)
		{
			this.top = this.top.next;
			// Reduce the size
			this.count--;
		}
		return value;
	}
	// Used to get top element of stack
	public int peek()
	{
		if (!this.isEmpty())
		{
			return this.top.operand;
		}
		else
		{
			return 0;
		}
	}
}
public class Execution
{
	public void evaluate(String e)
	{
		// Get the length of expression
		var size = e.Length;
		var a = 0;
		var b = 0;
		var s = new MyStack();
		var isVaild = true;
		// work with (+,-,/,*,%) operator
		for (var i = 0; i < size && isVaild; i++)
		{
			if (e[i] >= '0' && e[i] <= '9')
			{
				a = (int)(e[i]) - (int)('0');
				s.push(a);
			}
			else if (s.isSize() > 1)
			{
				a = s.pop();
				b = s.pop();
				// Perform arithmetic operations between 2 operands
				if (e[i] == '+')
				{
					s.push(b + a);
				}
				else if (e[i] == '-')
				{
					s.push(b - a);
				}
				else if (e[i] == '*')
				{
					s.push(b * a);
				}
				else if (e[i] == '/')
				{
					s.push((int)(b / a));
				}
				else if (e[i] == '%')
				{
					s.push(b % a);
				}
				else
				{
					// When use other operator
					isVaild = false;
				}
			}
			else if (s.isSize() == 1)
			{
				// Special case  
				// When use +, - at the beginning
				if (e[i] == '-')
				{
					a = s.pop();
					s.push(-a);
				}
				else if (e[i] != '+')
				{
					// When not use  +,-
					isVaild = false;
				}
			}
			else
			{
				isVaild = false;
			}
		}
		if (isVaild == false)
		{
			// Possible case use other operators
			// 1) When using special operators
			// 2) Or expression is invalid
			Console.WriteLine(e + " Invalid expression ");
			return;
		}
		Console.WriteLine(e + " = " + s.pop());
	}
	public static void Main(String[] args)
	{
		var task = new Execution();
		// Execute the following expression
		// (1+2)*(3+5)
		task.evaluate("12+35+*");
		// -((7*2) + (6/2) + 4)
		task.evaluate("72*62/+4+-");
	}
}

Output

12+35+* = 24
72*62/+4+- = -21