how to determine whether input is string or int

void Add()
{
    int a, b;
    //Check for integer
    if (int.TryParse(textBox1.Text, out a) && int.TryParse(textBox2.Text, out b))
    {
        txtResult.Text = (a + b).ToString();
    }    
    // The value in textboxes are string
    else
    {
        txtResult.Text = textBox1.Text + textBox2.Text
    }
}

txtResult.Text = HandleInput(textbox1.Text, textbox2.Text);

public string HandleInput(string s1, string s2)
{
int i1;
int i2;
if (int.TryParse(s1, out i1) && int.TryParse(s2, out int2))
return (i1 + i2).ToString();
else
return s1 + s2;
}