Compare two textboxes, donot enter same value in both textboxes

protected void button1_Click(object sender, EventArgs e)
       {
           if (TextBox1.Text.Trim() == "" || TextBox2.Text.Trim() == "")
               lblMessage.Text = "One or more input field is empty,Please enter values";

           if (TextBox1.Text.Trim() == TextBox2.Text.Trim())
               lblMessage.Text = "Please enter different values";

       }

string a =TextBox2.Text;
       string b = TextBox3.Text;
        if (a.Equals(b))
        {
//your first action
}
else {
//your second action
}

Below function will do it for you

function StringCompare()
{
    var string1 = document.getElementById("TextBox1").value;
    var string2 = document.getElementById("TextBox2").value;

    if(string1 == string2)
    {
        alert("Both values are same.");
    }
    else
       {
        alert("Values are different.");
    }
    return false;
}
//in the button event call the javascript
<asp:Button id="Button1" OnClientClick="StringCompare();" runat="server" Text="Button1"/>

if(TextBox1.Text.Equals(TextBox2.Text, StringComparison.Ordinal))
{
  //Don't enter same value
}
else
{
  //Continue your work
}