Validation Control in C#

You can use Compare Validator Control which allows you to compare the value entered by the user into an input control.

Look some more details: ASP.NET CompareValidator Control[^]

Try this code:

<asp:TextBox ID="txtPassword" runat="server" TextMode="Password" />

<asp:RequiredFieldValidator ID="req6" runat="server" ControlToValidate="txtPassword" Text="*" />

Confirm Password <asp:TextBox ID="txtPasswordC" runat="server" TextMode="Password" />

<asp:CompareValidator runat="server" ID="Comp1" ControlToValidate="txtPassword" ControlToCompare="txtPasswordC" Text="Password mismatch" />

Here is similar discussion on same:
Compare Validator Issue in Change Password Screen[^]
Use CompareValidator to check password field and confirm password field[^]

[EDIT]
Try this:

public bool IsPasswordsEqual(string password1, string password2)
{
	if (password1.Equals(password2))
        {
		return true;
	}

	return false;
}

use this method on submit button click event or confirmpassword textbox leave event
as

if(!IsPasswordsEqual(Textbox1.Text,TextBox2.Text))
{
    MessageBox.Show("Enter same password in both");
}