How to switch to iframe using Id - webdriver

Question

I am newbie to selenium..
I do have a home page which has create account. If i click Create Account which pops up me to enter Email address and password and submit button.
I have used below one navigate to first iframe.
driver.switchTo().frame(0);
It is working perfectly for me.
On click of submit button on iframe 1, there is another iframe which shows me to enter firstName, lastName, gender, and One time passsword.
I am using below snippet to navigate to the iframe and to enter firstName. it is not working
driver.switchTo().defaultContent();
driver.switchTo().frame("login-iframe");
driver.findElement(By.xpath("//input[#id='fname']")).sendKeys("Test");
Can any one tell me is it correct or not?? I have tried setting back to defaultContent and not setting to defaultContent() both..
Source code here:
<iframe id="login-iframe" style="width: 800px;height: 530px;">
</iframe></div></div>
<div id="user-verify-modal" class="md-modal md-effect-10">
<div class="md-content">
<!-- a.close(href="#")-->
<div class="tab-links">
<ul>
<li data-type="" class="hidden active">
Complete your details to create XXX Cash Wallet
<span class="arrow arrow-filled">
</span>
</li>
</ul>
<div class="logo pull-right">
</div>
<div class="clear">
</div>
</div>
<div class="tab-content">
<div id="new-wallet" class="tab-pane active">
<div class="pull-left border-rt">
<div class="row">
<div class="first">
<label>First name </label>
<input id="fname" type="text" value="" name="fname" maxlength="25"/>
<span class="blankfirstName">Enter your First Name</span>
</div>
<div>
<label>Last name</label>
<input id="lname" type="text" value="" name="lname" maxlength="25"/>
</div>
</div>
<div class="row radiobuttons">
<label><input id="male" type="radio" name="gender" value="Male"/>Male</label>
<label><input id="female" type="radio" name="gender" value="Female"/>Female</label>
</div>
<div class="row emailrow"><em class="verified"><i></i>Verified</em>
<input id="userEmail" type="text" value="" name="userEmail"/>
<span class="blankemail">Enter your Email where we will send you a verification link once you proceed</span>
<span class="invalidemail">Enter a valid Email where we will send you a verification link once you proceed</span>
<span class="invalidEmailEntered">The entered Email is invalid. Please enter a valid Email where we will send you a verification link once you procced.</span>
<span class="duplicateEmail">We already have a XXX account with this Email. Enter a different Email or Sign in using this.</span>
<span class="errorEmail">Sorry, there's been an error sending mail.</span>
</div>
<div class="row changeEmailContent">We have sent a verification link to <b> </b>. Please verify the same once you proceed or Change Email
</div>
<div class="row mobilerow"><em class="verified"><i></i>Verified</em><label>Mobile number </label>
<input id="userNumber" type="text" value="" name="userNumber" maxlength="10"/>
<span class="blankNumber">Enter your Mobile</span><span class="invalidNumber">Enter a valid Mobile</span>
<span class="invalidNumberEntered">The entered Mobile is incorrect, update this to verify the new Mobile</span>
<span class="duplicateNumber">This Mobile exists in other xXXX accounts. Update this and to verify the new Mobile</span>
<span class="errorNumber">Sorry, there's been an error sending OTP.</span>
</div>
<div class="row changeMobileContent">Enter One Time Password (OTP) sent to your Mobile <b> </b> or Change Mobile</div><div class="row enterOTP">Resend OTP
<label>Enter OTP</label>
<input id="enterOTP" type="text" name="enterOTP" value="" maxlength="6"/>
<span class="error"></span>
<span class="otpsent">One Time Password (OTP) has been resent. If you still do not receive the OTP within 15 minutes, please contact us at care#xxx.com</span>
</div>
<div style="margin-bottom:0px;" class="row"><div class="first"><button id="skipVerifyDetails" type="submit" class="btn btn-primary">Skip</button>
</div>
<div><button id="confirmVerifyDetails" type="submit" class="btn btn-primary">Confirm</button></div>

Answer 1

From your html source, it looks like input with id fname is NOT part of iframe with name login-iframe because it ends and the input is below that in the default content.
<iframe id="login-iframe" style="width: 800px;height: 530px;">
</iframe>
So I think you should search for the input within defaultContent. Did you try this?
driver.switchTo().defaultContent();
driver.findElement(By.xpath("//input[#id='fname']")).sendKeys("Test");
On a side note you could use just By.id instead of By.xpath to find the element since it has id.