In this article I will explain with an example, how to implement Google authentication in ASP.Net using C# and VB.Net.
This article makes use of ASPSnippets.GoogleAPI.
 
 

Download ASPSnippets Google API Library

You can download the latest ASPSnippets.GoogleAPI.dll from the following link.
Download DLL file
Note: You will need to add the reference of ASPSnippets.GoogleAPI.dll in your project.
 
 

Getting Google API Client ID and Client Secret

In order to use Google Account API for login, you will need to create an Application in Google Console and get Client ID and Client Secret. For more details, please refer the following article.
Google Developer Console: Generate Client ID and Client Secret for use with Google APIs
 
 

HTML Markup

Following HTML Markup consists of:
Button – For authenticate the user through Google.
The Button has been assigned with an OnClick event handler.
Panel – For displaying the Google Profile details.
The Panel contains Image and Labels.
Image – For displaying Profile picture.
Labels – For displaying Google Profile ID, Name, Email and Email Verification status.
<asp:Button ID="btnLog in" Text="Login" runat="server" OnClick="Login" />
<asp:Panel ID="pnlProfile" runat="server" Visible="false">
    <hr />
    <table>
        <tr>
            <td rowspan="6" valign="top">
                <asp:Image ID="imgProfile" runat="server" Width="50" Height="50" />
            </td>
        </tr>
        <tr>
            <td>ID:
                <asp:Label ID="lblId" runat="server" Text=""></asp:Label>
            </td>
        </tr>
        <tr>
            <td>Name:
                <asp:Label ID="lblName" runat="server" Text=""></asp:Label>
            </td>
        </tr>
        <tr>
            <td>Email:
                <asp:Label ID="lblEmail" runat="server" Text=""></asp:Label>
            </td>
        </tr>
        <tr>
            <td>Verified Email:
                <asp:Label ID="lblVerified" runat="server" Text=""></asp:Label>
            </td>
        </tr>
    </table>
</asp:Panel>
 
 

Namespaces

You will need to import the following namespaces.
C#
using ASPSnippets.GoogleAPI;
using System.Web.Script.Serialization;
 
VB.Net
Imports ASPSnippets.GoogleAPI
Imports System.Web.Script.Serialization
 
 

Property Class

JSON

Following JSON returned from Google API, it consists of some additional information which are not covered in this article and you can easily extend your class as per your requirement.
Google authentication in ASP.Net
 

Class

The following class will be used to hold Google details.
C#
public class GoogleProfile
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string Picture { get; set; }
    public string Email { get; set; }
    public string Verified_Email { get; set; }
}
 
VB.Net
Public Class GoogleProfile
    Public Property Id() As String
    Public Property Name() As String
    Public Property Picture() As String
    Public Property Email() As String
    Public Property Verified_Email() As String
End Class
 
 

Authenticating user using Google account

When the Login button is clicked, page will be redirected to Google for inputting user details for authentication using the Authorize method of GoogleConnect class.
Then, request is sent to get access to the Profile and Email details of the user by passing the profile and email scopes to Authorize method respectively and finally, sent back to the URL set as the RedirectUri.
Note: You need to set valid Redirect Uri in the Google Console App otherwise you will get redirect mismatch error discussed later in the article.
 
C#
protected void Login(object sender, EventArgs e)
{
    GoogleConnect.Authorize("profile", "email");
}
 
VB.Net
Protected Sub Log in(sender As Object, e As EventArgs)
    GoogleConnect.Authorize("profile", "email")
End Sub
 
 

Fetching the User’s Google Plus Profile details

Inside the Page Load event handler, the ClientId, ClientSecret and the RedirectUri properties are set.
Then, inside the Not IsPostBack condition, a check is performed to the Code returned from Google, if the Code is available, then using the Code, user’s Google profile details such as Google Profile ID, Name, Picture, Email and Verified Email status are fetched using the Fetch method of the GoogleConnect class.
Finally, the returned JSON string is deserialized to the GoogleProfile class object using the Deserialize method of JavaScriptSerializer class and set in the respective controls.
C#
protected void Page_Load(object sender, EventArgs e)
{
    GoogleConnect.ClientId = "<Google Client ID>";
    GoogleConnect.ClientSecret = "<Google Client Secret>";
    GoogleConnect.RedirectUri = Request.Url.AbsoluteUri.Split('?')[0];
 
    if (!this.IsPostBack)
    {
        string code = Request.QueryString["code"];
        if (!string.IsNullOrEmpty(code))
        {
            GoogleConnect connect = new GoogleConnect();
            string json = connect.Fetch("me", code);
            GoogleProfile profile = new JavaScriptSerializer().Deserialize<GoogleProfile>(json);
            lblId.Text = profile.Id;
            lblName.Text = profile.Name;
            lblEmail.Text = profile.Email;
            lblVerified.Text = profile.Verified_Email;
            imgProfile.ImageUrl = profile.Picture;
            pnlProfile.Visible = true;
            btnLogin.Enabled = false;
        }
    }
}
 
VB.Net
Protected Sub Page_Load(sender As Object, e As EventArgs)Handles Me.Load
    GoogleConnect.ClientId = "<Google Client ID>"
    GoogleConnect.ClientSecret = "<Google Client Secret>"
    GoogleConnect.RedirectUri = Request.Url.AbsoluteUri.Split("?"c)(0)
 
    If Not Me.IsPostBack Then
        Dim code As String = Request.QueryString("code")
        If Not String.IsNullOrEmpty(code)Then
            Dim connect As GoogleConnect = New GoogleConnect()
            Dim json As String = connect.Fetch("me", code)
            Dim profile As GoogleProfile = New JavaScriptSerializer().Deserialize(Of GoogleProfile)(json)
            lblId.Text = profile.Id
            lblName.Text = profile.Name
            lblEmail.Text = profile.Email
            lblVerified.Text = profile.Verified_Email
            imgProfile.ImageUrl = profile.Picture
            pnlProfile.Visible = True
            btnLogin.Enabled = False
        End If
    End If
End Sub
 
 

Errors

Following are some errors that you might encounter when using the ASPSnippets.GoogleAPI in your projects.
1. invalid_client
Google authentication in ASP.Net
 
The cause of this error is directly related to the Client ID and Client Secret and hence you need to make sure that you have supplied a valid Client ID and Client Secret.
2. invalid_request
Google authentication in ASP.Net
 
This error will occur when the Client ID and/or Client Secret is blank or not supplied.
3. redirect_uri_mismatch
Google authentication in ASP.Net
 
The above error will occur when the RedirectUri set in the code is not matching with the RedirectUri set while configuring the application in Google Developer Console.
For more details on setting RedirectUri, please refer Google Developer Console: Generate Client ID and Client Secret for use with Google APIs.
4. deleted_client
Google authentication in ASP.Net
 
The above error will occur when the Client ID is deleted from Google Developer Console.
 
 

Screenshot

Google authentication in ASP.Net
 
 

Demo

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Downloads

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