In this Post Check Already Email In this Example use in cname,city,emailaddress Insert table Data Value.

Important Namespaces used
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using System.Data;
using System.Data.SqlClient;

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Register Button Click Event Coding
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 if(checkemail()==true)
        {
            Label3.Text = "Your Email Already Registered with Us";
            TextBox3.BackColor = System.Drawing.Color.PaleGreen;
            

        }
        else
        {
            String query = "insert into registrationdetail(cname,city,emailaddress) values('" + TextBox1.Text + "','" + TextBox2.Text + "','" + TextBox3.Text + "')";
            String mycon = "Data Source=HP-PC\\SQLEXPRESS; Initial Catalog=RegisteredData1; Integrated Security=true";
            SqlConnection con = new SqlConnection(mycon);
            con.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.CommandText = query;
            cmd.Connection = con;
            cmd.ExecuteNonQuery();
            Label3.Text = "Registration Has Been Saved Successfully";
            TextBox1.Text = "";
            TextBox2.Text = "";
            TextBox3.Text = "";
        }

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User Defined Method
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private Boolean checkemail()
    {
        Boolean emailavailable = false;
        String mycon = "Data Source=HP-PC\\SQLEXPRESS; Initial Catalog=RegisteredData1; Integrated Security=True";
        String myquery = "Select * from registrationdetail where emailaddress='" + TextBox3.Text+"'";
        SqlConnection con = new SqlConnection(mycon);
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = myquery;
        cmd.Connection = con;
        SqlDataAdapter da = new SqlDataAdapter();
        da.SelectCommand = cmd;
        DataSet ds = new DataSet();
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count > 0)
        {
            emailavailable = true;
          
        }
        con.Close();

        return emailavailable;
    }