Get message 'System.Winform.ErrorProv'

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C#

SQL-Server-2008R2

I create the insert trigger in the registration table.

Registration table:

UserID | UserName | Password | Confirm | Email

Insert trigger:

ALTER TRIGGER [dbo].[LimitTtblReg]
on [dbo].[tblRegF]
after insert
as
    declare @tableCount int
    select @tableCount = Count(*)
    from tblRegF

    if @tableCount > 1
    begin
        rollback
    end

C# code on Registration button:

if (this.ValidateChildren())
    {

       string c = ConfigurationManager.ConnectionStrings["Constr"].ConnectionString;
       SqlConnection con = new SqlConnection(c);
             try
             {
                 con.Open();
                 SqlCommand cmd = new SqlCommand("RegistrationForm", con);
                 cmd.CommandType = CommandType.StoredProcedure;
                 cmd.Parameters.AddWithValue("@UserName", txtRegUserN.Text);
                 cmd.Parameters.AddWithValue("@Password", txtRegPass.Text);
                 cmd.Parameters.AddWithValue("@Confirm", txtRegPassConfirm.Text);
                 cmd.Parameters.AddWithValue("@Email", txtRegEmail.Text);
                 SqlDataReader dr = cmd.ExecuteReader();
                 MessageBox.Show("Data Inserted Succesfully");
                 con.Close();
                 clear();
                 this.Close();
             }
             catch (Exception)
             {
               MessageBox.Show("you already have been registered);
             }

       else
         {
             var listOfErrors = this.errorProv.ContainerControl.Controls
                                 .Cast<Control>()
                                 .Select(c => this.errorProv.GetError(c))
                                 .Where(s => !string.IsNullOrEmpty(s))
                                 .ToList();
             MessageBox.Show("please correct validation errors:\n - " +
                 string.Join("\n - ", listOfErrors.ToArray()),
                 "Error",
                 MessageBoxButtons.OK, MessageBoxIcon.Error);
         }

    }

I've set the validating event on text box and auto validate on form...
When I try to enter the information second time it shows me message box .
' System.Winform.ErrorProv'
I want to show the message
'You have already been registered'.
Please tell me how to resolve this problem.