title: Invalid cast from 'System.String' to 'System.Guid
body: <p>My page uses a GridView to show all clients to the user after he or she logs in. However I'm trying to get the GridView to show only those clients that belong to the respective user.</p>
  
  <p>...the code behind: </p>
  
  <pre><code>String strConnString = ConfigurationManager
      .ConnectionStrings["BA_2013ConnectionString"].ConnectionString;
  SqlConnection con = new SqlConnection(strConnString);
  SqlCommand cmd = new SqlCommand();
  cmd.CommandType = CommandType.StoredProcedure;
  cmd.CommandText = "getClients";
  cmd.Parameters
      .Add("@UserId", SqlDbType.UniqueIdentifier)
      .Value = txtUserId.Text.ToString();
  cmd.Connection = con;
  
  con.Open();
  
  gvClients.DataSource = cmd.ExecuteReader();
  gvClients.DataBind();
  
  con.Close();
  con.Dispose(); 
  </code></pre>
  
  <p>...the stored procedure:</p>
  
  <pre><code>ALTER PROCEDURE dbo.getClients
  @UserId uniqueidentifier
  AS
  BEGIN
      SELECT ClientId, UserId, ClientName, EmailAddress, PhoneNumber, ServicesNum 
      FROM  Client_tb 
      WHERE UserId = @UserId  
  
  END
  </code></pre>
  
  <p>when I run the page, I get the error: Invalid cast from 'System.String' to 'System.Guid
  ...could I get some help with this please?</p>