title: Invalid cast from 'System.String' to 'System.Guid'
body: <p>I have a submission form that consist of a DropDownList Control; it's ListItem values consist of an UniqueIdentifier, e.g. 524988aa-0594-4ebe-b7fa-61396855e17f.</p>
  
  <p>I'm attempting to insert the value of the ListItem into a table in the database via a stored procedure, but get a "Invalid cast from 'System.String' to 'System.Guid" error upon submission. </p>
  
  <p>Hypertext: </p>
  
  <pre><code> &lt;asp:DropDownList ID="ddlserver" runat="server"&gt;
      &lt;asp:ListItem Text="Server 1" Value="88dc9e7d-0904-4a49-a6d9-8d0badfc4d0b" /&gt;
      &lt;asp:ListItem Text="Server 2" Value="524988aa-0594-4ebe-b7fa-61396855e17f" /&gt;
   &lt;/asp:DropDownList&gt; 
  </code></pre>
  
  <p>Code Behind:</p>
  
  <pre><code>SqlCommand cmd = new SqlCommand();
  cmd.CommandType = CommandType.StoredProcedure;
  cmd.CommandText = "InsertClient";
  cmd.Parameters.Add("@UserId", SqlDbType.UniqueIdentifier).Value = ddlserver.SelectedValue.ToString();
  cmd.Parameters.Add("@ClientName", SqlDbType.NVarChar).Value = txtName.Text;
  cmd.Parameters.Add("@EmailAddress", SqlDbType.NVarChar).Value = txtEmail.Text;
  cmd.Parameters.Add("@PhoneNumber", SqlDbType.NVarChar).Value = txtPhone.Text;
  cmd.Parameters.Add("@ServicesNum", SqlDbType.NVarChar).Value = rbzero.Text;
  cmd.Connection = con;
  </code></pre>
  
  <p>stored Procedure:</p>
  
  <pre><code>ALTER PROCEDURE [dbo].[InsertClient]
  
      @UserId UniqueIdentifier,
      @ClientName nvarchar(150),
      @EmailAddress nvarchar(150),
      @PhoneNumber nvarchar(50),
      @ServicesNum nvarchar(50)
  
  AS
  BEGIN
      SET NOCOUNT ON;
      INSERT INTO  Client_tb (UserId, ClientName, EmailAddress, PhoneNumber, ServicesNum)
      VALUES (@UserId, @ClientName,  @EmailAddress, @PhoneNumber, @ServicesNum) 
  END
  </code></pre>
  
  <p>...could I get some assistance please as to what I'm doing wrong?</p>